UVCS/SOHO PAPERS - 2001

Numbers in green square brackets [ ] are the PUB ID. from the SOHO Bibliography & Publications Database at
http://sohowww.nascom.nasa.gov/publications/ (click on "database"). In-press and submitted papers are not included.

2001-29:    A 16-moment solar wind model: From the chromosphere to 1 AU

O. Lie-Svendsen, E. Leer and V. H. Hansteen

JGR, volume 106, pp 8217-8232

We present a solar wind fluid model extending from the chromosphere to Earth. The model is based on the gyrotropic approximation to the 16-moment set of transport equations, in which we solve for the density, drift speed, temperature parallel and perpendicular to the magnetic field, and transport of parallel and perpendicular thermal energy along the magnetic field (heat flux). The solar wind plasma is created dynamically through (photo) ionization in the chromosphere, and the plasma density in the transition region and corona is computed dynamically, dependent on the type of coronal heating applied, rather than being set arbitrarily. The model improves the description of proton energy transport in the transition region, where classical heat conduction is only retrieved in the collision-dominated limit. This model can serve as a ``test bed'' for any coronal heating mechanism. We consider heating of protons by a turbulent cascade of Alfvén waves in rapidly expanding coronal holes. The resulting high coronal proton temperatures lead to a downward proton energy flux from the corona which is much smaller than what classical transport theory predicts, causing a very low coronal density and an extremely fast solar wind with a small mass flux. Only when some of the wave energy is forcibly deposited in the lower transition region can a realistic solar wind be obtained. Because of the poor proton heat transport, in order to produce a realistic solar wind any viable heating mechanism must deposit some energy in the transition region, either directly or via explicit heating of coronal electrons.


2001-28:     Differential Flow Speeds of Ions of the Same Element: Effects on Solar Wind Ionization Fractions

Esser, R., and Edgar, R. J.

ApJ 563, pp. 1055-1062.

In previous studies of solar wind ion fractions it has always been assumed that ions of the same element flow with the same flow speed. In the present paper, we investigate the effects of differential flow speeds between different ionization states of the same element on the formation of O, C, Mg, Si, and Fe ions. We show that if the difference in flow speeds of adjacent ions, vi and vi+1, is large, the discrepancy between observed in situ ion fractions and low coronal electron temperature can be significantly reduced. The calculations are carried out for a radial profile of the electron temperature chosen in agreement with Solar Ultraviolet Measurement of Emitted Radiation (SUMER) measurements of coronal dark lanes and with two radial profiles of the electron density representing lower and upper observational limits. In order for the differential flow speeds to close the gap between the ion fractions observed in situ and the ones predicted from the low electron temperature observed in the corona, the differential flow speeds have to be extremely large in the regions where the ions are formed, e.g., below 1.5RS for O ions and below 2.5RS for Si ions; and in the case of Si, the coronal electron density has to be at the upper limit of observed values. In the cases of C, O, Mg, and Fe, the lower limit on the electron density is also acceptable. [4347]


2001-27:    Outflow speed of protons and O VI ions in a coronal hole

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

Memorie della Società Astronomica Italiana 72, pp.600-603
2nd National Meeting on the Italian Solar Research, L'Aquila, 3 - 5 July 2000, ed. P. Francia, E. Pietropaolo, B. Caccin

In this work we estimate the outflow speed of protons and O VI ions as a function of latitude and heliocentric distance within a coronal hole. The outflow speed of protons is obtained from the mass flux conservation along the coronal magnetic field lines, while that of O VI ions is derived by applying the Doppler dimming technique to the intensity ratio of the O VI doublet lines at 1031.9 Å and 1037.6 Å, observed by the UVCS experiment. To this end we develop a 2D semiempirical coronal hole model, also based on UVCS data. We obtain that the outflow speed of these ions increases with the heliocentric distance, and with latitude from regions close to the equatorial streamer to the pole. [4644]


2001-26:    Electromagnetic heavy ion cyclotron instability: Ansiotropy constraint in the solar corona

Gary, S. P., Yin, L., Winske, D., and Ofman, O.

JGR, 106, 10715

The electromagnetic proton cyclotron anisotropy instability is driven by Tp/Tp>1 where p represents protons and the directional subscripts denote directions relative to the background magnetic field. Fluctuating field growth leads to wave-particle scattering, which in turn imposes an upper bound on the anisotropy of the form Tp/Tp-1 = Sp/, where p8npkBTp/B, and the fitting parameters Sp1 and p0.4. Recent SOHO observations indicate that minority heavy ions are substantially hotter and more anisotropic than protons in the solar corona. Here linear theory and hybrid simulations have been carried out in a model of a homogeneous, magnetized, collisionless plasma with anisotropic minority oxygen ions (denoted by subscript O). These calculations show that the electromagnetic oxygen ion cyclotron anisotropy instability also leads to wave-particle scattering, which constrains that anisotropy by the form TO/TO-1 = SO/[(mp/mO)O]o, where O8nekBTO/B, So~10 and O~0.4. This constraint should be observable in the solar corona. [4645]


2001-25:    Oxygen Abundance in Coronal Streamers During Solar Minimum

Marocchi, D., Antonucci E., & Giordano, S.

Annales Geophysicae, 19, pp.135-145

We present a study of the oxygen abundance relative to hydrogen in the equatorial streamer belt of the solar corona during the recent period of activity minimum. The oxygen abundance is derived from the spectroscopic observations of the outer corona performed during 1996 with the Ultraviolet Coronagraph Spectrometer (SOHO) in the ultraviolet region. This study shows that the depletion of oxygen, by almost one order of magnitude with respect to the photospheric values, found in the inner part of streamers by Raymond et al. (1997) is a common feature of the solar minimum streamer belt, which shows an abundance structure with the following characteristics. In the core of streamers the oxygen abundance is 1.3 times 10^{-4} at 1.5 R{sun}, then it drops to 0.8 times 10^{-4} at 1.7~R{sun}, value which remains almost constant out to 2.2 R{sun}. In the lateral bright structures that are observed to surround the core of streamers in the oxygen emission, the oxygen abundance drops monotonically with heliodistance, from 3.5 times 10^{-4} at 1.5 R{sun} to 2.2 times 10^{-4} at 2.2 R{sun}. The oxygen abundance structure found in the streamer belt is consistent with the model of magnetic topology of streamers proposed by Noci et al. (1997). The composition of the plasma contained in streamers is not the same as observed in the slow solar wind. Even in the lateral branches, richer in oxygen, at 2.2 R{sun} the abundance drops by a factor 2 with respect to the slow wind plasma observed with Ulysses during the declining phase of the solar cycle. Hence the slow wind does not appear to originate primarily from streamers, with the exception perhaps of the plasma flowing along the heliospheric current sheet. [4646]


2001-24:    Stray light evaluation of the Ultraviolet and Visible-light Coronagraph Imager (UVCI) rocket prototype

Romoli, Marco; Landini, Federico; Fineschi, Silvano; Gardiol, Daniele; Naletto, Giampiero; Malvezzi, Marco; Tondello, Giuseppe; Noci, Giancarlo C.; Antonucci, Ester

Proc. SPIE 4498, pp. 27-38
UV / EUV and Visible Space Instrumentation for Astronomy and Solar Physics, H. Siegmund; S. Fineschi; M. A. Gummin; Eds.

The Ultraviolet and Visible-light Coronagraph Imager (UVCI) proposed for the European Space Agency (ESA) Solar Orbiter mission, is designed to image the visible and the ultraviolet coronal emissions, in order to diagnose the solar corona. The UVCI is an externally occulted reflection coronagraph that obtains monochromatic images in the neutral hydrogen HI 121.6 nm and in the single ionized helium He II 30.4 nm lines, and measures the polarized brightness (pB) of the K-corona in broadband visible light. One of the most stringent requirements in the design of a coronagraph is the stray light rejection. The stray light is produced by solar disk radiation which is several order of magnitude brighter than the coronal radiation in both visible and UV. The solar disk radiation enters the instrument through the external aperture and stray light is produced by diffraction off the edges of the apertures and of the optical components, non-specular reflections off the mirror surfaces, and scattering off the mechanical structure. In this paper, the features in the optical design that contribute to the stray light reduction are described, and an analysis of all possible stray light contributions is performed on the optical configuration of the UVCI sounding rocket prototype (UVC-SR). From this analysis, a stray light model has been developed and its results are compared with the minimum measurable signal expected from the solar corona. [4647]


2001-23:    Heating of Coronal Holes and Generation of the Solar Wind by Ion-Cyclotron Resonance

Isenberg, P. A.

Space Sci. Rev., 95, pp. 119-131

We discuss a new model to describe the heating of the magnetically open solar corona and the acceleration of the fast solar wind by the cyclotron resonant interaction of coronal ions with ion-cyclotron waves. This `kinetic shell' model includes important details of the wave-particle interaction which are not present in most other treatments. In this model, we approximate the evolution of the collisionless coronal ion distribution through the assumption that the pitch-angle scattering by the resonant ion-cyclotron waves is much faster than the other processes taking place. Under this assumption, the resonant ions uniformly populate velocity-space surfaces, or shells, of constant energy in the frame moving with the wave phase speed. We show that a fast solar wind can be generated by this process. Furthermore, we present a number of properties of the resonant interaction that are implied by this model: (1) The amount of wave energy that can be absorbed by the proton distribution at a given radial position is limited. (2) The proton distribution generated by the interaction with outward-propagating waves will necessarily be unstable to the generation of inward-propagating waves, so a complete description must include waves of both propagation directions. (3) The structure of the resonant shells for ions heavier than protons indicates that these ions will be perpendicularly heated by the second-order Fermi process, an energization channel that is not available to the protons. This last point is particularly intriguing, and may lead to a fundamentally new way to produce the preferential effects on heavy ions in the fast solar wind. [4658]


2001-22:     Solar Cycle 24: Variation of the Solar Corona in the Ultraviolet from Solar Minimum to Solar Maximum

Miralles, M. P.; Panasyuk, A. V.; Strachan, L.; Gardner, L. D.; Suleiman, R.; Cranmer, S. R.; Kohl, J. L.

ISCS 2001 Symposium: Solar Variability, Climate, and Space Weather. Longmont, Colorado, USA, p. 59., June 2001

UVCS/SOHO measurements of H I Ly-alpha and O VI (103.2 nm and 103.7 nm) intensities in the solar corona have been made from solar Cycle 23's minimum in 1996 to its current maximum. At solar minimum, the corona consisted of large coronal holes at the poles and quiescent streamers at the equator. During the approach to solar maximum, equatorial coronal holes and high latitude streamers became more conspicuous. Recently, coronal holes at higher latitudes have reappeared, allowing a comparison to be made of O VI intensities and line widths of coronal holes at different latitudes. We also characterize the variation of coronal hole properties with height, and location over the solar cycle. This work is supported by NASA under Grant NAG5-10093 to the Smithsonian Astrophysical Observatory, by the Italian Space Agency and by PRODEX (Swiss contribution) [4648]


2001-21:     A solar wind coronal origin study from SOHO/UVCS and ACE/SWICS joint analysis

Ko, Y.-K.; Zurbuchen, T.; Strachan, L.; Riley, P.; Raymond, J.C.

AIP Conf. Proc. 598 pp. 133-8
Solar and Galactic Composition. Joint SOHO/ACE Workshop, 6-9 March 2001, Bern, Switzerland

The solar wind ionic charge composition is a powerful tool to distinguish between the slow wind and the coronal-hole associated fast wind. The solar wind heavy ions are believed to be 'frozen-in' within 5 solar radii of the Sun which falls right in the range of SOHO/UVCS coronal observations. We present a joint analysis from SOHO/UVCS and ACE/SWICS which attempts to establish observational evidence of the coronal origin of the solar wind. To connect the solar wind with its coronal origin, we adopt a 3D MHD model as a guide to link the solar wind at 1 AU to structures in the inner corona. We relate in-situ measured properties of the solar wind (elemental abundances and charge state distributions) with remotely sensed signatures in the corona, namely outflow velocity, electron temperature and elemental abundance [4649]


2001-20:    Highlights from SOHO and Future Space Missions

Fleck, B

Astrophys. & and Space Sci Library, 259
The Dynamic Sun, ed. A Hanslmeier, M. Messerotti, & A, Veronig [Kluwer]
Proc. Summer School and Workshop, Solar Observatory Kanzelhöhe, Kärnten, Austria

The Solar and Heliospheric Observatory (SOHO) has provided an unparalleled breadth and depth of information about the Sun, from its interior, through the hot and dynamic atmosphere, out to the solar wind. Analysis of the helioseismology data from SOHO has shed new light on a number of structural and dynamic phenomena in the solar interior, such as the absence of differential rotation in the radiative zone, subsurface zonal and meridional flows, sub-convection-zone mixing, a possible circumpolar jet, and very slow polar rotation. Evidence for an upward transfer of magnetic energy from the Sun's surface toward the corona has been established. The ultraviolet instruments have revealed an extremely dynamic solar atmosphere where plasma flows play an important role. Electrons in coronal holes were found to be relatively ``cool", whereas heavy ions are extremely hot and have highly anisotropic velocity distributions. The source regions for the high speed solar wind has been identified and the acceleration profiles of both the slow and fast solar wind have been measured. This paper tries to summarize some of the most recent findings from the SOHO mission. Present plans for future solar space missions are also briefly discussed. [4164,4144]


2001-19:     Impact of SOHO, TRACE and Yohkoh on Solar Physics

Brekke, P.

ASP Conf. Proc. 223, ed. R. J. Garcia Lopez, R. Rebolo & M. R. Z. Osorio
11th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun [4371]


2001-18:     Coronal and Solar Wind Elemental Abundances

Raymond, J. C.; Mazur, J. E.; Allegrini, F.; Antonucci, E.; del Zanna, G.; Giordano, S.; Ho, G.; Ko, Y.-K.; Landi, E.; Lazarus, A.; Parenti, S.; Poletto, G.; Reinard, A.; Rodriguez-Pacheco, J.; Teriaca, L.; Wurz, P.; Zangrilli, L.

AIP Conf. Proc. 598, pp.49-57
Solar and Galactic Composition, Joint SOHO/ACE Workshop, 6-9 March 2001, Bern, Switzerland

Coronal elemental abundances, as compared with abundances in the solar wind and solar energetic particles, provide the means for connecting solar wind gas with its coronal source. Comparison of coronal abundances with photospheric values shows fractionation with the ionization potential of the atom, providing important, though not yet fully understood, information about the exchange of material between corona and chromosphere. Fractionation due to gravitational settling provides clues about flows within the corona. In this paper, we discuss the uncertainties of abundance determinations with spectroscopic techniques and in situ measurements, we survey the ranges of abundance variations in both the corona and solar wind, and we discuss the progress in correlating solar wind features with their coronal sources. [4650]


2001-17:    Measuring Solar Abundances

von Steiger, R.; Vial, J.-C.; Bochsler, P.; Chaussidon, M.; Cohen, C. M. S.; Fleck, B.; Heber, V. S.; Holweger, H.; Issautier, K.; Lazarus, A. J.; Ogilvie, K. W.; Paquette, J. A.; Reisenfeld, D. B.; Teriaca, L.; Wilhelm, K.; Yusainee, S.; Laming, J. M.; Wiens, R. C.

Publication: AIP Conf. Proc. 598, p.13-22
Solar and Galactic Composition. Joint SOHO/ACE Workshop, 6-9 March 2001, Bern, Switzerland

[4363]


2001-16:   Oxygen abundance in the extended corona at solar minimum

E. Antonucci and S. Giordano

AIP Conf. Proc. 598 pp.77-81
Solar and Galactic Composition: Joint SOHO/ACE Workshop, 6-9 March 2001, Bern, Switzerland

We present a study on the abundance of oxygen relative to hydrogen in the solar minimum corona and for the first time we measure this quantity in polar coronal holes. The results are derived from the observations of the extended corona obtained with the Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO. The diagnostic method used to obtain the oxygen abundance is based on the resonant components of the O VI 1032 A and H I 1216 A emission lines. This method fully accounts for the effects of the outflow velocity of the solar wind, which can be determined through Doppler dimming, and of the width of the absorbing profiles of the coronal ions or neutral atoms involved in resonant scattering. The oxygen abundance is higher in the polar coronal hole regions, where the fast wind is accelerated, than in the streamer belt. In the polar regions the observed oxygen abundance is consistent with the photospheric value and with the composition results obtained with Ulysses for the fast wind. The oxygen abundance values derived with UVCS suggest that the plasma remains substantially contained in quiescent streamers, that therefore do not contribute significantly to the solar wind. [4651]


2001-15:    Preliminary results from coordinated SOHO-Ulysses observations

Parenti, S.; Poletto, G.; Bromage, B.J.I.; Suess, S.T.; Raymond, J.C.; Noci, G.; Bromage, G.E.

AIP Conf. Proc. 598, 83
Solar and Galactic Composition. Joint SOHO/ACE Workshop, 6-9 March 2001, Bern, Switzerland

SOHO-Ulysses quadratures occur at times when the SOHO-Sun-Ulysses angle is 90 degrees and offer a unique possibility to compare properties of plasma parcels observed in the low corona with properties of the same parcels measure, in due time, in situ. The June 2000 quadrature occurred at a time Ulysses was at 3.35 AU and at a latitude of 58.2 degrees in the south-east quadrant. Here we focus on the UVCS observations made on June 11, 12, 13, 16. UVCS data were acquired at heliocentric altitudes ranging from 1.6 to 2.2 solar radii, using different grating positions, in order to get a wide wavelength range. The radial direction to Ulysses, throughout the 4 days of observations, traversed a region where high latitude streamers were present. Analysis of the spectra taken by UVCS along this direction shows a variation of the element abundances in streamers over our observing interval: however, because the radial to Ulysses crosses through different part of streamers in different days, the variation could be ascribed either to a temporal or to a spatial affect. The oxygen abundance, however, seems to increase at the edge of streamers, as indicated by previous analyses. This suggests the variation may be a function of position within the streamer, rather than a temporal effect. Physical conditions in streamers, as derived from UVCS observations, are also discussed. [4652]


2001-14:    Oxygen abundance in streamers above 2 solar radii

Zangrilli, L.; Poletto, G.; Biesecker, D.; Raymond, J.C.

AIP Conf. Proc. 598, 71
Solar and Galactic Composition. Joint SOHO/ACE Workshop, 6-9 March 2001, Bern, Switzerland

The oxygen abundance in streamers has been evaluated by several authors see, e.g. 1,2, 3 who found, in the core of streamers, an oxygen abundance lower by a factor 3-4 than in the lateral branches legs. All estimates were made at heliocentric distances 2.2 solar radii. In this paper we analyze UVCS observations of two streamers, observed during solar minimum at altitudes h 2.4 solar radii to derive the oxygen abundance, relative to hydrogen, and its latitude dependence within streamers, in the range 2.4 r 4 solar radii. To this end, electron densities have been derived form LASCO data, taken at the time of UVCS observations, and the radial temperature profile has been taken from literature. These parameters allow us, after the collisional contribution to the OVI 1032 and 1037 A line intensities has been identified, to determine the oxygen abundance that reproduces the observed collisional components. Our results are compared with previous abundance determinations and the relationship between coronal and in situ abundances is also discussed. [4653]


2001-13:    Oxygen abundance in polar coronal holes

Teriaca, L.; Poletto, G.; Falchi, A.; Doyle, J.G.

AIP Conf. Proc. 598, 65
Solar and Galactic Composition. Joint SOHO/ACE Workshop, 6-9 March 2001, Bern, Switzerland

Fast solar wind is known to emanate from polar coronal holes. However, only recently attention has been given to the problem of where, within coronal holes, fast wind originates. Information on whether the fast solar wind originates from plumes or interplume regions may be obtained by comparing the elemental abundances in these regions with those characterizing the fast wind. Here we present a first attempt to determine the oxygen abundance in the interplume regions by using spectra taken at times of minimum in the solar cycle when it is easier to identify these structures by the SUMER spectrograph aboard SOHO. To this end, we analyze spectra taken in 1996 in polar regions, at altitudes ranging between 1.05 and 1.4 solar radii, finding a value 8.5 for the oxygen abundance in interplume regions. From the analysis of the OVI 1032 to 1037 line intensity ratio we also find no evidence of outflow velocities below 1.2 solar radii in interplume regions, while there are indications that outflow motions start to be significant above 1.5 solar radii. The method used and the assumptions made are discussed in light of the derived values. Our values are compared with previous determinations in the corona and solar winds. [4547]


2001-12:    Ultraviolet Coronagraph Spectrometer Observations of a High-Latitude Coronal Hole with High Oxygen Temperatures and the Next Solar Cycle Polarity

Miralles, M. P.; Cranmer, S. R.; Kohl, J. L.

ApJL, 560, pp.L193-L196

We announce the resurgence of extreme ion properties in a large, high-latitude coronal hole observed above the north heliographic pole in 2001 February at solar maximum. The observations were taken with the Ultraviolet Coronagraph Spectrometer on the Solar and Heliospheric Observatory. These observations are part of an ongoing campaign to determine the plasma properties of coronal holes during the current solar cycle. In this Letter, we compare the observations and analysis of O VI lambdalambda1032, 1037 spectral lines of a high-latitude coronal hole in 2001 with observations of an equatorial solar maximum hole in 1999 and polar coronal holes observed near solar minimum 1996-1997. These lines provide spectroscopic diagnostics of O+5 velocity distributions and outflow velocities. The O VI line profiles show a narrow core and broad wings. The narrow core is attributed to foreground and background streamers and, possibly, dense polar plumes at the lowest observed heights. The broad wings are attributed to the coronal hole. The comparison of the coronal hole line widths shows that the O+5 perpendicular kinetic temperatures in the 2001 high-latitude hole are similar to those observed in polar coronal holes at solar minimum. These observations of extremely high ion kinetic temperatures exceeding 108 K at the north pole in 2001 occurred nearly simultaneously with the polarity change of the Suns magnetic field, as seen in recent magnetogram data. This coronal hole in 2001 may represent the first manifestation of the negative polarity polar coronal holes that will dominate the Suns open magnetic flux tubes at the next solar minimum. The reappearance of broad O VI profiles at a time when not all of the new polarity magnetic flux has migrated to the poles was an interesting development. The variations in coronal hole parameters with the solar cycle provide constraints on models of extended coronal heating. [4126]


2001-11:    Constraints on the O+5 Anisotropy in the Solar Corona

Ofman, L.; Viñas, A.; Gary, S. P.

ApJL, 547, pp. L175-L178

Velocity distributions of O+5 ions derived from Ultraviolet Coronagraph Spectrometer (UVCS) observations in coronal holes indicate that the O+5 ions are highly anisotropic (T⊥i/T∥i~30-300 at 3.5 Rsolar). The observations provide empirical values for the electron density and the ion temperatures. It is well known that the electromagnetic ion cyclotron instability is driven by temperature anisotropy. The instability leads to the rapid decrease of anisotropy and transfer of part of the kinetic energy of the particles into the magnetic field fluctuations. Here we use linear theory and hybrid simulations combined with the empirical values of the densities and the temperatures to investigate the ion cyclotron instability of the anisotropic minor ions in the coronal hole plasma. We find that an initial O+5 anisotropy of 50 decreases by an order of magnitude within ~300-900 proton cyclotron periods. Thus, the ion cyclotron instability constrains the anisotropy of O+5 ions that can be sustained in the solar corona without continuous perpendicular heating. [4655]


2001-10:    SOHO Observations of a Coronal Mass Ejection

Akmal, A.; Raymond, J. C.; Vourlidas, A.; Thompson, B.; Ciaravella, A.; Ko, Y.-K.; Uzzo, M.; Wu, R.

ApJ, 553, pp. 922-934.

We describe a coronal mass ejection (CME) observed on 1999 April 23 by the Ultraviolet Coronagraph Spectrometer (UVCS), the Extreme-Ultraviolet Imaging Telescope (EIT), and the Large-Angle and Spectrometric Coronagraphs (LASCO) aboard the Solar and Heliospheric Observatory (SOHO). In addition to the O VI and C III lines typical of UVCS spectra of CMEs, this 480 km s-1 CME exhibits the forbidden and intercombination lines of O V at λλ1213.8 and 1218.4. The relative intensities of the O V lines represent an accurate electron density diagnostic not generally available at 3.5 Rsolar. By combining the density with the column density derived from LASCO, we obtain the emission measure of the ejected gas. With the help of models of the temperature and time-dependent ionization state of the expanding gas, we determine a range of heating rates required to account for the UV emission lines. The total thermal energy deposited as the gas travels to 3.5 Rsolar is comparable to the kinetic and gravitational potential energies. We note a core of colder material radiating in C III, surrounded by hotter material radiating in the O V and O VI lines. This concentration of the coolest material into small regions may be a common feature of CMEs. This event thus represents a unique opportunity to describe the morphology of a CME, and to characterize its plasma parameters. [4199]


2001-09:    Three-Fluid 2.5-dimensional Magnetohydrodynamic Model of the Effective Temperature in Coronal Holes

Ofman, L.; Davila, J. M.

ApJ, 553, p.935

Recent SOHO Ultraviolet Coronagraph Spectrometer (UVCS) observations show that protons and minor ions are hot (Tp>106 K, Ti>107 K) and anisotropic in coronal holes. A possible cause of the large perpendicular motions is unresolved Alfvénic fluctuations in the solar wind. Using the three-fluid 2.5-dimensional MHD model, we have shown that the unresolved Alfvénic fluctuations lead to apparent proton temperature and anisotropy consistent with UVCS observations. However, Alfvén waves with realistic amplitudes cannot reproduce the O5+ perpendicular temperature and anisotropy deduced from UVCS observations. This suggests that the minor ions are heated by a different mechanism than protons. [4656]


2001-08:    A two-dimensional Alfven wave-driven solar wind model

Chen, Y.; Hu, Y. Q.

Solar Phys., 199, pp. 371-384

This paper presents a two-dimensional, Alfven wave-driven solar wind model, in which the wave energy is assumed to cascade from the low-frequency Alfven waves to high-frequency ion cyclotron waves and to be transferred to the solar wind protons by cyclotron resonance at the Kolmogorov rate. A typical structure in the meridional plane consisting of a coronal streamer near the Sun, a fast wind in high latitudes, and a slow wind across the heliospheric current sheet, is procured. The fast wind obtained in the polar region is essentially agreeable with that derived by previous one-dimensional flow tube models, and its density profile in the vicinity of the Sun roughly matches relevant observations. The proton conditions at 1 AU are also consistent with observations for both the fast and slow winds. The Alfven waves appear in the fast and slow wind regions simultaneously and have comparable amplitudes, which is accordant with Helios observations. The acceleration and heating of the solar wind by the Alfven waves are found to occur mainly in the near Sun region. It is demonstrated in terms of one-dimensional calculations that the distinct properties of the fast and slow winds are mainly attributed to different geometries of the flow tubes associated with the two sorts of winds. In addition, the 2-D and 1-D simulations give essentially the same results for both the fast and the slow winds. [4200]


2001-07:    Results from the UVCS and LASCO Observation of the Sungrazing Comet C/2000 C6

Uzzo, M.; Raymond, J. C.; Biesecker, D.; Marsden, B.; Wood, C.; Ko, Y.-K.; Wu R.

ApJ, 558, pp. 403-410

On February 9-10, 2000 the LASCO and UVCS instruments aboard SOHO observed Comet C/2000 C6, a member of the Kreutz family of Sungrazing comets. A tail nearly 0.5 R_{\odot} in length was detected in Ly alpha emission. UVCS was able to observe the comet at four heights as it approached the Sun. A jump in the Ly alpha brightness between 5.71 and 4.56 R_{\odot} suggests that the nucleus fragmented, exposing more area to solar illumination and increasing the outgassing rate. The only previously published UVCS observation of a Sungrazing comet was that of C/1996 Y1, which showed a broad Ly alpha profile from H I produced by charge transfer behind the comet's bow shock. C/2000 C6 shows a narrower profile with no indication of a bow shock. We interpret the Ly alpha luminosity in terms of the outgassing rate, and use this to estimate the diameter of the nucleus. The Ly alpha emission fades as H I is ionized, providing estimates of the density in the coronal streamer the comet encountered. [4657]


2001-06:    UVCS Observations of Velocity Shear at Streamer Boundaries in the Corona

Habbal, S. R.; Woo, R.; Vial, J.-C.

Space Sci. Rev. 97, pp. 5-8

Measurements of the intensities of the O IV 1032 and 1037 A spectra lines in the southern solar hemisphere, from 1.5 to 5 R_sun, were made with the SOHO Ultraviolet Coronagraph Spectrometer (UVCS) in May 2000 close to solar maximum. The ratio of the intensity of the two oxygen lines is used as a proxy for solar wind velocity in the inner corona. White light images of the corona taken with the SOHO/LASCO-C2 during the same time period, and used to place the UVCS observations in the context of coronal structures, show streamers extending to high latitudes. The measured radial and azimuthal gradients of the intensity ratio reflect strong velocity shears in the corona with the slowest wind coinciding with the axis of streamers. Comparison of these results with ratios measured with UVCS during solar minimum indicates that the transition from fast to slow wind in the inner corona occurs within 20° of the axis of streamers at both phases of the solar cycle. [4383]


2001-05:    Comparison of Empirical Models for Polar and Equatorial Coronal Holes

Miralles, M. P.; Cranmer, S. R.; Panasyuk, A. V.; Romoli, M.; Kohl, J. L.

ApJ 549, L257-L260

We present a self-consistent empirical model for several plasma parameters of a large equatorial coronal hole observed on 1999 November 12 near solar maximum. The model was derived from observations with the Ultraviolet Coronagraph Spectrometer on the it Solar and Heliospheric Observatory. In this Letter, we compare the observations of ionO6 lambdalambda1032, 1037 emission lines with previous observations of a polar coronal hole observed near solar minimum. At the time of the 1999 observations, there was no evidence of large polar coronal holes. The resulting empirical model for the equatorial coronal hole describes the outflow velocities and most probable speeds for O5+, and we compared the derived ion properties to the empirical model for a solar minimum polar coronal hole. The comparison of the empirical models shows that the 1999 equatorial hole has lower O5+ outflow speeds and perpendicular temperatures than its polar counterpart from 1996--1997 at heights between 2 and 3R_sun. However, it in situ asymptotic speeds of the wind streams coming from the 1996--1997 polar hole and from the 1999 equatorial hole are only sim15 different. Thus, the bulk of the solar wind acceleration must occur above 3R_sun for the equatorial coronal hole. The equatorial hole also has a higher density than the polar hole at similar heights. It is not yet known if the higher densities are responsible for the seeming inhibition of the fast ion outflow speeds and extremely large perpendicular temperatures that occur in polar coronal holes at solar minimum. We discuss the constraints and implications on various theoretical models of coronal heating and acceleration. [3741]


2001-04:     The Dec. 12, 1997 Helical Coronal Mass Ejection: Density, Energy Estimates and Hydrodynamics

Ciaravella, A.; Raymond, J. C.; Reale, F.; Strachan, L.; Peres, G.

ApJ 557, pp. 351-365

We use UVCS spectra to investigate the density range of the plasma ejected during the the CME on Dec 12, 1997. Time-dependent ionization states for several phenomenological models, with the boundary conditions derived from EIT and UVCS observations, were computed and constraints on the density and temperature of the plasma at the early stage of the ejection are obtained. The role of physical mechanisms such as thermal conduction, radiation and heating is also studied with a 2D hydrodynamics simulation. The kinetic, thermal and gravitational energies are estimated as well as the plasma heating. Whenever the ejected plasma has density ge 109 rm cm-3 a continuous supply of heat is required to meet the conditions observed at 1.7 R_sun. Moreover heating mechanisms which release energy gradually during the outward motion of the plasma seem to be more appropriate of those which dump most of the energy when the plasma is lower down in the corona. Our simulations also indicate that a 3-D self similar expansion does not fit the UVCS observations. Comparisons with some CME models from the dynamical and energetics points of view are discussed. [4441]


2001-03:    Ion Cyclotron Diffusion of Velocity Distributions in the Extended Solar Corona

Cranmer, S. R.

J. Geophys. Res. 106, pp.24937-24954

The Ultraviolet Coronagraph Spectrometer UVCS aboard the Solar and Heliospheric Observatory SOHO has revealed strong kinetic anisotropies and extremely large perpendicular temperatures of heavy ions in the extended solar corona. These observations have revived interest in the idea that the high-speed solar wind is heated and accelerated by the dissipation of ion cyclotron resonant Alfven waves. This process naturally produces departures from Maxwellian and bi-Maxwellian velocity distributions. Here it is argued that these departures must be taken into account in order to understand the resonant velocity-space diffusion, the wave damping, and the formation of ultraviolet emission lines. Time-dependent ion velocity distributions are computed for a fixed spectrum of waves in a homogeneous plasma, and the moments of the distributions are compared with simple bi-Maxwellian models. The existence of a boundary, in parallel velocity space, between resonance and nonresonance produces an effective saturation of the velocity-space diffusion that bi-Maxwellian models could not predict. The damping of an input wave spectrum is computed for a coronal population of 1000 ion species with the above saturation effect included. For realistic levels of fluctuation power, it is concluded that waves propagating solely from the coronal base would not be able to heat and accelerate the ions that have been observed to exhibit strong energization, and that local wave generation is required. Ultraviolet emission line profiles are computed for the derived non-Maxwellian distributions, and possible unique identifiers of the ion cyclotron resonance mechanism are noted. [4659]


2001-2:    Observations and Models of the Fast and Slow Solar Wind

Marsch, E.

Proc IAU Sym 203, ed. P. Brekke, B. Fleck & J. B. Gurman
Recent Insights into the Physics of the Sun and Heliosphere: Highlights from SOHO and Other Space Missions

There are two major types of solar wind. The steady fast wind originates on open magnetic field lines in coronal holes, which may last for many solar rotations. In contrast, the unsteady slow wind is coming from the bulk or boundary layer of streamers, which are mostly magnetically closed and open up only temporarily. Many observations of the solar wind have in the past been made, e.g., in situ by Helios and Ulysses and remotely by SOHO. Correspondingly, many models for the fast and slow wind have been developed to different levels of sophistication. The majority of the models is concerned with the fast wind. Essential properties of fast streams can be reproduced by 1-D multi-fluid models involving broad-band waves. Yet, the integration of the fluid equations must start low in the corona in the magnetic funnels at transition region level. Also, 3-D MHD models have recently been developed. Owing to its time-variable nature, no robust understanding of the slow wind exist. Apparently, its acceleration starts only beyond two solar radii. Key empirical constraints, which are imposed on the models by the Helios (near-Sun, in-ecliptic) and Ulysses (high-latitude) interplanetary measurements and by the SOHO plasma-spectroscopy results, are discussed with respect to the fluid as well as kinetic properties of the wind. Selected results from modelling and observations are presented and discussed. [4410]


2001-01:    Element Abundance in Different Corona Streamers

Parenti, S., B. I. J. Bromage, G. Poletti, G. Noci, J. C. Raymond & G. E. Bromage

Proc. IAU Symp. 203, pp.413-415, ed. P. Brekke, B. Fleck, & J. Gurman
Recent Insights into the Physics of the Sun and Heliosphere: Highlights from SOHO and other Space Missions,

Element abundance in equatorial and mid-latitude streamers have been derived from data taken by SOHO/CDS and UVCS experiments. Observations were made at 1.1, 1.5, 1.6 Rodot, to allow us to check a possible variation of elemental composition with altitude. Part of the CDS data were taken at the border of the south Corona Hole, so that the variation of coronal composition at the streamer edge is investigated. UVCS spectra have been acquired using three different grating positions to cover lines from low and high First Ionization Potential. Absolute abundance of Oxygen and Iron have been determined via the evaluation of the radiat ive and collisional components of the H-Lyman β and O VI (1032 Å) lines and of th e intensities of lines from Fe X-XIII-XV-XVIII. Abundance of Fe, Si, Al, Mg were estimated using the Differential Emission Measure technique. This method gives us information also on the plasma electron temperature (Te), which is compared with temperatures estimates from line ratio technique. The variability of (Te) and of abundances in the observed streamers are discussed and compared with estimates from the literature. [4660]