UVCS/SOHO ABSTRACTS
ESLAB MEETINGS

ESLAB Symposium, Noordwijk, 22 - 25 September, 1997
Correlated Phenomena at the Sun, in the Heliosphere, and in Geospace

Ultraviolet Observations of Coronal Mass Ejections

A. Ciaravella^1,2, J. Raymond² C. Benna³, S Fineschi², L. Gardner², S. Giordano, A. Modigliani⁴, R. O'Neal², M. Romoli⁴, E. Antonucci, J. Kohl², G. Noci⁴

¹ ESA Space Science Department, ESTEC, Noordwijk, The Netherlands
² Harvad-Smithsonian Center for Astrophysics
³ University of Turin , Italy
⁴ University of Florence, Italy

The Ultraviolet Coronagraph Spectrometer (UVCS) has observed several coronal mass ejections (CME) and, for some of them, the evolution of the ejected plasma has been followed. We present two CME events caused by prominence eruption on December 23 1996 and March 5 1997, respectively. In both cases we were able to detect many spectral lines providing very detailed diagnostics of physical and dynamical characteristics of plasma. The Lyman lines of hydrogen are very bright but also other low temperature lines as C III (977.02 Å), N III (989.79 Å, 991.58 Å), N II (1084.56 Å), N V(1242.80 Å) are very intense as well. Absorption has been detected in the hot coronal line of Si XII (499 Å) probably due to the He I and H I in the cold ejected plasma. Doppler line shifts give an accurate diagnostics of the velocity field along the line of sight.

The Impact of UVCS/SOHO Observations on Models of Ion-Cyclotron Resonance Heating of the Solar Corona

S.R.Cranmer¹, G.B.Field¹, G.Noci² and J.L.Kohl¹

¹ Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138
² Universita di Firenze, I-50125 Firenze, Italy

We examine the compatibility between theoretical models and observations of the temperatures and anisotropic velocity distributions of hydrogen and minor ions in the solar corona. The UVCS instrument on board SOHO has measured hydrogen kinetic temperatures along lines of sight in coronal holes in excess of K, and O⁺⁵ ion kinetic temperatures of at least K. In addition, the velocity distributions in the radial direction (mainly perpendicular to the line of sight) are smaller, possibly implying temperature anisotropies of order for the oxygen ions. These properties can only be understood in terms of a mechanism which heats and accelerates heavier ions more than lighter ones (possibly proportionally to the ion mass to a power higher than unity), and preferentially in directions perpendicular to the magnetic field. We examine various features of plasma heating by the dissipation of high-frequency ion-cyclotron resonance Alfvén waves, which may be the most natural physical mechanism to produce such plasma conditions. We present preliminary quantitative models of the ion motions in polar coronal holes, and show that such models can be used to predict the spectrum of waves required to reproduce the observations. Indeed, the more ionic species that are observed spectroscopically, the greater the extent in frequency space the wave spectrum can be inferred.

Solar Wind Acceleration in the Solar Corona

S.Giordano¹, E.Antonucci², C.Benna¹, J.L.Kohl³, G.Noci⁴, J.Michels³, and
S.Fineschi³

¹ University of Torino, Torino, Italy
² Astronomical Observatory of Torino, Torino, Italy
³ Harvard-Smithsonian Center for Astrophysics, Cambridge, US
⁴ University of Firenze, Firenze, Italy

The ratio of O VI 1037/1032 lines observed with the Ultraviolet Coronagraph Spectrometer (UVCS) onboard SOHO can be used as a diagnostics to determine the solar wind outflow velocity in the extended corona. A study of the OVI ratio measured in the solar corona between 1.5 and 3.8 R on the basis of observations performed during August 19- September 1, 1996, shows that solar wind acceleration is much more rapid at the poles than in the mid-latitude and equatorial regions. At the poles the level corresponding to a velocity of 100 km s^-1 (ratio 0.5) is reached at approximately 2.0 R. At lower latitudes the 100 km s^-1 level is approximately tracing the separation between polar coronal holes, where the high-speed solar wind is expected to flow, and the streamer belt. Since the 100 km s^-1 level is running along the streamer borders, the acceleration of the solar wind is also high in regions between streamers. In the central part of streamers, the outflow velocity of the coronal plasma remains below 100 km s^-1 at least within 3.8 R. The regions at the North and South poles where solar wind accelerates more rapidly also correspond to regions where the UVCS observes enhanced OVI line broadenings, that is, an increase in the width of the line-of-sight velocity distribution, which is probably related to the mechanism of the solar wind acceleration.

Electron Temperature Determination in Coronal Streamers

L. Maccari¹, G. Noci¹, J. Kohl², M. Romoli¹, C. Benna⁴, L. Gardner²,
E. Antonucci³, S. Fineschi², S. Giordano⁴, and A. Modigliani¹

¹ Dipartimento di Astronomia e Scienza dello Spazio, Università di Firenze, Firenze, Italy
² Harvard Smithsonian Center for Astrophysics, Cambridge, MA, USA
³ Osservatorio Astronomico di Torino, Pino Torinese, Italy
⁴ Istituto di Fisica Generale, Università di Torino, Torino, Italy

This paper presents 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 and Ly lines, in order to separate the collisional and radiative components of the latter. Since the emissivity of the radiative component of the Ly 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. The main limits of this method are: (i) the disturbing effect of the integration along the line of sight; (ii) the rather slow dependence of the hydrogen ionization fraction on the electron temperature in the region of interest ( The influence of these factors are discussed.

Absolute Abundances in Streamers from UVCS

J.Raymond, R.Suleiman, A.van Ballegooijen and J.Kohl

Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA

The Ultraviolet Coronagraph Spectrometer on SOHO covers the 940-1350 range as well as the 470-630 range in second order. It has detected coronal emission lines of H, N, O, Mg, Al, Si, S, Ar, Ca, Fe and Ni in coronal streamers. We are able to determine the ionization state and the absolute elemental abundances in the gas. We find a strong First Ionization Potential (FIP) dependence in the abundances. We find an overall depletion relative to hydrogen, reaching a full order of magnitude for oxygen in the center of a quiescent streamer. Here, we extend our earlier work by considering greater heights in the streamer and by estimating the projection effects (the line of sight passing through the streamer edges). We suggest that the FIP fractionation occurs in the chromosphere, but that in addition an overall depletion at large heights in the closed-field region results from gravitational settling. We present numerical results of diffusion models.

Physical Parameters in Plume and Interplume Regions from UVCS Observations

Corti, G., Poletto, G., Romoli, M., Michels, J., Kohl, J.L., Noci, G.

UVCS data acquired in coronal holes between 1.5 and 2.3 $R_{sun}$ have been analyzed with the aim of deriving some information about the physical parameters of the plume and interplume plasma at these heights. To this end, intensity and profiles of Hydrogen Lyman-$\alpha$ and of OVI 1032 and 1037 A lines have been used to infer the profiles of electron density, kinetic temperature and flow speed vs. height. The density profile is in good agreement with previous estimates obtained from white light observations and represents the first determination of densities from UV lines in this range of altitudes; plasma flow speeds range between ~ 50 km/s at 1.5 R_sun and coronal hole area where there is a concentration of plumes. Kinetic temperatures derived from line profiles, in perfect agreement with prior analyses of UVCS observations, show OVI kinetic temperatures to be much higher than H kinetic temperatures. Both increase with heliocentric distance. Interplume regions have about the same speed, lower densities and larger kinetic temperatures than plume regions. These estimates, based on several assumptions which are here discussed, put constraints on solar wind theories. Future developments of this work are also briefly illustrated.

ESA SP-404, 289, 1997

Determination of 3D Coronal Structures from UVCS/SOHO Synoptic Observations

L.Strachan¹, A.V.Panasyuk¹, S.Fineschi¹, L.D.Gardner¹, J.C.Raymond¹,
E.Antonucci², S.Giordano³, M.Romoli⁴, G.Noci⁴, and J.L.Kohl¹

¹ Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., USA
² Osservatorio Astronomico di Torino, Torino, Italy
³ Universita di Torino, Torino, Italy
⁴ Universita di Firenze, Firenze, Italy

Daily synoptic observations made with the SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) are used to create Carrington maps of line-of-sight intensities and spectral line widths for H I Lyman alpha and the O VI 1032/1037 doublet. These 2D maps can then be used as inputs to a tomographic inversion routine to produce 3D models of the corona out to 2.5 R in the polar regions (3.0 R in equatorial regions). The initial results for the determination of densities and velocities for H⁰ and O⁵⁺ as a function of heliographic height, latitude, and longitude will be presented.