1998-33:
Far Ultraviolet and visible light scatter measurements for CVD SiC mirrors for SOHO
Leviton, D. B., Saha, T. T., Gardner, L. D.
Proc. SPIE, 3443, 19
Chemically-vapor-deposited (CVD) silicon carbide (SiC) has become a popular mirror material for spaceborne solar instrumentation for the vacuum ultraviolet wavelength range due to its appreciable broadband reflectance and favorable thermal and opto-mechanical properties. Scatter from surfaces of mirrors operating in this wavelength range can destroy otherwise good image contrast especially for extended targets such as the sun. While valid far ultraviolet (FUV) scatter measurements are entirely non-trivial to conduct and so are rarely performed, visible light scatter measurements are comparatively easy. Unfortunately, it is not straightforward to predict FUV scatter performance based on visible light scatter measurements for mirrors made of CVD SiC. It is hoped that by carrying out scatter measurements in both wavelength regimes for the same CVD SiC mirror, that the ability to make such predictions may be enhanced. Visible light (633 nm) scatter measurements were performed at Goddard Space Flight Center (GSFC) by two different means on CVD SiC telescope mirrors (from the same process and same vendor) for two instruments on the Solar and Heliospheric Observatory (SOHO) - - the Ultraviolet Coronagraph Spectrometer (UVCS) and Solar Ultraviolet Measurement of Emitted Radiation (SUMER). Additionally, extensive FUV scatter measurements were made for SUMER telescope mirrors. In this paper, we correlate the results for those FUV and visible light scatter measurements for this important material.
1998-32: Models of Plumes: Their Flow, Their Geometric Spreading,and Their Mixing with Interplume Flow
Suess, S. T.
in "Solar Jets and Coronal Plumes",
ESA SP-421, pp 223-229, 1998.
1998-31: Research on Solar Activity in the Last 50 Years: The Space Era
Antonucci, E.
SIF (Societa Italiana de Fisica)
Conf. Proc. 63, 59 (1998)
In the past fifty years the understanding of solar activity has been
rapidly progressing thanks to the great technological advances in space
solar physics that started shortly after the second world war. We briefly
review the major results obtained with the large solar space observatories
launched in the last three decades: Skylab, the
Solar Maximum
Mission, and SOHO. We emphasize the scientific achievements
of Italian scientists and the Italian participation in the Solar and
Heliospheric Observatory that consists in providing the spectrometer
for the Ultraviolet Coronagraph Spectrometer (UVCS), and supporting
the operations of the UVCS.
1998-30: Origin of the slow solar wind
G. Noci and L. Maccari
Mem. Soc. Astron. Ital. 69, 741 (1998)
We model the flow of plasma in the solar corona in a flux
tube whose cross-section initially increases with height and then decreases
up to at least 5 r_0 of heliocentric distance. We show that the proton
flow in the low corona is strongly reduced, which could be the cause of
the reduction of the heavy ion abundance observed by UVCS in streamers.
1998-29: Heating and acceleration of minor ions in the solar wind
Czechowski, A., Ratkiewicz, R., McKenzie, J. F., Axford, W. I.
1998, A&A, 335, 303
We make use of the results of a one-ion model of McKenzie et al. (1997) to
obtain the critical solution describing the acceleration of a two-ion solar wind.
The model includes preferential heating of the heavy ion species and the
temperature anisotropy expected if the heating is predominantly due to
ion-cyclotron damping. The relationship of the model to the picture emerging from
UVCS/SOHO observations is discussed.
1998-28: Electron temperature determination in streamers
L. Maccari and G. Noci
Mem. Soc. Astron. Ital. 69, 689 (1998).
In this paper we expose a method to evaluate the electron
temperature in coronal streamers through the determination of the ratio
between neutral hydrogen atoms and electrons. The method is based on a
comparison between the intensities of the Ly-alpha and Ly-beta lines, in
order to separate the collisional and radiative components of the
latter. Since the emissivity of the radiative component of the Ly-beta
line is proportional to the density of neutral hydrogen, and that of the
collisional component to the density of the neutrals times the electron
density, the two components of the line permit the evaluation of both the
electron and the neutral hydrogen densities.
1998-27: UVCS Observations of Polar Regions
Poletto, G., Corti, G., Romoli, M., Kohl, J.L.,Noci, G
in "Solar Jets and Coronal Plumes" T-D. Guyenne, Ed., ESA-SP 421, 69, 1998
In order to facilitate the interpretation
of UVCS data, the first section of this paper gives a short summary of
the mechanisms of line formation in the extended corona and of the diagnostic
techniques adopted to analyze UVCS observations. This allows us to interpret,
in a semi-qualitative way, the morphology of polar regions, as revealed
by UVCS, which are next illustrated. Results of data analysis, i.e. densities,
kinetic temperatures and flow speeds at heliocentric distances between
1.5 and 3.5 $R_{sun}$ are then discussed. The ensuing scenario has a strong
impact on the theories of heating and acceleration of the solar wind plasma:
we briefly outline the theoretical implications emerging from UVCS results
and the theories which may comply with the observational requirements.
1998-26: High Minor Ion Outflow Speeds in the Inner Corona and Observed Ion Charge States in Interplanetary Space
Ruth Esser, Richard J. Edgar, and
Nancy S. Brickhouse
1998, Ap.J., 498, 448
The interpretation of charge state observations in interplanetary space is only possible in the context of models since the ion ratios are extremely sensitive functions of the electron density, electron temperature and minor ion outflow speeds. In past studies it has always been assumed that the minor ion outflow speeds in the inner corona are of the order of a few km s^{-1} or even less. In the present study we investigate whether higher ion outflow speeds can also be in agreement with the observed ion ratios. We first evaluate the uncertainties in the calculated ion ratios due to uncertainties in atomic data, the possible presence of non-Maxwellian electron velocity distribution functions and mass outflows. Assuming a Maxwellian velocity distribution function we show that the high minor ion outflow speeds of the order of the O^{+5} outflow speed (about 200 km s^{-1} at 2 solar radii) recently derived from Doppler dimming experiments in a polar coronal hole region (Kohl et al. 1996) are also in agreement with charge state observations. The coronal model recently presented by Feldman et al (1996), on the other hand, requires electron emperatures much higher than the ones usually observed in coronal hole regions.
1998-25: The geometric spreading of coronal plumes and coronal holes
Suess, S.T., Poletto, G., Wang,
A.H., Wu, S.T. and Cuseri, I.
Solar Physics, 180, 231 (1998)
1998-24: A new heating and acceleration mechanism for the fast solar wind
Cuseri, I., Mullan, D.J., Noci, G., Poletto, G.
Mem. Soc. Astron. Ital. 69, 745, 1998
In the fast solar wind models here
presented the heating and acceleration of the plasma is mainly due to Alfven
waves dissipating energy by gravitational damping. This mechanism, proposed
by Khabibrakhmanov and Mullan (1994), has never been used in solar wind
modeling. An extension of this theory has been made to give the evolution
of the Alfven wave amplitude as a function of the local parameters of the
atmosphere. The models, comparing favorably with recent data at low coronal
heights, show that gravitational damping of Alfven waves can heat the solar
plasma to coronal values and accelerate the solar wind to ~ 600-700 km/s.
1998-23: Coronal Plumes and the Solar Wind: Observations and Theories
Poletto, G., Corti, G., Romoli, M.
in "Advances in Solar Connection with Transient Interplanetary Phenomena" X.S. Feng, F.S. Wei, M. Dryer, Eds., International Academic Publ., Beijng, China, p. 235, 1998
Coronal plumes are thin, ray-like
features, which are rooted in coronal holes and extend outwards towards
the interplanetary space. Available data -- in white light, and, more recently,
in XUV radiation -- allow plumes to be traced out to a few solar radii
and there is some evidence that plume remnants can be identified also in
the solar wind, at distances of at least 0.3 AU. In this paper, after reviewing
plume observations and models, we illustrate a technique which offers a
still unexplored tool for checking model predictions against observational
data. We conclude with a brief discussion on SOHO observations as a means
of gaining more knowledge about the physics of plumes.
1998-22:
O5+ in High Speed Solar Wind Streams:
SWICS/Ulysses Results
Wimmer-Schweingruber, R. F.,
von Steiger, R., Geiss, J.,
Gloeckler, G., Ipavich, F. M., Wilken, B. Space Science Reviews, 85, 387 Recent observations with UVCS on SOHO of high outflow velocities of O5+ at
low coronal heights have spurred much discussion about the dynamics of solar
wind acceleration. On the other hand, O6+ is the most abundant oxygen charge
state in the solar wind, but is not observed by UVCS or by SUMER because this
helium-like ion has no emission lines falling in the wave lengths observable by
these instruments. Therefore, there is considerable interest in observing O5+ in
situ in order to understand the relative importance of O5+ with respect to the
much more abundant O6+. High speed streams are the prime candidates for the
search for O5+ because all elements exhibit lower freezing-in temperatures in
high speed streams than in the slow solar wind. The Ulysses spacecraft was
exposed to long time periods of high speed streams during its passage over the
polar regions of the Sun. The Solar Wind Ion Composition Spectrometer
(SWICS) on Ulysses is capable of resolving this rare oxygen charge state. We
present the first measurement of O5+ in the solar wind and compare these data
with those of the more abundant oxygen species O6+ and O7+. We find that our
observations of the oxygen charge states can be fitted with a single coronal
electron temperature in the range of 1.0 to 1.2 MK assuming collisional
ionization/recombination equilibrium with an ambient Maxwellian electron gas.
1998-21: Ultraviolet Coronagraph Spectrometer Observations of Density Fluctuations in the Solar Wind
Ofman, L., Romoli, M., Poletto, G., Noci, G., Kohl, J.L.
ApJ 507, L189, 1998
In the Letter ``Ultraviolet Coronagraph
Spectrometer Observations of Density Fluctuations in the Solar Wind'' by
L. Ofman, M. Romoli, G. Poletto, G. Noci, and J. L. Kohl (ApJ, 491, L111
[1997]), there was an error in the data reduction of the polarized brightness
(pB). It was assumed that the cadence of the data and the exposure time
are equal. However, the correct cadence is30 s longer than the exposure
time because of the time it takes the polarizer to change orientation between
exposures. This error does not affect the main result of the Letter, i.e.,
the detection of quasi-periodic density fluctuations in the solar wind.
However, the correct cadences of the data in Table 1 are 30 s longer. This
correction can be taken into account in Figure 1 by multiplying the times
by 1.5 and dividing the frequencies by the same factor. Thus, the highest
peak in the power spectrum is at 1.8+/-0.07 mHz (the corresponding period
is 9.3+/-0.4 minutes). The correction factor is 1.1 in Figure 2 because
of the longer exposure time in this observation.
1998-20: The White Light Polarimeter of SOHO/UVCS
Romoli, M., Benna, C., Fineschi, S., Gardner, L.D. Kohl, J.L. Noci, G.
Mem. Soc. Astron. Ital. 69, N. 3, 703-706, 1998
The White Light Polarimeter (WLC)
of the Ultraviolet Coronagraph Spectrometer (UVCS) on board of the Solar
Heliospheric Observatory (SOHO) is a coronagraph polarimetrer that measures
the polarized brightness (pBeta) of the K-corona between 1.6 and 5. Ro
in the 450-600 nm wavelength range. The pBeta gives a direct measurement
of the electron density integrated along the line of eight (LOS), which
is a key parameter for coronal plasma diagnostics. The WLC observations,
combined with the UVCS ultraviolet spectrometers observations, provide
a unique diagnostic tool for densities, temperatures and velocities of
the plasma in the extended corona. This paper is devoted to the interpretation
of the WLC observations from the present calibration of the data to the
determination of the polarized brightness and the electron density using
the complete set of synoptic observations, which now span almost two years
of solar activity.
1998-19: Solar Wind Velocity and Anisotropic Coronal Kinetic Temperature Measured With the O~VI Doublet Ratio
Dodero, M.A., Antonucci, E., Giordano, S., Martin, R.
Solar Physics, 183, 77-90, 1998
Doppler dimming of the Ovi resonance
lines (lambda1032Å, lambda1037Å) in an expanding corona is
calculated including the pumping effect on the Ovi lambda1037.61 Å
of both Cii lines at lambda1036.34Å and lambda1037.02Å, 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
lambda1036.34Å allows us to extend to approximately 450kms-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.
1998-18: Ultraviolet Spectroscopy of Coronal Mass Ejection
A. Ciaravella, J.C. Raymond, S. Fineschi, M. Romoli, C. Benna, L.D. Gardner, S. Giordano, R.H. O'Neal, J. Michels, E. Antonucci, J.L Kohl, G. Noci.
IAU Colloquium 167, Vol. 150, p. 370, 1998
A coronal mass ejection (CME) event
was observed on December 23, 1996 with the Ultraviolet Coronagraph Spectrometer
in both ultraviolet and visible light channels at 0.5 R_sun over the solar
limb. The CME was followed during its evolution, in the bright lines
of Ly alpha (1216 A), Ly beta (1026 A), Ly gamma (972 A), CIII(977
A) and the O VI doublet (1032, 1037 A) as well as in several other weaker
lines. The Lya (1216 A) peak intensity shows an excursion of two orders
of magnitude during the CME evolution, and blue shifts up to 0.8A\/ (200
km/sec). The data provide the emission measure in the Log T range 4.0--5.5
with a 0.3 sampling. Line intensities and profiles have been measured,
providing important diagnostics for a very detailed study of the physical
and dynamical parameters of the CME.
1998-17: Tomographic Reconstructions of the Corona From UVCS/SOHO Synoptic Observations
Panasyuk, A.V., Strachan, L., Fineschi, S., Gardner, L.D., Raymond, J., Kohl, J.L., Antonucci, E., Giordano, S., Romoli, M.
Synoptic Solar Physics, ASP Conference Series; Vol. 140, 407, 1998
The Solar Heliospheric Observatory
(SOHO) has broadened our capability to observe the outer solar corona by
gaining access to an unexplored electromagnetic domain, the ultraviolet
region, with the Ultraviolet Coronagraph Spectrometer (UVCS), and to a
much larger field of view, 30 solar radii, and better temporal coverage
in visible light with the Large Angle Spectroscopic Coronagraoph (LASCO).
The main results obtained with the SOHO coronagraphs concern the identification
of the sources and regions of acceleration of the slow and fast solar wind,
of possible signatures of the mechanisms of coronal expansion, and of instabilities
leading to coronal transients. Synoptic observations from the Ultraviolet
Coronagraph Spectrometer on SOHO are used to obtain spectral profiles and
intensities (integrated along the line-of-sight) for a number of emission
lines in the corona including H I Lyman alpha and the O VI 1032/1037 doublet.
The UVCS synoptic program makes a complete scan around the Sun once per
day, covering heights from 1.5 to 3 solar radii in the equatorial regions
and heights from 1.5 to 2.5 solar radii in the polar and midlatitude regions.
These data provide a direct measure of the line-of-sight velocities for
the emitting atoms or ions. Further analysis can be used to derive densities,
temperatures, and outflow velocities for hydrogen and O5+ ions in particular.
Here we present the results from our efforts to resolve the line-of-sight
distribution of intensities with the help of 3D reconstruction techniques.
In theory, it is possible to resolve 3D spatial structures using these
techniques but the analysis of the synoptic data can be complicated due
to the fact that the corona is not stationary, but evolving over time.
We will discuss these complications and some first results for the tomographic
reconstructions. This work is supported by NASA under Grant NAG5-3192 to
the Smithsonian Astrophysical Observatory, by the Italian Space Agency,
and by Swiss funding sources.
1998-16:
RODOMA: The Rome network for Doppler and magnetic oscillations
Cacciani, A., Hanslmeier, A., Messerotti, M., Moretti, P. F., Otruba, W., Pettauer, Th., Rodgers, W. ESA SP-418, 131 The development of the RODOMA network is now at the point to start taking
regular observations from two sites: the Austria site in Kanzelhoehe and the
California site in Apple Valley. The final test is connected with a UVCS(SOHO)
campaign operating between april 20 and May 05 1998. The data consist of
Doppler and simultaneous magnetic and intensity full-disk images taken at
intervals of 30 or 60 seconds, depending on the final desired precision. The
contribution is aimed to illustrate the state and the characteristics of the
instrumentation and to show samples of results. A third site will be Tashkent in
Uzbekistan, in the framework of the IRIS collaboration.
1998-15: Momentum deposition by a spectrum of Alfven waves in fast solar wind: effects on the mission lines observed by SOHO/ UVCS
Orlando, S.; Ventura, R.; Peres, G.; Spadaro, D.
Mem. Soc. Astron. Ital. 69; 777-780, 1998
We investigate how the Alfven waves propagating in the solar wind may affect some spectral lines observed by SOHO/UVCS. We use a detailed model of solar wind, which includes the transition region and accounts for the momentum deposition by a spectrum of non-WKB Alfven waves. We synthesize the emission in the Ly alpha, Ly beta and O VI (1032 A and 1038 A) lines considering wind models for a set of model parameter values.
1998-14: White light stray light test of the SOHO UVCS
Leviton, D.B.; Gardner, L.D.; Fineschi, S.; Jhabvala, M.D.; Kohl, J.L.; Romoli, M.; Noci, G.
Proc. SPIE, 3443, 50-60 (1998)
During the late stages of integration
at MATRA-Marconi in Toulouse, France of the Ultraviolet Coronagraph Spectrometer
(UVCS) for the joint NASA/ESA (European Space Agency) Solar and Heliospheric
Observatory (SOHO), project management for the International Solar and
Terrestrial Physics Project (ISTP) at Goddard Space Flight Center (GSFC)
became concerned that the instrument's elaborate stray light rejection
system had not been tested and might possibly be misaligned such that the
instrument could not deliver promised scientific returns. A white light
stray light test, which would place an upper bound on UVCS's stray light
rejection capability, was commissioned, conceived, and carried out. This
upper bound value would be indicative of the weakest coronal features the
spectrometer would be capable of discerning. The test was rapidly developed
at GSFC, in parallel with spacecraft integration, in coordination with
science team members from Harvard- Smithsonian Center for Astrophysics
(CFA) and was carried out at MATRA in late February 1995. The outcome of
this test helped justify later impact to integration schedule to conduct
similar much needed testing with visible and far ultraviolet light at CFA
in a facility specifically designed to perform such tests.
1998-13: Doppler dimming of the OVI 1032, 1037 doublet in the solar corona
M.A. Dodero, E. Antonucci, R. Martin.
Mem. Soc. Astron. Ital. 69, 757-760, 1998
The coronal O VI lines are Doppler dimmed in the presence of outflows and the dimming, which is a function off the solar wind velocity, is different for the two lines. We compute the ratio of the O VI (103.7 and 103.2 nm) resonance line emissivities as a function of the velocity. The calculation include the pumping effect on the O VI 103.761 line of the two C II lines at 103.702 nm and 103.634 nm. The presence of these lines in the spectrum exciting the oxygen coronal ions allows the measurement of solar wind velocities up to 400-450 km s-1. The present calculations account for the effects on the O VI ratio of the following quantities: width of the coronal absorbing profiles, electron density and temperature.
1998-12: Grating Stray Light Analysis and Control in the UVCS/SOHO
S. Fineschi, L.D. Gardner, J.L. Kohl, M. Romoli, G. Noci
Proc. 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 (UL spectrometers, optimized for the H I Lyman alpha (Ly-a) line (1216 A) and for the O VI doublet (1023/1037 A). The stray light profile of the Ly-a 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- a from the solar corona. These observations have resulted, for the first time, in the most direct measurement of the coronal electron temperatures.
1998-11: Thermal Coupling of Protons and Neutral Hydrogen in the Fast Solar Wind
L. Allen, S. R. Habbal: Harvard-Smithsonian
CfA Y.-Q. Hu: University of Science and Technology of China
1998, J. Geophys. Res., 103, 6551-6569
The coupling of neutral hydrogen to protons is investigated for a range of proton temperatures reaching a maximum of 6x10^6 K between 3 and 5 R_s, as recently inferred from observations of the Lyman alpha spectral line profiles by Kohl et al. [1996]. The approach used is adopted from Olsen et al. [1994] whereby the neutral hydrogen atoms are treated as test particles in a background electron-proton solar wind. Charge exchange between neutrals and protons, radiative recombination, collisional ionization of the neutrals, and the effects of Alfven waves, described by a single frequency or a spectrum, are included. The model computations show that an anisotropy in the neutral hydrogen temperature in the directions parallel and perpendicular to the magnetic field develops in the inner corona below 10 R_s. While the proton temperature and the neutral hydrogen temperature in the parallel direction remain strongly coupled, the perpendicular neutral hydrogen temperature exceeds these two in all cases studied. The temperature, T_H, incorporating both random and wave motions of neutral hydrogen, is found to be independent of frequency and less than the proton effective temperature T_p(eff) in the inner corona. Thus, without additional information about the waves, which would allow T_H and T_p to be extracted from the models, T_H(eff) provides an upper limit on T_p and a lower limit on $T_p(eff). However, as the peak of the proton temperature increases, the anisotropy in the inner corona decreases, with a temperature difference of 8x10^5 K between the protons and neutrals when the latter reach 6x10^6 K. In reference to the recent Ly alpha measurements, T_H ($\approx T_{H(eff)}^\perp$ for a wave spectrum of frequencies), is a better indicator of T_p than for lower inferred temperatures. The difference between T_{H(eff) and T_{p(eff) is 5x10^5 K below 3 R_s for all solutions, and becomes significant only beyond this point.
1998-10: Scientific achievements of SOHO: outer corona
Antonucci, E.
"A Crossroads for European Solar and Heliospheric Physics", ESA SP 417, 25-34, 1998
The Solar Heliospheric Observatory (SOHO) has broadened our capability to observe the outer solar corona by gaining access to an unexplored electromagnetic domain, the ultraviolet region, with the Ultraviolet Coronagraph Spectrometer (UVCS), and to a much larger field of view, 30 solar radii, and better temporal coverage in visible light with the Large Angle Spectroscopic Coronagraoph (LASCO). The main results obtained with the SOHO coronagraphs concern the identification of the sources and regions of acceleration of the slow and fast solar wind, of possible signatures of the mechanisms of coronal expansion, and of instabilities leading to coronal transients.
1998-09: UVCS/SOHO: The First Two Years
Cranmer, S. R., Kohl, J. L., and Noci, G.
Space Science Reviews, 85, 341 (1998)
The SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) has observed the extended solar corona between 1 and 10 solar radii for more than two years. We review spectroscopic and polarimetric measurements made in coronal holes, equatorial streamers, and coronal mass ejections, as well as selected non-solar targets. UVCS/SOHO has provided a great amount of empirical information about the physical processes that heat and accelerate the solar wind, and about detailed coronal structure and evolution.
1998-08: Space Observations of the source Regions of the Solar Wind: Implications for the Acceleration Mechanisms
D. Spadaro
Mem. Soc. Astron. Ital. 69, 677, 1998
This paper presents and discusses some of the main observational results concerning the extended solar atmosphere recently obtained by the coronal remote--sensing instruments on board the SOHO satellite, in particular LASCO and UVCS. It concentrates on the contribution given by these observations to a more detailed understanding of the physical conditions (magnetic field configuration, temperature, density, flow velocity, chemical composition and so on) in the solar wind source regions. Some emphasis is given to the results got by UVCS in the extended corona concerning line intensities and profiles, which can be important for the understanding of the solar wind heating and acceleration processes. These results, coupled to plasma diagnosis with in--situ particle instruments, can help to establish the nature of the relationship between conditions in the regions of origin of the solar wind and the plasma flow properties observed at 1 A.U.
1998-07: Physical Structure of a Coronal Streamer in the Closed Field Region Observed from UVCS/SOHO and SXT/YOKOH
J. Li, J.C. Raymond, L.W Acton, J.L. Kohl, M. Romoli, G. Noci & G. Naletto
Ap.J. 506, 431-438 (1998)
We analyze a coronal helmet streamer observed on July 25, 1996 using instruments aboard two solar spacecraft, the UltraViolet Coronagraph Spectrometer (UVCS) on board SOlar and Heliospheric Observatory (SOHO) and the Soft X-ray Telescope (SXT) on board Yohkoh. We derive temperatures and electron densities at 1.15 solar radii from SXT/Yohkoh observations. At this height, the streamer temperature is about Log T [K] = 6.28$\pm 0.05$, and electron density is about Log $n_e$ [$cm^{-3}$] = 8.09$\pm 0.26$, while at 1.5 $\RSUN$ a temperature of Log T [K]= 6.2 and density of Log $n_e$ [$cm^{-3}$] = 7.1 are obtained by UVCS/SOHO. Within the measurement uncertainty, this suggests a constant temperature from the base of the streamer to 1.5 $\RSUN$. Electron density measurements suggest that the gas in the streamer core is close to hydrostatic equilibrium. Comparison with potential field models for the magnetic field suggests a plasma $\beta$ larger than 1 in the closed field region in the streamer. In deriving electron densities and temperatures from the SXT/Yohkoh data, we include the effects of abundance anomalies on the SXT filter response. We use the elemental abundances derived from the UVCS/SOHO observations to estimate the First Ionization Potential (FIP) and gravitational settling effects. We give the set of abundances for the solar corona which agrees with our observations. In addition, we analyzed the SXT data from 6 consecutive days. We found that from July 22 to 27, the streamer physical properties are nearly constant. We conclude that we may be observing the same loop system over 6 days.
1998-06: Elemental Abundances in Coronal Structures
John C. Raymond, Raid Suleiman, John L. Kohl, Giancarlo Noci
Space Science Reviews, 85, 283 (1998) (eds. C. Frolich, M.C.E. Huber, S. Solanki and R. von Steiger)
A great deal of evidence for elemental abundance variations among different structures in the solar corona has accumulated over the years. Many of the observations show changes in the relative abundances of high- and low-First Ionization Potential elements, but relatively few show the absolute elemental abundances. Recent observations from the SOHO satellite give absolute abundances in coronal streamers. Along the streamer edges, and at low heights in the streamer, they show roughly photospheric abundances for the low-FIP elements, and a factor of 3 depletion of high-FIP elements. In the streamer core at 1.5 Rsun, both high- and low-FIP elements are depleted by an additional factor of 3, which appears to result from gravitational settling.
1998-05: Solar Wind at 6.8 Solar Radii from UVCS Observation of Comet C/1996 Y1
J. C. Raymond, S. Fineschi, P.L. Smith, L. Gardner, R. O'Neal, A. Ciaravella, J.L. Kohl, B. Marsden, G.V. Williams,C. Benna, S. Giordano, G. Noci & D. Jewitt
Ap.J., 508, 410 (1998)
The comet C/1996 Y1, a member of the Kreutz family of sungrazing comets, was observed with the Ultraviolet Coronagraph Spectrometer (UVCS) aboard the SOHO satellite. The Ly-alpha$- line profile and spatial distribution are interpreted in terms of the theory of bow shocks driven by mass-loading. At the time of the observation, the comet was 6.8 Rsun from the sun in a region of high-speed wind, a region difficult to observe directly with the the SOHO instruments, but an important region for testing models of solar wind acceleration and heating. We find a solar wind speed below 640 km s^{-1} and a constraint on the combination of solar wind speed and proton temperature. The total energy per proton at 6.8 Rsun is 50-75% of the energy at 1 AU, indicating that significant heating (perhaps by MHD waves) occurs at larger radii. The centroid and width of the Ly-alpha line generally confirm the predictions of models of the cometary bow shock driven by mass-loading as cometary molecules are ionized and swept up in the solar wind. We estimate an outgassing rate of 20 kg/s, which implies an active area of the nucleus only about 6.7 m in diameter at 6.8 Rsun. This is likely to be the size of the nucleus, as any inert mantle would have probably been blown off during the approach to the Sun.
1998-04: Non-Maxwellian Redistribution in Solar Coronal Lyman Alpha Emission
Cranmer, S. R.
Ap. J., 508, 925 (1998)
This paper presents theoretical models of H I Lyman alpha emission from the extended solar corona, taking into account various plasma kinetic effects which induce departures from Maxwellian velocity distributions. Such phenomena as suprathermal tails, strong temperature anisotropies, and skewed or double-peaked distributions have been observed in the solar wind, and UV spectroscopy is beginning to be able to detect their signatures in the corona. For resonantly scattered lines like H I Lyman alpha most of the physics is contained in the frequency-dependent redistribution function. The dependence of this function on the local plasma parameters is presented analytically for four different non-Maxwellian distributions, and optically thin line intensities are computed for a representative model of the fast solar wind. Isotropic power-law ``kappa'' tails in the velocity distribution should be detectable between 2 and 5 Angstroms from line center. Although existing observations which appear to have broad tails do not resemble those arising from kappa distributions, their presence is still possible. Anisotropic bi-Maxwellian distributions affect line profile shapes and total intensities via both their parallel and perpendicular components, and it is important to include an accurate description of the photon redistribution for large anisotropies. Skewness caused by a Chapman-Enskog expansion in the conductive heat flux is detectable as a unique non-Gaussian profile shape, but other types of collisionally beamed or skewed distributions may not noticeably affect the emission lines.
1998-03: A Global Model of the Corona with Heat and Momentum Addition
A.H. Wang, S.T. Wu, S.T. Suess, G. Poletto
J. Geophys. Res. 103, 1913 (1998)
We have been developing a series of global coronal models directed a better simulation of empirical coronal hole and streamer properties. In a previous study, a volumetric heat source was used to produce a thin current sheet above streamers and high solar wind speed in the coronal hole. This improved the preexisting coronal structure for coronal mass ejection simulations even when not using a polytropic energy equation. Here we report on the addition of a momentum source to the model with volumetric heating and thermal conduction. Most theoretical acceleration models in coronal holes are driven either by thermal pressure or waves (magnetosonic, Alfven, and sonic waves). In the thermal pressure driven models an artifically high effective temperature is assumed. In the wave driven models the force is generally not big enough to accelerate the solar wind as observed. In the present modes, in comparison to earlier calculations (Suess, et al., 1996), we reduce the heat source and add momentum. These changes appear to further improve the numerical simulation results in comparison to empirical properties. We have high solar wind speed in the hole without using unrealistic high plasma temperature. We also demonstrate the the deposition height of the momentum addition affects the mass flux. The model still predicts a slow-speed solar wind source in the streamer and high plasma beta at the top of the streamer.
1998-02 UVCS/SOHO Empirical Determinations of Anisotropic Velocity Distributions in the Solar Corona
Kohl, J. L., Noci, G., Antonucci, E., Tondello, G., Huber, M. C. E., Cranmer, S. R., 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. Letters (1998), 501, L127
We present a self-consistent empirical model for several plasma parameters of a polar coronal hole at solar minimum using observations from the SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO). The model describes the radial and latitudinal distribution of density, outflow velocity, and microscopic velocity for electrons, neutral hydrogen, and ionized oxygen during the period between November 1996 and April 1997, and will be developed at length in a forthcoming paper. In this Letter we compare observations of H I Lyman alpha and O VI 1032,1037 emission-line intensities and profiles with coronal models, and iterate for optimal consistency in the microscopic and outflow velocities. The unexpectedly large line-widths of H^0 atoms and O^(5+) ions at most radii are the result of anisotropic velocity distributions which cannot be explained with either thermal equilibrium or common fluid motions. Above 2 R_sun, the observed horizontal microscopic velocities for O^(5+) are significantly larger than the corresponding motions for H^0, and the outflow velocities for O^(5+) are also significantly larger than the corresponding velocities of H^0. We discuss the constraints and implications on various theoretical models of coronal heating and acceleration.
1998-01: The Effect of Temperature Anisotropy on Observations of Doppler Dimming and Pumping in the Inner Corona
Li, X., Habbal, S. R., Kohl, J. L., and Noci, G.
Ap. J. Letters (1998), 501, L133
Recent observations of the spectral line profiles and intensity ratio of the O VI lambda lambda 1032 and 1037.6 doublet by the Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO), made in coronal holes below 3.5 R_{{S}} , provide evidence for Doppler dimming of the O VI lambda 1037.6 line and pumping by the chromospheric C II lambda 1037.0182 line. Evidence for a significant kinetic temperature anisotropy of O5+ ions was also derived from these observations. We show in this Letter how the component of the kinetic temperature in the direction perpendicular to the magnetic field, for both isotropic and anisotropic temperature distributions, affects both the amount of Doppler dimming and pumping. Taking this component into account, we further show that the observation of the O VI doublet intensity ratio less than unity can be accounted for only if pumping by C II lambda 1036.3367 in addition to C II lambda 1037.0182 is in effect. The inclusion of the C II lambda 1036.3367 pumping implies that the speed of the O5+ ions can reach 400 km s-1 around 3 R_{{S}} , which is significantly higher than the reported UVCS values for atomic hydrogen in polar coronal holes. These results imply that oxygen ions flow much faster than protons at that heliocentric distance.