UVCS/SOHO PAPERS - 1999

1999-75:   Astudy of solar wind acceleration based on gyrotropic transport equations

P. Young and R. Esser

1999-73:   On the discrepancy between atmospheric and coronal electron densities

R. Esser and D. Sasselov

Ap J Lett., volume 521, L145

Electron densities in the lower solar atmosphere (photosphere and chromosphere) are derived using a number of different atmospheric models which are constrained by observed spectral lines. Comparing these atmospheric densities to coronal electron densities derived from polarization brightness measurements in the region from about 1.1 to several solar radii, it is shown that there is a discrepancy between the two sets of densities. The atmospheric electron densities are in agreement with a density of maximum 10^7 cm^-3 at 1.1 R_solar. The polarized brightness densities given in the literature are typically 5x10^7 cm^-3 or higher. It is shown that this discrepancy might be due to an overestimation of the coronal electron densities below 1.5-2 R_solar.

1999-72:   The Impact of Ion-Cyclotron Wave Dissipation on Heating and Accelerating the Fast Solar Wind

Cranmer, S. R., Field, G. B., and Kohl, J. L.

AIP Conf. Proc. 471, page 35

1999-71:   Spectroscopic Constraints on Models of Ion-Cyclotron Resonance Heating in the Polar Solar Corona

Cranmer, S. R., Field, G. B., and Kohl, J. L.

Space Sci. Rev., 87, 149

1999-70:   Heating and cooling of protons by turbulence-driven ion cyclotron waves in the fast solar wind

Li, Xing; Habbal, Shadia R.; Hollweg, Joseph V.; Esser, Ruth

JGR 104, A2, 2521-2536

1999-69:    Density Fluctuations in Polar Coronal Holes: Mechanism and Observational Consequences

Kaghashvili, E. Kh.; Esser, R.

AIP Conference Proceedings, Vol. 471, p.341

1999-68:    Whole Sun Month at solar minimum: An introduction

Galvin, Antoinette B., Kohl, John L.

Journal of Geophysical Research, 104, 9673

1999-67:   Flat field of UVCS detectors for early part of SOHO mission

Cosmo, Mario L.; Smith, Peter L.; Atkins, Nigel; Suleiman, Raid M.; Gardner, Larry D.; Kohl, John L.

Proc. SPIE Vol. 3764, p. 161-167, Ultraviolet and X-Ray Detection, Spectroscopy, and Polarimetry III, Silvano Fineschi; Bruce E. Woodgate; Randy A. Kimble; Eds.

The Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO) comprises two telescopes and two spectrometer channels for spatially resolved ultraviolet spectral diagnostics of the solar corona. The principal lines for which the two channels are optimized are the H I 'Lyman-(alpha)' line at 121.5 nm and the O VI (O5+) doublet at 103.2 and 103.7 nm. An 'in-flight' method, using observations of stars and scattered solar disk light, has been devised to determine the flat field function, i.e., the relative detection efficiency of the detector pixels.  We present the details and results of this process. Local pixel-to-pixel efficiency variation is found to be, typically, about plus or minus 9% to plus or minus 17% (1 (sigma) ) for the H I Lyman-(alpha) channel and plus or minus 9% for the O VI channel.

1999-66:   Ultraviolet and Optical Observations of a Coronal Transient With SOHO

Ciaravella, A., Raymond, J.C., Strachan, L., Thompson, B., St-Cyr, O.C., Gardner, L., Modigliani, A., Antonucci, E., Kohl, J., Noci, G.

ApJ 510, 1053 (1999)

A coronal transient was observed on 1997 March 6 at 1.6 R over an active region on the east limb. We observed both the edge of horizontally compressed gas and the diffuse curtain of coronal material. The region was monitored for 4 hr, and the H I Ly, O VI 1031.91, 1037.61, N V 1242.80, 1238.80, and O V] 1218.35 lines were detected during the ejection evolution. The density, velocity, temperature, and oxygen abundance of the ejected plasma have been obtained from the observed spectra. Intermediate temperature lines of N V, O VI, and O V show a large enhancement, suggesting a quite narrow range of plasma temperature around 4×10 K. Doppler shifts of the ejected material evolve from an initial blueshift of 100 km s-1 to a redshift of 145 km s-1. The outflow velocity, as determined by Doppler dimming analysis of the O VI doublet, is only about 20 km s-1.

1999-65:   Radiation scattering in the solar corona

G. Noci and L. Maccari

Astron. Astrophys. 341, 275 (1999).

This paper discusses the scattering in the solar corona of the radiation emitted by the solar disk. We obtain expressions for the emissivity which permit to deduce the spectral characteristics of the scattered radiation. With these expressions, assuming a Maxwellian distribution for the velocities of the scattering ions and a simple spectral shape of the exciting radiation, we produce approximate analytic formulae  which show some interesting properties of the scattered radiation that do not clearly emerge from the exact expressions. Among them, the expression for the Doppler shift of the radiatively excited component of a line, and that for its width. We also examine the case of bi-Maxwellian velocity distributions and  the scattering by the free electrons.

1999-64:    Coronal hole diagnostics out to 8R_{sun}

Doyle, J. G., Teriaca, L., Banerjee, D.

A&A, 349, 956

The Si viii line width measurements and N_e estimates based on SUMER observations are combined with LASCO and UVCS output to provide an overview of its variations with height above a polar coronal hole. From the combined dataset we find a radial dependence of the electron density, in the range 1-2 R_{sun} as r(-8) , from 2 to 4 R_{sun} as r(-4) and then as r(-2) . Combining the Si viii half width at 1/e of the peak intensity with the UVCS O vi half width, we find a small increase of the half width from 1 to 1.2 R_{sun}, then a plateau until 1.5 R_{sun}, thereafter a sharp increase until 2 R_{sun}, finally a more gradual increase reaching 550 km s(-1) at 3.5 R_{sun}. Our data suggests that the MHD waves responsible for the excess line broadening tends to become non-linear as it reaches 1.2 R_{sun}.

Y.-K. Ko and C. P. T. Groth

Space Science Reviews 87: 227-231, 1999

The electron temperature profile in the polar coronal hole inferred by the solar wind ionic charge state data exhibits a local maximum of about 1.5 million degrees (Ko et al. 1997). This indicates the existence of electron heating in the inner coronal region. In this paper, a two-fluid solar wind model, which incorporates additional 'mechanical' heating, is used to investigate the heating of the electrons in the coronal hole. We find that the classical collision-dominated expressions or the electron conduction heat flux are not strictly valid and need to be severely limited in order or the electron temperatures predicted by the model to agree with constraintssupplied by both the solar wind ionic charge state data and the solar wind plasma properties observed at 1 AU. The corresponding constraints on the magnitude of the coronal heating will also be discussed.

1999-62:   Ulysses-SWICS Constraints on Non-Maxwellian Electron Distributions in a Polar Coronal Hole

S. P. Owocki and Y.-K. Ko

AIP Conf. Proc. 471, 263 (1999) [Solar Wind Nine],

The Solar Wind Ion Composition Spectrometer (SWICS) on board Ulysses has compiled an extensive collection of ion charge state measurements in high-speed-wind streams. These provide important diagnostic constraints for the acceleration region of the large south polar coronal hole in which these charge states were ``frozen-in". Initial analyses of these data have inferred that the coronal electron distribution may deviate modestly from a Maxwellian (Ko et al. 1996),  or that the coronal outflow speeds of heavy ion may vary inversely with the ion mass  (Ko et al. 1997).  In this paper we apply a generalized freezing-in analysis to examine carefully the robustness and uniqueness of these inferences. In particular, we emphasize that careful attention to the ionization states of both Oxygen and Carbon provides the best potential diagnostic for a non-Maxwellian distribution of coronal electrons, since the similarity in their overall rate coefficients suggests a similar freezing-in location, while differences in their (comparitively high) ionization potentials provide a differential sensitivity to a high-energy electron tail. We also discuss the possibility that the freezing-in of the ionization state of these elements may begin in the underlying transition region of their source coronal hole.

1999-61:   Three-dimensional  reconstruction of UV emissivities in the solar corona using Ultraviolet Coronagraph Spectrometer data from the Whole Sun Month

Panasyuk, A.V.

JGR 104, 9721-6 (1999)

UV coronal intensity data collected by the Ultraviolet Coronagraph Spectrometer on SOHO in a daily synoptic program during the Whole Sun Month are used to reconstruct the 3D distribution of emissivities in the solar corona for H I Ly\alpha  and the O VI doublet at 1032 and 1037 \AA . We discuss the specifics that complicate a tomographic reconstruction of the solar corona and estimate the uncertainties introduced in the reconstruction by them.

1999-60:   Solar Wind Acceleration Region

E. Antonucci

ESA SP-446, 53 (1999) [SOHO 8 Workshop]

abstract

1999-59:   Study of the heating mechanism of solar wind ions in coronal holes

Tu, C.-Y.; Marsch, E.

AIP Conf. Proce. 471, 373 (1999) [Solar Wind Nine]

1999-58:   Spectroscopic diagnostics for remote detection of particle acceleration regions at coronal shocks

Kahler, S. W.; Raymond, J. C.; Laming, J. M.

AIP Conf. Proc. 471, 685 (1999) [Solar Wind Nine]

We suggest a possible technique for remotely sensing the acceleration of solar energetic ions to E > 1 MeV at coronal shock fronts.  It involves the observation of the wings of line emission from ions accelerated in shocks, which could be done with the UVCS experiment on the SOHO spacecraft.  UVCS has two UV channels producing stigmatic spectra from 42 arc min slits that can observe the Ly alpha and O VI lines, both of which could be broadened with the passage of a shock. In that case, the Ly alpha line formation results from proton charge exchange and collisional excitation of hydrogen atoms, while the broadened O VI lines are due to directly accelerated O5+ ions. The search criterion for candidate SOHO events consists of UVCS observations of coronal regions flanking or ahead of fast (v > 400 km/s) white light CMEs observed with the LASCO coronagraph.

1999-57:   Ion-cyclotron wave dissipation channel for Alfven waves

E.K. Kaghashvili

Geophys. Res. Lett. 26, 1817

The effect of inhomogeneous velocity flow on Alfvén wave dynamics in the solar wind is investigated. Unlike existing studies in this field that model energy deposition from high-frequency waves to solar wind particles, we consider here the possibility of obtaining high-frequency waves (in this case fast magnetosonic waves) from the ordinary Alfvén wave spectrum. It is shown that the spatial inhomogeneity of the velocity field can lead to dissipation of Alfvén waves through ion-cyclotron resonance. Recent Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) observations indicate that preferential heating and acceleration of heavy ions occurs close to the Sun. The process described here provides a possible interpretation of such solar wind observations.

1999-56:   Working Group 3 Report: Coronal hole boundaries and interaction with adjacent regions

R. Esser

Space Science Reviews 87: 93-104 (1999)

Summarized below are the discussions of working group 3 on ``Coronal hole boundaries and interactions with adjacent regions'' which took place at the 7th SOHO workshop in Northeast Harbor, Maine, USA, 28 September to 1 October 1998. A number of recent observational and theoretical results were presented during the discussions to shed light on different aspects of coronal hole boundaries. The working group also included presentations on streamers and coronal holes to emphasis the difference between the plasma properties in these regions, and to serve as guidelines for the definition of the boundaries. Observations, particularly white light observations, show that multiple streamers are present close to the solar limb at all times. At some distance from the sun, typically below 2 R_Sun, these streamers merge into a relatively narrow sheet as seen in for example LASCO and UVCS images. The presence of multiple current sheets in interplanetary space was also briefly addressed. Coronal hole boundaries were defined as the abrupt transition from the bright appearing plasma sheet to the dark coronal hole regions. Observations in the inner corona seem to indicate a transition of typically 10 to 20 deg, whereas observations in interplanetary space carried out from Ulysses, show on one hand,an even faster transition of less than 2 deg which is in agreement with earlier HELIOS results. On the other hand, these observations also show that the transition happens on different scales, some of which are significantly larger.  The slow solar wind is connected to thestreamer belt/plasma sheet, even though, the discussions were still not conclusive on the point where exactly the slow solar wind originates. Considered the high variability of plasma characteristics in slow wind streams, it seems most likely that several types of coronal regions produce slow solar wind, such as streamer stalks, streamer legs and open field regions between active regions, and maybe even regions just inside of the coronal holes. Observational and theoretical studies presented during the discussions show evidence that each of these regions may indeed contribute to the solar slow wind.

1999-55:   Physical properties of coronal streamers as observed by Spartan 201

Miralles, M. P.; Strachan, L.; Gardner, L. D.; Smith, P. L.; Kohl, J. L. ; Guhathakurta, M.; Fisher, R. R.

AIP Conf. Proc. 471, 239 (1999) [Solar Wind Nine]
 

The three Spartan 201 flights from 1993 to 1995 provided us with observations in HI Lyman-$\alpha$ of several streamers in the declining phase of the current solar cycle. Analysis of the heliocentric height dependence of the HI Ly-$\alpha$ LOS velocity distribution width clearly show that there is a maximum in the proton 1/e velocities at heights ranging from 2.1 to 2.5 R$_{\odot}$ for all of the observed streamers. We compare UVCS/Spartan results with proton velocity distributions  from UVCS/SOHO during solar minimum. We also discuss differences in the proton velocity distributions for different types of streamers over the three year period of Spartan 201 observations.

This work is supported by the National Aeronautics and Space Administration under grants NAG 5-613 and 5-3192 to the Smithsonian Astrophysical
Observatory. M. G. & R. F. acknowledge grant from NASA suborbital research program of the Space Physics Division and NASA grant 5-2881 to the Catholic University of America.

1999-54:   Properties of coronal hole/streamer boundaries and adjacent regions as observed by Spartan 201

Miralles, M.P.; Strachan,  L.; Gardner, L.D.; Smith, P.L.; Kohl, J.L.; Guhathakurta, M.; Fisher, R.

Space Science Reviews 87: 277-281 (1999)
 

The Spartan 201 flights from 1993 to 1995 provided us with observations in H I Lyman-$\alpha$ of several coronal hole/streamer boundaries and adjacent streamers during the declining phase of the current solar cycle. Analysis of the latitudinal dependence of the line intensities clearly shows that there is a boundary region at the coronal hole/streamer interface where the H I Lyman-$\alpha$ intensity reaches a minimum value. Similar results are also found in UVCS/SOHO observations.  We also discuss differences in the coronal hole/streamer boundaries for different types of streamers and their changes over the three year period of Spartan 201 observations.

This work is supported by NASA under grants NAG 5-613 and 5-3192 to the Smithsonian Astrophysical Observatory. M. G. and R. F. acknowledge grant from NASA suborbital research program of the Space Physics Division and NASA grant 5-2881 to the Catholic University of  America.

1999-53:   UVCS / SOHO observations of spectral line profiles in polar coronal holes

Kohl, J.L.; Fineschi, S.; Esser, R.; Ciaravella, A.; Cranmer, S.R.; Gardner, L.D.; Suleiman, R.; Noci, G.; Modigliani, A.

Space Science Reviews 87: 233-236 (1999)

Ultraviolet emission line profiles have been measured on 15-29 September 1997 for H~I 1216 A, O~VI 1032, 1037 A and Mg~X 625 A in a polar coronal hole, at heliographic heights R_sun (in solar radii) between 1.34 and 2.0. Observations of H~I 1216 A and the O~VI doublet from January 1997 for R_sun = 1.5 to 3.0 are provided for comparison. Mg~X 625 A is observed to have a narrow component at R_sun = 1.34 which accounts for only a small fraction of the observed spectral radiance, and a broad component that exists at all observed heights. The widths of O~VI broad components are only slightly larger than those for H~I at R_sun= 1.34, but are significantly larger at R_sun = 1.5 and much larger for R_sun > 1.75.  In contrast, the Mg~X values are less than those of H~I up to 1.75 and then increase rapidly up to at least R_sun = 2.0, but never reach the values of O~VI.

1999-52:   Ly-alpha observation of a coronal streamer with  UVCS/ SOHO

Maccari,  L.; Noci, G.; Fineschi, S.; Modigliani, A.; Romoli, M.; Kohl, J.L.

Space Science Reviews 87: 265-268 (1999)

In this paper we discuss some characteristics of an equatorial streamer observed by UVCS in July 1997. We determine the height distribution of  the Ly-alpha total intensity and of its width. We focus our attention, in particular, on the time variability of these parameters.

1999-51:   Comparison of Outflow Velocity Determinations with UVCS and LASCO for the Coronal Mass Ejection of 13-14 Aug 1997

L. Strachan, A. Ciaravella, J. C. Raymond, S. Fineschi, R. O'Neal, J. L. Kohl, A. Modigliani, G. Noci, and M. D. Andrews

AIP Conf. Proc. 471, 637 (1999) [Solar Wind Nine]
 

The Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO observed a Coronal Mass Ejection (CME) on 13-14 August 1997. The event was observed simultaneously with the LASCO white light coronagraphs. This paper describes the results from a comparison of outflow velocities determined from UVCS Doppler dimming studies with the velocities determined by examining the proper motions of the CME as measured by LASCO. In addition, estimates of expansion velocities and line of sight motions are discussed. This work is supported in part by NASA under grant NAG-3192 to the Smithsonian Astrophysical Observatory, by the Italian Space Agency and by Swiss funding agencies.

1999-50:   Constraints on Coronal Outflow Velocities derived from UVCS Doppler Dimming Measurements and In-Situ Charge State Data

L. Strachan, Y-K. Ko, A.V. Panasyuk, D. Dobrzycka, J.L. Kohl M. Romoli, G. Noci, S.E. Gibson, D. A. Biesecker

Space Sci Rev, 87, 311 (1999)
 

We constrain coronal outflow velocity solutions, resolved along the line-of-sight, by using Doppler dimming models of H I Lyman alpha and O VI 1032/1037 Angstrom emissivities obtained with data from the Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO. The local emissivities, from heliocentric heights of 1.5 to 3.0 radii, were determined from 3-D reconstructions of line-of-sight intensities obtained during the first Whole Sun Month Campaign (10 Aug. to 8 Sep. 1996). The models use electron densities derived from polarized brightness measurements made with the visible light coronagraphs on UVCS and LASCO, supplemented with data from Mark III at NCAR/MLSO. Electron temperature profiles are derived from `freezing-in' temperatures obtained from an analysis of charge state data from SWICS/ Ulysses. The work concentrates on neutral hydrogen outflow velocities which depend on modeling the absolute coronal emissivities. We use an iterative method to determine the neutral hydrogen outflow velocity with consistent values for the electron temperatures derived from a freezing-in model.

1999-49:    Composition Variations in the Solar Corona and Solar Wind

Raymond, J. C.

Space Science Reviews, 87, 55

1999-48:   Synoptic Sun during the first Whole Sun Month Campaign: August 10 to September 8, 1996

BIESECKER, D. A.; THOMPSON, B. J.; GIBSON, S. E.; ALEXANDER, D.; FLUDRA, A.; GOPALSWAMY, N.; HOEKSEMA, J. T.; LECINSKI, A.; STRACHAN, L.,

Space Science Reviews 87: 277-281 (1999)

1999-47:   The Three-dimensional Coronal Magnetic Field during Whole Sun Month

GIBSON, S. E.; BIESECKER, D.; GUHATHAKURTA, M.; HOEKSEMA, J. T.; LAZARUS, A. J.; LINKER, J.;MIKIC, Z.; PISANKO, Y.; RILEY, P.; STEINBERG, J.; STRACHAN, L.; SZABO, A.; THOMPSON, B. J.; ZHAO, X. P.,

ApJ, 520, 871, 1999

Combining models and observations, we study the three-dimensional coronal magnetic field during a period of extensive coordinated solar observations and analysis known as the Whole Sun Month (WSM) campaign (1996 August 10-September 8). The two main goals of the WSM campaign are addressed in this paper, namely, (1) to use the field configuration to link coronal features observed by coronagraphs and imaging telescopes to solar wind speed variations observed in situ and (2) to study the role of the three-dimensional coronal magnetic field in coronal force balance. Specifically, we consider how the magnetic field connects the two fastest wind streams to the two regions that have been the main foci of the WSM analysis: the equatorial extension of the north coronal hole (known as the Elephant's Trunk) and the axisymmetric streamer belt region on the opposite side of the Sun. We then quantitatively compare the different model predictions of coronal plasma and solar wind properties with observations and consider the implications for coronal force balance and solar wind acceleration. "Latitudinal Dependence of Outflow Velocities from O VI Doppler Dimming Observations during the Whole Sun Month" L. Strachan, A. V. Panasyuk, D. Dobrzycka, John L. Kohl, G. Noci, S. E. Gibson, D. A. Biesecker, Submitted JGR 1999 Abstract Empirical determinations of outflow velocities in the solar corona provide a much needed constraint, along with density and temperature determinations, of the acceleration and heating mechanisms in the extended corona. Much progress has been made on density determinations from white light polarized brightness observations but outflow velocities have been more difficult to determine. We present the first determinations of outflow velocities versus height and latitude based on a 3D reconstruction of the O VI 1032 and 1037 Angstrom emissivities. The Doppler dimming (and pumping) of the local emissivities give true localized outflow velocities at the selected locations in the extended corona from about 1.75 to 2.75 solar radii. The velocities are based on using the O VI emissivity ratios from SOHO/UVCS synoptic observations and an empirical model which uses densities determined from SOHO/LASCO and MLSO Mk III white light measurements.

1999-46:   A Kinetic Model of Coronal Heating and Acceleration by Ion-Cyclotron Waves: Preliminary Results

Philip A. Isenberg, Martin A. Lee, and Joseph V. Hollweg

Solar Physics, Topical Issue on the Physics of the Solar Corona and Transition Region, in press

We present a kinetic model of the heating and acceleration of coronal protons by outward-propagating ion-cyclotron waves on open, radial magnetic flux tubes. In contrast to fluid models which typically insist on bi-Maxwellian distributions and which spread the wave energy and momentum over the entire proton population, this model follows the kinetic evolution of the collisionless proton distribution function in response to the combination of the resonant wave-particle interaction and external forces.  The approximation is made that pitch-angle scattering by the waves is faster than all other processes, resulting in proton distributions which are uniform over the resonant surfaces in velocity space.  We further assume, in this preliminary version, that the waves are dispersionless so these resonant surfaces are portions of spheres centered on the radial sum of the Alfven speed and the proton bulk speed.  We incorporate the fact that only those protons with radial speeds less than the bulk speed will be resonant with outward-propagating waves, so this rapid interaction acts only on the sunward half of the distribution.  Despite this limitation, we find that the strongperpendicular heating of the resonant particles, coupled with the mirror force, results in substantial outward acceleration of the entire distribution.  The proton distribution evolves towards an incomplete shell in velocity space, and appears vastly different from the distributions assumed in fluid models.  Evidence of these distinctive distributions should be observable by instruments on Solar Probe.

1999-45:    Solar Wind Acceleration Region

Antonucci, E.

ESA , SP-446, 53

1999-44:   Physical conditions in a high--latitude streamer from UVCS and CDS observations

S. Parenti, G. Poletto, J. Raymond and B. Bromage

ESA SP-446, 531 (1999) [SOHO 8 Workshop]

UVCS and CDS observations of a streamer in the southern hemisphere, at a latitude of 40 Degrees, have been acquired on March 8, 1998. UVCS data have been taken at an heliocentric altitude of 1.6 solar radii and cover the spectral range from 950 to 1250 Angstrom; CDS data have been taken at an altitude of 1.1 solar radii and cover the range from 308 to 381, and 513 to 633, Angstrom. These data have been used to determine the physical conditions of a streamer structure: in particular, we give an estimate of the electron temperature, electron density and element abundance in the structure. The electron temperature has been evaluated from lines of different ions from the same elements, crude values of densities have been derived from an analysis of the OVI doublet lines at 1032 and 1037 Angstrom and element abundances have been estimated both from a DEM (Differential Emission Measure) analysis and from other techniques. Because UVCS spectra have been taken at different times, during the day, we have been looking also at temporal variations in the physical parameters of the streamer. Changes across the streamer have been analyzed as well. Coordinated CDS and UVCS observations allow us also to compare results from the two experiments and look for variations, with solar distance, of the streamer physical parameters.

1999-43:    A study of solar wind acceleration based on gyrotropic transport equations

Olsen, E. L., Leer, E

Journal of Geophysical Research, 104, 9963

1999-42:   Resonant heating and acceleration of protons and ions in coronal holes: two-proton closure

Hollweg, J. V.

ESA SP-446, 357 (1999) [SOHO 8 Workshop]

The ion-cyclotron mode does not extend above the proton cyclotron frequency. Thus only roughly half of the protons can be in resonance. We calculate the trajectories of individual protons in the electric, magnetic, and gravitational fields in a coronal hole, and we include the resonant heating and acceleration for an average resonant particle. To provide closure we consider two protons, which are proxies for the resonant and non-resonant halves of the distribution. For dispersive waves, k_res becomes large. If the dissipation is determined by a turbulent cascade, k_res controls the relative importance of resonant acceleration and resonant heating. Such models yield good agreement with what is known about the behavior of protons in coronal holes. We also consider, in a cruder model, the behavior of minor ions such as O+5. We show qualitatively how such ions can be more than mass-proportionally heated, and that dispersion is important. We also show that the observed radial profile of T_perp for O+5 requires a steep power spectrum. It is concluded that the cyclotron resonance can account for many observed features of protons and heavy minor ions in coronal holes. However, the source of the resonant waves remains uncertain.

1999-41:   Study of the latitudinal dependence of H I Ly-alpha and O VI emission in the solar corona - evidence for the super-radial geometry of the outflow in the polar coronal holes

D. Dobrzycka, S. R. Cranmer, A. V. Panasyuk, L. Strachan & J. L. Kohl

1999, JGR, 104, 9791

We study the latitudinal distribution of the H I Ly-alpha and O VI (103.2 nm,103.7 nm) line emission during the period of the Whole Sun Month campaign (August, 10 -- September,8, 1996) when the Sun was close to the minimum of its activity. The H I Ly-alpha and O VI line intensities appeared to be almost constant with latitude within the polar coronal holes and have abrupt increases towards the streamer region. We found that both north and south polar coronal holes had similar line intensities, line--of--sight velocities, as well as kinetic temperatures of H^{0} and O^{5+}. The dependence of these parameters on latitude and radius is provided. We derived boundaries of the polar coronal holes based on the H I Ly-alpha and O VI line intensity distributions for several days during the Whole Sun Month campaign. We found that the polar coronal hole boundaries clearly have a superradial geometry with diverging factor f_{max} ranging from 6.0 to 7.5 and they are consistent with boundaries previously derived from the electron density distributions. We also found that, in general, they are not symmetric with respect to the heliographic poles and their size and geometry change over periods of days. The H I Ly-alpha, O VI (103.2 nm), and the O VI (103.7 nm) line intensities showed similar boundaries within the uncertainties of our data. We modeled the latitudinal distribution of the H I Ly-alpha and O VI (103.2 nm,103.7 nm) line intensities in the south polar coronal hole.

1999-40:   Coronal Holes and Solar Wind Acceleration: Proceedings of the SOHO-7 Workshop

J. L. Kohl and S. R. Cranmer  (eds.)

Kluwer Academic Publishers, Dordrecht, 1999, ISBN 0-7923-5828-7.
Also published as vol. 87, no. 1-2 of Space Science Reviews.

S. T. Suess and S. Nerney.

ESA SP-448, 1101-1106 (1999) [European Solar Physics Meeting, Florence, Italy]

Flow tubes adjacent to closed magnetic field lines on the boundaries of streamers can have extremely large geometric spreading factors. Numerical models in this thin layer are subject to grid definition uncertainties. Therefore, we compute flow tube geometry using the analytic model of streamer structure described by Pneuman (1968). This model has been found to be more widely applicable than commonly believed as a consequence of observations made with SOHO/UVCS and YOKHOH/SXT (Li et al., 1998). We use the model to compute the radial dependence of flow tube geometry (the ``spreading factors'') for several different streamer models. The results are used to analyze the hypothesis that extremely slow flows in these open flow tubes may cause high densities relative to adjacent coronal hole flow. Such high density could mean that the streamer brightness boundary is defined by the open flow tubes adjacent to streamers rather than closed field lines.

1999-38:   A two-fluid, MHD coronal model

Suess, S. T., A.-H. Wang, S. T. Wu, G. Poletto, and D. J. McComas

1999 JGR 104, 4697

We describe first results from a numerical two-fluid MHD model of the global structure of the solar corona. The model is two-fluid in the sense that it accounts for the collisional energy exchange between protons and electrons. As in our single-fluid model, volumetric heat and momentum sources are required to produce high speed wind from coronal holes, low speed wind above streamers, and mass fluxes similar to the empirical solar wind. By specifying different proton and electron heating functions we obtain a high proton temperature in the coronal hole and a relatively low proton temperature in the streamer (in comparison with the electron temperature). This is consistent with inferences from SOHO/UVCS [ Feldman et al., 1977;  Kohl et al., 1997], and with the Ulysses/SWOOPS proton and electron temperature measurements which we show from the fast latitude scan. The density in the coronal hole between 2 solar radii and 5 solar radii ($2R_S$ and $5R_S$) is similar to the density reported from SPARTAN 201-01 measurements by {\it Fisher and Guhathakurta} [1994]. The proton mass flux scaled to 1 AU is $2.4\times 10^8 cm^{-2}s^{-1}$, which is consistent with Ulysses observations [ Phillips et al., 1995]. Inside the closed field region, the density is sufficiently high so that the simulation gives equal proton and electron temperatures due to the high collision rate. In open field regions (in the coronal hole and above the streamer) the proton and electron temperatures differ by varying amounts. In the streamer, the temperature and density are similar to those reported empirically by  Li et al. [1998] and the plasma $\beta$ is larger than unity everywhere above $\sim 1.5\ R_S$, as it is in all other MHD coronal streamer models [e.g. Steinolfson et al., 1982; Gary and Alexander, 1998]. 

1999-37:   UVCS/SOHO Ion Kinetics in Coronal Streamers

Frazin, R.A., A. Ciaravella, E. Dennis, S. Fineschi, L. Gardner, J. Michels, R. O'Neal, J. Raymond, R. Wu, J. Kohl, A. Modigliani & G. Noci.

Space Science Reviews 87:189-192, 1999

We made streamer observations with the Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO) during the early part of 1998, which was a time of moderate solar activity. We present an empirical study of coronal ion kinetics using the line profiles from these observations. Our first and most striking result is that the mid-latitude (ML) streamers have much narrower O VI 1032A line profiles than the solar minimum equatorial (SME) streamers. Our second result is that the line profiles from a small collection of ions in ML streamers do not seem to be consistent with the ions having a single temperature and turbulent velocity. We discuss several interpretations, including line of sight (LOS) effects. This work is supported by the National Aeronautics and Space Administration under grant NAG-3192 to the Smithsonian Astrophysical Observatory.

1999-36:   UVCS/SOHO Observations of Coronal Streamers

Frazin, R.A., et al.

AIP Conf. Proc. 471 235 (1999) [Solar Wind Nine]

We used the Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO) to obtain line profiles in mid-latitude coronal streamers between 1.3 R(sun) and 5.5 R(sun) during a period of moderate solar activity. We present a summary of the preliminary results. These results clearly indicate that the mid-latitude streamers observed during this time period have very different spectral properties than the equatorial streamers observed near solar minimum.

1999-35:   CORONAL MAGNETIC FIELD DIAGNOSTICS WITH UV SPECTRO-POLARIMETRY

S. Fineschi, A. van Ballegoijen, J. L. Kohl

ESA SP-446, 317 (1999) [SOHO 8 Workshop]
 

In the extended corona, the state of linear polarization of ultraviolet (UV) line emission, is affected by the presence of magnetic fields. This provides a useful diagnostics of the coronal magnetic field, that, to date, is still largely unknown. The interpretation through the Hanle effect of the strength and direction of the polarization vector of the H~I~Lyman series lines (i.e., Ly-alpha, 1216 A, Ly-beta, 1025 A, Ly-gamma, 972 A) yields information on the vector magnetic field in corona. The polarization direction of the O~VI, 1032 A, line is affected by non-radial outflow velocities of the solar wind. This may be used to obtain information on the coronal field topology. The parameter study presented here shows that the H~I~Lyman series lines have sensitivities to the field stregths larger than 1~gauss, at heliocentric heights comprised between 1.2 to 2~solar radii. The Advanced Solar Coronal Explorer mission, briefly described here, is designed to carry out spectro-polarimetric measurements of these UV coronal lines. UVCS/SOHO

1999-34:   Observations of H I Lyman Alpha Line Profiles in Coronal Holes at Heliocentric Heights above 3.0 R_sun

R. M. Suleiman, J. L. Kohl, A.V. Panasyuk, A. Ciaravella, S. R. Cranmer, L. D. Gardner, R. Frazin, R. Hauck, P. L. Smith

Space Science Reviews 87: 327-330, 1999

The Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO) has been used to measure spectral line profiles for  H I Ly alpha in the south polar coronal hole at projected heliocentric heights from 3.5 to 6.0 R_sun during 1998 January 5 - 11. Observations from 1.5 to 2.5 R_sun were made for comparison. The H I Ly alpha profile is the only one observable  with UVCS above 3.0~R_sun  in coronal holes. Within this region the outflowing coronal plasma becomes nearly collisionless and the ionization  balance is believed to become frozen.

In this paper, the 1/e half widths of the coronal velocity distributions are provided for the observed heights. The velocity distributions include all motions contributing to the velocities along the line of sight (LOS). The observations have been corrected for instrumental effects and interplanetary H I Ly alpha. The half widths were found to increase  with projected heliographic height from 1.5 to 2.5~R_sun and decrease  with height from 3.5 to 5 R_sun.

1999-33:   Coronal Heating: a comparison of ion-cyclotron and gravity damping models

Cuseri, I., Mullan, D.J., Poletto, G.

Space Science Reviews 87: 153-156, 1999

SOHO-UVCS data indicate that minor ions in the corona are heated more than hydrogen, and that coronal heating results in $T_{\perp}$ larger than $T_{//}$. Analogous behavior has been known from in situ measurements in solar wind for many years. Here we compare and contrast two mechanisms which have been proposed to account for the above behavior: ion--cyclotron resonance, and gravity damping.

1999-32:   Two-fluid 2.5D MHD Simulations of the Fast Solar Wind in Coronal Holes and the Relation to UVCS Observations

Davila, J.M.; Ofman, L.

Space Science Reviews vol.87, no.1-2 165-8.

SOHO/UVCS observations indicate that the perpendicular proton and ion temperatures are much larger than electron temperatures. The authors simulate numerically the solar wind flow in a coronal hole with the two-fluid approach. The authors investigate the effects of electron and proton temperatures on the solar wind acceleration by nonlinear waves. In the model the nonlinear waves are generated by Alfven waves with frequencies in the 10/sup -3/ Hz range, driven at the base of the coronal hole. The resulting electron and proton flow profile exhibits density and velocity fluctuations. The fluctuations may steepen into shocks as they propagate away from the Sun. The authors calculate the effective proton temperature by combining the thermal and wave velocity of the protons, and find qualitative agreement with the proton kinetic temperature increase with height deduced from the UVCS Ly alpha observations by Kohl et al. (1998).

1999-31:   Two-fluid 2.5D MHD Model of the Fast Solar Wind and the Effective Proton Temperature

Ofman, L., Davila, J.M

Solar Wind Nine, S. Habbal et al. (eds), AIP Conference Proceedings, 471, AIP, New York, 1999

Recent SOHO/UVCS observations indicate that the perpendicular proton and ion temperatures are much larger than electron temperatures (Kohl et al 1997). In the present study we simulate numerically the solar wind flow in a coronal hole with the two-fluid approach. For simplicity, we neglect electron inertia. We investigate the effects of electron and proton temperatures on the solar wind acceleration by nonlinear waves. In the model the nonlinear waves are generated by Alfven waves with frequencies in the 10^-3 Hz range, driven at the base of the coronal hole. The resulting electron and proton flow profile exhibits density and velocity fluctuations. The fluctuations may steepen into shocks as they propagate away from the sun. We construct the proton velocity distribution and a synthetic Ly-alpha line profile by including the combined effects of temperature and velocity fluctuations in the model, and compare them to the UVCS observations.

1999-30:   Heating and cooling of protons by turbulence-driven ion cyclotron waves in the fast solar wind

Li, Xing; Habbal, Shadia R.; Hollweg, Joseph V.; Esser, Ruth

JGR 104, A2, 2521-2536

1999-29:   Density Fluctuations in Polar Coronal Holes: Mechanism and Observational Consequences

Kaghashvili, E. Kh.; Esser, R.

AIP Conference Proceedings, Vol. 471, p.341

1999-28:   Streamer HI Ly-alpha Line Profiles During the Spartan 2O1-05/SOHO Coordinated Observations

M. P. Miralles, L. Strachan, L. D. Gardner, D. Dobryzcka, Y.-K. Ko, J. Michels, A. Panasyuk, R. Suleiman, J. L. Kohl

ESA SP-448, p.1193-1197 (1999) [European Solar Physics Meeting, Florence]

We present H I Ly-alpha observations of coronal streamers obtained by UVCS/SOHO during the coordinated observations with UVCS/Spartan on 1 - 3 November 1998. Two different streamer regions were observed, one relatively stable above the Northeast limb, and a post-CME streamer complex above the Southwest limb. H~I Ly-alpha profiles and intensities were measured at ~1.5 and 2 Ro in the NE streamer and its boundary regions, and also at heights ranging from 1.5 to 2.5 Ro in the SW streamer complex. We analyze integrated intensities and line widths in each streamer structure. 

1999-27:   Variation of Polar Coronal Hole Profiles with Solar Cycle

 Dobrzycka, D.,  Strachan, L.,  Miralles, M.P.,  Kohl, J.L., & Gardner, L.

Space Science Reviews, 87: 177-180, 1999

We compared the H I Lyman alpha polar coronal hole profiles obtained during the three Spartan 201 flights (in 1993, 1994, and 1995) and, during the more recent, UVCS/SOHO mission. We found that at 2.1 Rs there are no significant variations of the line shape over the several years of the descending phase of the solar cycle. However, there may be some evidence for the 1.8 Rs profiles being broader towards solar minimum. The profiles at 2.1 Rs are different from profiles obtained at 1.8 Rs - they have clearly narrower cores and wide wings. We fitted the profiles with single and/or multiple Gaussian functions and calculated their typical 1/e half widths.

1999-26:   Comparison of Polar and Equatorial Coronal Holes observed by UVCS/SOHO - Geometry and Physical Properties

Dobrzycka, D.,  Panasyuk, A.V.,  Strachan, L., & Kohl, J.L.

Space Science Reviews:  87,  173-176, 1999

We analyzed the UVCS/SOHO data and compared the H I Lyman alpha (121.6 nm) and O VI (103.2 nm, 103.7 nm) emission in the polar and equatorial coronal holes. We found that the emission lines have similar characteristics in these two types of coronal holes. Both types show evidence for superradially diverging boundaries. The latitudinal distribution of the O VI line ratio may indicate that the equatorial coronal hole has O+5 outflow velocities lower than in the polar coronal holes.

1999-25:   Geometry, Physical Properties and Outflow Velocities from the Polar and Equatorial Coronal Holes - UVCS/SOHO Observations

Dobrzycka, D.,  Panasyuk, A.V., Strachan, L., &  Kohl, J.L

AIP Conf. Proc. 471, 305 (1999) [Solar Wind 9]

We analyzed the UVCS/SOHO data obtained during the Whole Sun Month campaign in 1996 and compared the H I Lyman alpha (1216 A) and O VI (1032 A,1037 A) emission in the polar and equatorial coronal holes. We found that the emission lines have similar characteristics in these two types of coronal holes. Both types show evidence for superradially diverging boundaries. We used the O VI (1032 A) and O VI (1037 A) local emissivities, determined from the 3D tomographic reconstruction of the solar corona, to investigate the outflow velocities from the polar and equatorial coronal holes. The latitudinal distribution of the O VI ratio at 2.21 Rs, free from the line-of-sight effect, shows evidence that the equatorial coronal hole may have O+5 outflow velocities lower than in the polar coronal holes.

1999-24:   Heating and Acceleration of the Solar Wind via Gravity Damping of Alfven Waves

Cuseri, I., Mullan, D.J., Noci, G., Poletto, G.

ApJ 514, 989 (1999)

In this paper we present a two-fluid model for the heating of the solar corona and acceleration of the solar wind, based on the dissipation of Alfv\'en waves by gravity damping. This mechanism was proposed by Khabibrakhmanov and Mullan (1994) but has not previously been applied in modeling efforts. After extending Khabibrakhmanov and Mullan theory to give an expression for the evolution of the Alfv\'en wave amplitude as a function of the local parameters of the atmosphere, we show how gravity damping compares with other mechanisms which have been proposed for dissipation of Alfv\'en waves. Then we introduce the system of equations which we use for the wind model: this includes, in the energy equation, a gravity dissipation term, and, in the momentum equation, a different wave acceleration term from that which is usually adopted. Initial conditions for the integration of the equations are compatible with recent Ulysses measurements and the integration proceeds from 1 AU towards the base of the solar corona, and into the transition region (where T = (1-2) 10^5 K). Our results show that the gravity damping of Alfv\'en waves heats protons in the solar plasma to several million degrees, and accelerates the solar wind to 600-700 km/s. Model predictions at low heliocentric distances compare favorably with recently acquired data. One prediction of our model is that the damping process is most effective in regions where the Alfv\'en speed is low. Another prediction is that although the energy is deposited mainly into protons, the deposition occurs close enough to the Sun that collisional coupling also leads to effective heating of the electrons (to T_e \approx 10^6$ K). We compare and contrast the present model with models based on ion-cyclotron resonant processes.

1999-23:   On the Ly-alpha and OVI line profiles in streamers and coronal holes

 Zangrilli, L.,  Nicolosi, P.,  Poletto, G.,  Noci, G.

Space Science Reviews 87: 349-352, 1999

The profiles of the HI Ly (alpha) line at 1215.6 A and of the OV I doublet at 1031.9 A and 1037.6 A in the extended solar corona have been analyzed versus latitude and radial direction. Observations have been performed with the UltraViolet Coronagraph Spectrometer (UVCS) on board the ESA-NASA solar satellite SOHO (SOlar and Heliospheric Observatory). The results show that these lines have quite a different behaviour with latitude: the HI (alpha) line has larger full width at half maximum (FWHM) values in the streamer region and narrower ones towards polar latitudes, while the FWHM of OV I lines has a minimum at the center of the streamer, and slightly increases towards polar regions. The impact of our observations on coronal heating theories is also briefly illustrated. Coronal Heating: a comparison of ion-cyclotron and gravity damping models Cuseri, I., Mullan, D.J., Poletto, G. Space Sci. Rev., in press SOHO-UVCS data indicate that minor ions in the corona are heated more than hydrogen, and that coronal heating results in $T_{\perp}$ larger than $T_{//}$. Analogous behavior has been known from {\it in situ} measurements in solar wind for many years. Here we compare and contrast two mechanisms which have been proposed to account for the above behavior: ion--cyclotron resonance, and gravity damping.

1999-22:   SOHO Observations of Density Fluctuations in Coronal Holes

Ofman, L., Romoli, M., Poletto, G., Noci, G., Kohl, J.L. Howard, R.A., St. Cyr, C., DeForest, C.E.

Space Science Reviews 87: 287-290, 1999
 

In recent UVCS/SOHO White Light Channel (WLC) observations we found quasi-periodic variations in the polarized brightness (pB) in the polar coronal holes at heliocentric distances of 1.9-2.45 solar radii. The motivation for the observation is the 2.5D MHD model of solar wind acceleration by nonlinear waves, that predicts compressive fluctuations in coronal holes. In February 1998 we performed new observations using the UVCS/WLC in the coronal hole and obtained additional data. The new data corroborates our earlier findings with higher statistical significance. The new longer observations show that the power spectrum peaks in the 10-12 minute range. These timescales agree with EIT observations of brightness fluctuations in polar plumes. We performed preliminary LASCO/C2 observations in an effort to further establish the coronal origin of the fluctuations.

1999-21:   Grating Stray Light Analysis and Control in the UVCS/SOHO

Fineschi, Silvano, Gardner, Larry D., Kohl, John L., Romoli, Marco Nochi, Giancarlo C.

SPIE, 3443, 67-74 (1998)

The Ultraviolet Coronagraph Spectrometer (UVCS) of the Solar and Heliospheric (SOHO) mission has been developed for spectroscopic and polarimetric determinations of temperatures, densities and flow velocities in the extended solar corona. The instrument consists of a visible light (VL) polarimeter, and two ultraviolet (UV) spectrometers, optimized for the H I Lyman (alpha) (Ly-(alpha) ) line ((lambda) 1216 angstrom) and for the O VI doublet ((lambda) (lambda) 1032/1037 angstrom). The stray-light profile of the Ly-(alpha) holographic grating has been measured, in a laboratory set-up, in both the spatial and spectral directions. The observed profile has been found to be a combination of two components: the scattering from the grating surface, and the Fraunhofer diffraction due to the vignetting of the grating. An analytical expression for the scattering component of the grating point spread function (PSF) has been derived from a simple model of the grating surface roughness. The stray-light profile generated by the analytical expression of the grating PSF gives a good fit of stray-light profile measured in the laboratory. This instrument function has been used in the analysis of in-flight UVCS observations of the profile of electron scattered Ly- (alpha) from the solar corona. These observations have resulted, for the first time, in the most direct measurement of the coronal electron temperature.

1999-20:   UVCS observations and modelling of streamers

Vasquez, A.M., Raymond, J.C., and van Ballegooijen, A.A.

Space Science Reviews 87: 335-338, 1999

We present results derived from the analysis of an equatorial streamer structure as observed by the UVCS instrument aboard SOHO. >From observations of the HI Ly-alpha and Ly-beta lines we infer the density and temperature of the plasma. We develop a preliminary axisymmetric, magnetostatic model of the corona which includes the effects of gas pressure gradients on the magnetic structure. We infer a coronal plasma beta > 1 in the closed field regions and near the cusp of the streamer. We add to the model a parallel velocity field assuming mass flux conservation along magnetic flux tubes. Then we compute the Ly-alpha emissivity and the LOS integrals to obtain images of Ly-alpha intensity, taking into account projection effects and doppler dimming. The images we obtain from this preliminary model are in a good general agreement with the UVCS observations, both qualitatively and quantitatively.

1999-19:   Model of Solar Wind Flow near an Equatorial Coronal Streamer

Vasquez, A.M., van Ballegooijen, A.A., and Raymond, J.C.

AIP Conf. Proc. 471, 243 (1999) [Solar Wind Nine]

In a previous work (SOHO7 Proceedings) we developed a semiempirical axisymmetric and magnetostatic model of the minimum activity corona, accounting for the gas pressure gradient effects on the magnetic structure. The model is able to reproduce the streamer belt closed region and the streamer sourrounding open field lines, predicting high plasma beta values (>1) in the closed region, low values in the sourrounding open-field regions (streamer legs) and even lower values for the polar hole region. In this work we add to that model a solar wind solution under the assumption of mass and momentum flux conservation. For open field lines arising from the polar hole region we find a fast wind solution that reaches values of about 600 km/sec at 10 R_sun. For open field lines arising from the surroundings of the closed region (streamer legs), the morphology of the field lines is fast diverging, we find that this results in the existence of two posible sonic points. The first sonic point, located below the streamer cusp, gives a relatively fast solar wind solution, reaching values of about 400 km/sec at 10 R_sun. The second sonic point, located above the streamer cusp, gives a slow solution, reaching values of up to 200 km/sec for lines close to the streamer core, becoming supersonic only well above the streamer cusp.

1999-18:   Latitudinal properties of the Ly-(alpha)  and O VI profiles in the extended solar corona

 Zangrilli, L., Nicolosi, P., Poletto, G., Noci, G., Romoli, M., and Kohl, J.L.

Astronomy & Astrophysics 342, 592 (1999)

We have analysed the latitudinal properties of the profiles of the HI Lyman alpha line at 1215.6 A and of the O VI doublet at 1031.9 A and 1037.6 A in the extended solar corona, between 1.5 Ro and 2.0 Ro. Observations have been performed with the UltraViolet Coronagraph Spectrometer (UVCS) on board the ESA-NASA solar satellite SOHO (SOlar and Heliospheric Observatory). The results show that these lines have quite a different behaviour with latitude: the Ly alpha line has larger full width at half maximum (FWHM) values in the streamer region and narrower ones towards polar latitudes, while the OVI lines have a minimum FWHM at the center of the streamer, which almost steadily increases towards polar regions. The observations have been analysed looking also for an interpretation in terms of selective heating mechanisms. The implications of our results for coronal heating theories are also examined. In particular we discuss the possibility for the presence of the ion-cyclotron coronal heating mechanism. Moreover, we point out an interesting correlation between the intensity of the coronal lines and their widths, which may be relevant to the open question of the different morphological features visible in the Ly alpha and OVI lines.

1999-17:   EUV Spectral Line Profiles in Polar Coronal Holes from 1.3 to 3.0 Solar Radii

Kohl, J. L., Esser, R., Cranmer, S. R., Fineschi, S., Gardner, L. D., Panasyuk, A. V., Strachan, L., Suleiman, R. M., Frazin, R., and Noci, G.

Ap. J. Letters, 510, L59 (1999)

Spectral line profiles have been measured for H I 1216, O VI 1032, 1037, and Mg X 625 in a polar coronal hole observed during 1997 September 15-29, at projected heliographic heights rho between 1.34 and 2.0 R_solar. Observations of H I 1216 and the O VI doublet from 1997 January for rho=1.5-3.0 R_solar are provided for comparison. The O VI lines are well fit to a narrow and broad component which appear to be associated with regions of higher and lower spectral radiance, respectively. The narrow components dominate at low heights and become a small fraction of the lines at higher heights. Mg X 625 is observed to have a narrow component at rho=1.34 R_solar which accounts for only a small fraction of the observed spectral radiance. In the case of the broad components, the values of v_1/e for O VI are only slightly larger than those for H I at rho=1.34 R_solar but are significantly larger at rho=1.5 R_solar and much larger for rho>1.75 R_solar. In contrast, the Mg X values are less than those of H I up to 1.75 and then increase rapidly up to at least rho=2.0 R_solar but never reach the values of O VI.

1999-16:   Plasma Properties in Coronal Holes Derived from Measurements of Minor Ion Spectral Lines and Polarized White Light Intensity

Esser, Ruth, Fineschi, Silvano, Dobrzycka, Danuta, Habbal, Shadia R., Edgar, Richard J., Raymond, John C., Kohl, John L., Guhathakurta, Madhulika

Ap. J. Letters, 510, L63 (1999)

Recent observations of the Lyalpha 1216, Mg X 625, and O VI 1038 spectral lines carried out with the Ultraviolet Coronagraph Spectrometer (UVCS) on board SOHO at distances in the range 1.35-2.1 R_S in the northern coronal hole are used to place limits on the turbulent wave motions of the background plasma and the thermal motions of the protons and Mg^+9 and O^+5 ions. Limits on the turbulent wave motion are estimated from the measured line widths and electron densities derived from white light coronagraph observations, assuming WKB approximation at radial distances covered by the observations. It is shown that the contribution of the turbulent wave motion to the widths of the measured spectral lines is small compared to thermal broadening. The observations show that the proton temperature slowly increases between 1.35 and 2.7 R_S and does not exceed 3x10^6 K in that region. The temperature of the minor ions exceeds the proton temperature at all distances, but the temperatures are neither mass proportional nor mass-to -charge proportional. It is shown, for the first time, that collision times between protons and minor ions are small compared to the solar wind expansion times in the inner corona. At 1.35 R_S the expansion time exceeds the proton Mg^+9 collision time by more than an order of magnitude. Nevertheless, the temperature of the Mg ions is significantly larger than the proton temperature, which indicates that the heating mechanism has to act on timescales faster than minutes. When the expansion time starts to exceed the collision times a rapid increaseof the O^+5 ion spectral line width is seen. This indicates that the heavier and hotter ions lose energy to the protons as long as collision frequencies are high, and that the ion spectral line width increases rapidly as soon as this energy loss stops.

1999-15:   Spectroscopic Constraints on Models of Ion-Cyclotron Resonance Heating in the Polar Solar Corona and High Speed Solar Wind

Cranmer, S. R., Field, G. B., and Kohl, J. L.

ApJ, 518, 937

Using empirical ion velocity distributions derived from UVCS and SUMER ultraviolet spectroscopy, we construct theoretical models of the nonequilibrium plasma state of the polar solar corona. The primary energy deposition mechanism we investigate is the dissipation of high frequency (10-10000 Hz) ion-cyclotron resonant Alfven waves which can heat and accelerate ions differently depending on their charge and mass. We solve the internal energy conservation equations for the ion temperature components parallel and perpendicular to the superradially expanding magnetic field lines and use empirical constraints for the remaining parameters. We find that it is possible to explain many of the kinetic properties of the plasma (such as high perpendicular ion temperatures and strong temperature anisotropies) with relatively small amplitudes for the resonant waves. There is suggestive evidence for steepening of the Alfven wave spectrum between the coronal base and the largest heights observed spectroscopically, and it is important to take Coulomb collisions into account to understand observations at the lowest heights. Because the ion-cyclotron wave dissipation is rapid, the extended heating seems to demand a constantly replenished population of waves over several solar radii. This indicates that the waves are generated gradually throughout the wind rather than propagated up from the base of the corona.

1999-14:   Beta in Streamers

Suess, S. T., G. A. Gary, and S. Nerney

Solar Wind Nine Proceedings,  247-250, AIP Conf. Proc. 471,  Woodbury, NY, 1999.

Streamers are generally described as regions of the corona in which the density is higher than in coronal holes because the plasma is trapped by closed loops of magnetic flux. In contrast, MHD models of the global corona (Steinolfson, Suess, \& Wu, {\it ApJ, 255}, 730, 1982) show the plasma $\beta>1$ in streamers above $\sim1.2R_S$ (heliocentric). There are three recent contributions to this topic. The first is that heating near the cusp further drives $\beta$ up and results in release of new slow solar wind plasma from the top of the streamer (Suess, Wang, \& Wu, {\it JGR, 101}, 19,957, 1996). The second is SOHO/UVCS observations, in combination with a potential field/source surface model of the magnetic field, that show $\beta>1$ above $1.2R_S$ in a streamer observed near solar sunspot minimum (Li et al., {\it ApJ}, 1998, in press). The third is a magnetic field reconstruction technique that uses field deforming algorithms and is more versatile for local fields than potential field models (Gary \& Alexander, {\it Sol.Phys.}, submitted, 1998). The field reconstruction algorithm was applied to an isolated active region (AR 7999) and to the Pneuman \& Kopp ({\it Sol.Phys., 18}, 258, 1971) global MHD model ($\beta$ has never been published for their model). In the active region, $\beta$ becomes larger than unity at $\sim1.2 R_S$. In the Pneuman \& Kopp model, $\beta=1.0$ at the base of the streamer and rises with increasing height, becoming 15-20 at $1.6R_S$ and 35-50 at $1.7R_S$. Global simulations go on to show that the reason streamers do not simply explode under such high $\beta$ conditions is that they are held down by pressure from the sides due to the magnetic fields (and low $\beta$) in adjacent coronal holes. The main role of the closed magnetic loop near the cusp is to keep the steamer from continuously leaking plasma, as otherwise happens in a magnetic pinch which is similar but has no closed loops. Awareness of MHD physical conditions in streamers is causing us to focus more attention on the details of the heating. On obvious suggestion is that heating is at much lower heights in streamers than in coronal holes. Also, energy which is directly delivered as momentum in coronal holes might all be deposited as heat in streamers.

1999-13:   Can Kelvin-Helmholtz Instabilities of Jet-like Structures and Plumes Cause Solar Wind Fluctuations at 1 AU ?

Parhi, S.,  Suess, S.T., and Sulkanen, M.

J. Geophys. Res.,  v104, p14,781, 1999.

The long high latitude sampling of Ulysses provides the opportunity to study fine structures. At latitudes poleward of $\sim -60^0$, the solar wind had fluctuations in velocity gradients which were attributed to `` microstreams''. The data also suggested fluctuations characterized by magnetic plus thermal pressure balance structures (``PBS''). At higher frequencies, MHD turbulence was observed and found to be less evolved than in the ecliptic, but essentially independent of heliographic latitude. It is argued here that microstreams, PBS, and MHD turbulence could all be the remnants of mixing due to shear instabilities associated with plumes and other filamentarystructures (``jets'') in coronal holes. To show this we simulate a plume-like jet in the presence of an ambient magnetic field. We find the presence of the ambient field reduces the growth rate of the instability, but the shear between a jet and its ambient still becomes unstable to the MHD Kelvin-Helmholtz (``KH'') instability when the shear velocity is larger than the largest local fast mode speed - a condition probably satisfied for plumes.

1999-12:   Streamer Evaporation

Suess, S. T.,  Wang, A.-H., Wu, S.T., and  Nerney, S.

Space Science Reviews 87: 323-326, 1999

Streamer evaporation is the consequence of heating {\it in ideal MHD models} because plasma is weakly contained by the magnetic field. Heating causes inflation, opening of field lines, and release of solar wind. It was discovered in simulations and, due to the absence of loss mechanisms, the ultimate end point is the complete evaporation of the streamer. Of course streamers do not behave in this way because of losses by thermal conduction and radiation. Heating is also expected to depend on ambient conditions. We use a global MHD model with thermal conduction to examine the effect of changing the heating scale height. We also extend an analytic model of streamers developed by Pneuman (1968) to show that {\it steady} streamers are unable to contain plasma for temperatures near the cusp greater than $\sim2\times10^6$ K.

1999-11:   Ulysses-UVCS Coordinated Observations

Suess, S. T., Poletto G.,  Corti, G.,  Simnett, G., Noci, G.,  Romoli, M., Kohl, J., and Goldstein, B.,

Space Science Reviews 87: 319-322, 1999

We present results from SOHO/UVCS measurements of the density and flow speed of plasma at the Sun and again of the same plasma by Ulysses/SWOOPS in the solar wind. UVCS made measuremnts at 3.5 and 4.5 solar radii and Ulysses was at 5.1 AU. Data were taken for nearly 2 weeks in May-June 1997 at 9-10 degrees north of the equator in the streamer belt on the east limb. Density and flow speed were compared to see if near Sun characteristics are preserved in the interplanetary medium. By chance, Ulysses was at the very northern edge of the streamer belt. Nevertheless, no evidence was found of fast wind or mixing of slow wind with fast wind coming from the northern polar coronal hole. The morphology of the streamer belt was similar at the beginning and end of the observing period, but was markedly different during the middle of the period. A corresponding change in density (but not flow speed) was noted at Ulysses.

1999-10:   The Generation of Smooth High Speed Solar Wind from Plume-Interplume Mixing

Parhi, S., Suess, S.T., and  Sulkanen, M.

AIP Conf. Proc. 471, 433 (1999) [Solar Wind Nine]

Plumes and rays are magnetic field aligned density striations in coronal holes with different values of plasma $\beta$. The overall plasma $\beta$ is very small in the low corona but exceeds unity beyond 15-20 $R_\odot$. High speed solar wind reported beyond 0.3 AU is relatively smooth and uniform and known to originate from the much filamented coronal hole. Thus the obvious question is how to generate a smooth solar wind from seemingly filamentary structure. Hence one has to find a mechanism to substantiate this apparent observed (Ulysses) phenomenon. To do this we model plumes as jets (or wakes) of plasma emitted from the solar surface. The shear between a jet and its ambient is known to become unstable to the MHD Kelvin-Helmholtz (``KH'') instability if the Alfv\'{e}n Mach number of the jet is greater than one and the uniform external magnetic field is small. Starting with a simple configuration we consider a jet of half thickness $R$, having uniform density and uniform internal magnetic field. The external medium has also a uniform density and uniform magnetic field. The jet is perturbed at the boundary with a linear amplitude and fixed frequency. We simulate the coronal jet using the 3D ZEUS code. The first results indicate the slab jet is unstable to the MHD KH instability at 5-10 $R_\odot$ for some angle of wave propagation. The propagating instability may smooth the filamented flow. It may also produce the entrained Alfv\'{e}nic fluctuations observed by Ulysses in the high speed wind. We are at present determining the parameters which induce large growth rate. This may clarify the mystery behind the emergence of fast smooth solar wind from very filamentary structures in coronal holes. Also, using the dispersion relation already available for such a flow we obtain some general description of the instability criteria for the KH instability at a jet interface.

1999-09:    Fast solar wind acceleration by Alfvén waves: observable effects on the EUV lines detected by SOHO/UVCS

Ventura, R., Orlando, S., Peres, G., Spadaro, D

Astronomy and Astrophysics, 352, 670

SOHO/UVCS observations of the most intense EUV spectral lines emitted by the solar corona have been providing us a good opportunity to study in detail the acceleration regions of the solar wind. In this work we aim at deriving useful diagnostics and identifying possible signatures of Alfvén waves momentum deposition. More specifically we investigate, with the help of a detailed wind model Orlando et al. 1996), the insight and the constraints that these observations give on the presence of Alfvén waves, as deduced from the influence of the waves on the solar wind structure and dynamics. The model developed by Orlando et al. (1996) accounts for the momentum deposition by a spectrum of non-WKB Alfvén waves, generated in the Sun's lower atmosphere and undergoing significant reflection across the transition region. We compute a set of win solutions characterized by different physical conditions, synthesize, from them, the emission in the Lyalpha, Lybeta and O VI doublet (1032 Ä, 1038 Ä) lines and derive possible diagnostics. We finally compare our results with the most recent SOHO/UVCS data.

1999-08:   An Empirical Model of a Polar Coronal Hole at Solar Minimum

Cranmer, S. R., Kohl, J. L., Noci, G., Antonucci, E., Tondello, G., Huber, M. C. E., Strachan, L., Panasyuk, A. V., Gardner, L. D., Romoli, M., Fineschi, S., Dobrzycka, D., Raymond, J. C., Nicolosi, P., Siegmund, O. H. W., Spadaro, D., Benna, C., Ciaravella, A., Giordano, S., Habbal, S., Karovska, M., Li, X., Martin, R., Michels, J. G., Modigliani, A., Naletto, G., O'Neal, R. H., Pernechele, C., Poletto, G., Smith, P. L., and Suleiman, R. M.

Ap. J., 511, 481 (1999)

We present a comprehensive and self-consistent empirical model for several plasma parameters in the extended solar corona above a polar coronal hole. The model is derived from observations with the SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) during the period between 1996 November and 1997 April. We compare observations of H I Lyman alpha and O VI 1032, 1037 emission lines with detailed three-dimensional models of the plasma parameters, and iterate for optimal consistency between measured and synthesized observable quantities. Empirical constraints are obtained for the radial and latitudinal distribution of density for electrons, H^{0}, and O^{5+}, as well as the outflow velocity and unresolved anisotropic most-probable speeds for H^{0} and O^{5+}. The electron density measured by UVCS/SOHO is consistent with previous solar-minimum determinations of the white light coronal structure; we also perform a statistical analysis of the distribution of polar plumes using a long time series. From the emission lines we find that the unexpectedly large line-widths of H^0 atoms and O^{5+} ions at most heights are the result of anisotropic velocity distributions. These distributions are not consistent with purely thermal motions or the expected motions from a combination of thermal and transverse wave velocities. Above 2 R_sun, the observed transverse most-probable speeds for O^{5+} are significantly larger than the corresponding motions for H^0, and the outflow velocities of O^{5+} are also significantly larger than the corresponding velocities of H^0. Also, the latitudinal dependence of intensity constrains the geometry of the wind velocity vectors, and superradial expansion is more consistent with observations than radial flow. We discuss the constraints and implications on various theoretical models of coronal heating and acceleration.

1999-07:   Ultraviolet and Optical Observations of a Coronal Transient with SOHO

A. Ciaravella, J.C. Raymond, L. Strachan, B. Thompson, O.C. St. Cyr, L. Gardner, A. Modigliani, E. Antonucci, J. Kohl and G. Noci

Ap.J. 510, 1053 (1999)

A coronal transient was observed on March 6, 1997 at 1.6 Rsun over an active region on the East limb. We observed both the edge of horizontally compressed gas and the diffuse curtain of coronal material. The region was monitored for 4 hours and the H I Ly-alpha, O VI 1031.91 and 1037.61 N V 1242.80 and 1238.80 and O V] 1218.35 A lines were detected during the ejection evolution. The density, velocity, temperature and oxygen abundance of the ejected plasma have been obtained from the observed spectra. Intermediate temperature lines of N V, O VI and O V show a large enhancement, suggesting a quite narrow range of plasma temperature around 4 x 10^5 K. Doppler shifts of the ejected material evolve from an initial blue-shift of 100 km/sec to a red-shift of 145 km/sec. The outflow velocity, as determined by Doppler dimming analysis of the O VI doublet, is only about 20 km/sec.

1999-06:   Potential wells, the cyclotron resonance, and ion heating in coronal holes

Hollweg, J. V.

J. Geophys. Res., 104, 505, 1999.

We consider the motions of protons and O+5 ions in coronal holes. We first consider the effects of a potential well, which arises from the combination of gravity, the electrostatic electric field, and the mirror force. We show that if the potential well is time-dependent, then ions which are initially trapped will undergo a time-averaged energy gain. They can eventually gain enough energy to escape out of the potential well, and be ejected out of the corona. The process is analogous to Fermi acceleration of cosmic rays by reflections off of moving magnetic clouds, except here the trapped ions can be regarded as reflecting off of moving walls. There is evidence that the trajectories of the particles are chaotic. However, the timescales are long, the potential wells are not very deep, and the process is probably not important for coronal heating. We also point out that the potential wells can provide a population of particles which are moving inward relative to waves which are propagating outwards from the Sun. These particles are the ones which can interact most strongly with ion-cyclotron waves, since they resonate with the lowest frequency waves which have the highest phase speeds and presumably the most power. We present some simple arguments, invoking energy-conserving pitch-angle scattering in the wave frame, which show how O+5 ions can in principle acquire perpendicular temperatures which are more than mass-proportionally hotter than the protons. The basic principles are demonstrated by calculating trajectories for average particles interacting with dispersive ion-cyclotron waves. We also present a strongly-driven case which gives perpendicular energies and parallel flow speeds qualitatively resembling those believed to exist in coronal holes, but there are significant differences between the model results and the SOHO/UVCS data. In this case the particles are not trapped in a potential well.

1999-05:   The cyclotron resonance: heating of protons and oxygen in coronal holes

Hollweg, J. V.

Solar Wind Nine edited by S. R. Habbal, R. Esser, J. V. Hollweg and P. A. Isenberg, pp.369-372, AIP, Woodbury, New York, 1999.

The UVCS/SOHO data have offered remarkable evidence that the coronal holes and acceleration region of the fast solar wind are heated by ion-cyclotron waves. We here summarize the basic physics of the cyclotron resonance, and show why ions such as 0+5 can be heated to more than mass-proportional temperatures compared to the protons. The mirror force provides the main acceleration out of the corona, yielding heavy ions which flow faster than the protons. We quantify these ideas by following an average test particle. Agreement with observation is achieved, but only if we take a steep power spectrum. Particular attention is given to the behavior of T_perp for 0+5, which seems to increase with distance from the Sun out to the limits of the observations; this observation is a major constraint.

1999-04:   The cyclotron resonance in coronal holes: heating and acceleration of protons, O+5, and Mg+9

Hollweg, J. V.

J. Geophys. Res. 104, 24781 (1999)

The resonant heating and acceleration of protons and selected heavy ions in coronal holes are investigated by calculating trajectories of individual test particles under the influence of gravity, the electrostatic electric field, the mirror force, and the resonant acceleration due to interaction with dispersive ion-cyclotron waves. The transverse heating due to the resonance is also included. We show in general terms how heavy ions can be more than mass-proportionally heated, emphasizing that wave dispersion may play an important part in producing very hot heavy ions. We pay particular attention to the UVCS/SOHO observation that the transverse temperature of 0+5 is still increasing out to the outer limit of observation at about 3.5 solar radii. Using both approximate analytical expressions and the trajectory calculations, we find that this observation can only be reproduced if the magnetic power spectrum falls off at least as steeply as k^-2, where k is wavenumber. Surprisingly, this conclusion holds even when the power spectrum consists of two power laws, if the inner scale is proportional to the proton inertial length. Once the particles are heated transversely by the resonance, the mirror force provides the dominant outward acceleration, and leads to heavy ions which flow faster than the protons. It is shown that it is possible to construct a model which gives reasonable agreement with the UVCS/SOHO data for both protons and 0+5. Overall, we conclude that it is highly likely that the cyclotron resonance is responsible for heating protons and heavy ions in coronal holes. However, we also briefly discuss some data for Mg+9, which does not fit the overall picture.

1999-03:   The cyclotron resonance in coronal holes: 2. a two- proton description

Hollweg, J. V.

J. Geophys. Res. 104, 24793 (1999)

In a cold plasma, the ion-cyclotron mode does not extend above the proton cyclotron frequency. As a consequence, when viewed in the average proton frame, only protons which are moving opposite to the wave propagation direction can resonate with this mode. Thus only roughly half of the proton distribution function can be in resonance at any instant of time. The proton distribution function is then expected to depart significantly from a bi-Maxwellian, which is usually assumed to provide closure to a set of fluid equations. Here we consider the effects of the ion- cyclotron resonance on protons in a coronal hole. We calculate the trajectories of individual protons in the electric, magnetic, and gravitational fields, and we include the resonant heating and acceleration for an average particle which is diffusing in phase space. To provide closure we consider two protons, which are proxies for the resonant and non-resonant halves of the distribution. Elementary arguments show that the two protons tend to approach nearly the same radial velocity. When the waves are dispersive, this means that the resonant wavenumber, k_res, increases. For a power spectrum which is a power law in wavenumber, and if the dissipation is determined only by the resonant particles, then the resonant effects become very weak as k_res becomes large, and there is little heating or acceleration of the coronal plasma. On the other hand, if the dissipation is determined by a turbulent cascade, k_res mainly controls the relative importance of resonant acceleration and resonant heating. Such models yield good agreement with what is known about the behavior of protons in coronal holes.

1999-02:   Wavelength shifts of emission line profiles and velocity fields in the solar corona

R. Ventura* & D. Spadaro*

A&A, 341, 264 (1999)

We have investigated the dependence of the wavelength shift of the coronal emission line profiles on the line of sight velocity of the emitting plasma. The results of our numerical calculations point out that, while the wavelength shift of the collisionally excited component of the line is related to the line of sight velocity by the usual formula for the Doppler effect, that of the resonantly scattered component also depends on the angle of scatter and on the angle between the velocity vector and the line of sight. For the same outflow velocity, the absolute value of the resonantly scattered component shift is significantly smaller than that of the collisional component. Since both mechanisms generally contribute to the formation of a coronal line, we conclude that the results of this work should be taken into account when deducing line of sight velocities from the analysis of emission line profiles observed in the extended solar corona. * Osservatorio Astrofisico di Catania, CNR-Gruppo Nazionale di Astronomia, UdR di Catania, Citt\`a Universitaria, Viale A.Doria, 6 I-95125, Catania, Italy

1999-01:   Solar wind velocity and anisotropic coronal kinetic temperatures measured with the O VI doublet ratio

M.A. Dodero, E. Antonucci, R. Martin

Solar Physics, 183, 77 (1999)

Doppler dimming of the OVI resonance lines (1032, 1037) in an expanding corona is calculated including the pumping effect on the O VI 1037.61 of both CII lines at 1036.34 and 1037.02, and the effect of the width of the absorption profiles of the coronal oxygen ions along the incident radiation. The pumping effect of the CII line at 1036.34 allows us to extend to approximately 450 km s-1 the measurement of solar wind velocities with the OVI line ratio technique. Since the emissivity ratio of the OVI doublet depends on the width of the oxygen coronal absorbing profiles, this ratio can provide an accurate measurement of the solar wind velocity in the case that the width of the absorbing profile along the direction of the incident radiation is independently determined. However, if on the one hand the ratio of the emissivities of the OVI doublet has limitations in probing the wind velocity, on the other hand it can be used as a diagnostics for inferring the velocity distribution of the coronal OVI ions along the radial, and detecting possible velocity anisotropies. This diagnostics, applied to recent observational results, allows us to infer that the velocity distribution of the oxygen ions is much broader in the direction perpendicular to the magnetic field direction, and that the acceleration of the fast solar wind in the first 2 solar radii is high.