2000 I.A.U.
1997 I.A.U.

UVCS/SOHO ABSTRACTS
I.A.U. MEETINGS

 


IAU General Assembly, Manchester 7-18 August 2000

Ultraviolet Spectroscopy versus White Light observations of Coronal Mass Ejections

A. Ciaravella et al.

The UltraViolet Coronagraph Spectrometer observed several tens of CMEs providing new insights into the classical white light scenario. Temperature, density, abundances, ionization stage, velocity along the line of sight can be determined. On the other side, the different perspectives provided by images in the UVCS spectral lines and LASCO white light can reveal the three dimensional velocity structure of a CME and constrain the magnetic topology. Moreover, the UVCS observations, higher in the corona ($\ge$ 1.5 Rsun), can be also compared to the early stage of a CME, detected by EIT, to investigate the temporal evolution of the ejected plasma and put constraints on the heating rate required to match the physical conditions obtained from the UVCS spectra. We present a CME observed on Feb 12, 2000 by UVCS and LASCO along with the preliminary results of the analysis.


Comparison of Empirical Models for Polar and Equatorial Coronal Holes

M. P. Miralles et al

A large equatorial coronal hole was observed with UVCS during 12 - 14 November 1999. Observations in H~I Ly$\alpha$ and O~VI 103.2 and 103.7 nm provided spectroscopic diagnostics of proton and O$^{5+}$ velocity distributions and outflow velocities. Values of electron density derived from the UVCS visible light channel will be presented. The resulting empirical model will be compared to an empirical model for a polar coronal hole observed near solar minimum. The several obvious differences in the two structures may be associated with the different magnetic field configurations and flux tube expansion factors. \\ \noindent (Co-authors: {\it S. R. Cranmer, A. V. Panasyuk, M. Romoli, R. Suleiman, and J. L. Kohl}.)


Element Abundance in Different Corona Streamers

S. Parenti et al.

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 $R_\odot$, 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 $\beta$ and O VI (1032 $\AA$) 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 ($T_e$), which is compared with temperatures estimates from line ratio technique. The variability of ($T_e$) and of abundances in the observed streamers are discussed and compared with estimates from the literature.


A semi-empirical model for a solar coronal hole

L. Zangrilli et al.

On the basis of UVCS observations of the Lyman alpha and ${\rm O~VI}$ doublet lines at $1031.9\AA$ and $1037.6\AA$, we developed a semi-empirical model for a solar coronal hole observed on 5-11 August 1996. Making use of the collisional part of the ${\rm O~VI}$ line at $1037.6\AA$ we estimated the spatial distribution of the electron density for the North-East sector of the solar corona, at heliocentric distances between 1.5 and $3.0~{\rm R\odot}$. Assuming a magnetic field geometry, from the mass flux conservation and the derived electron densities, we determined the expansion velocity of the solar wind. Kinetic temperatures are given by the profiles of the Lyman alpha and ${\rm O~VI}$ lines, and electron temperatures have been adopted a priori from values given in the literature. We tested the reliability of our results by comparing the observed ${\rm Lyman \alpha}$ emission with that predicted by our model. We found that, in agreement with observations, there is no sensible variation of the ${\rm Lyman \alpha}$ emission with latitude in the coronal hole, at least within the heliocentric distances between 1.5 and $3.0~ {\rm R\odot}$. The model has also been used to derive the ${\rm O~VI}$ line intensities. {\noindent Co-authors: {\it G. Poletto, P. Nicolosi, G. Noci}}


Variation of H I Ly-alpha and O VI 103.2 nm in the Solar Corona from 1996 to 2000

P. L. Smith et al.

UVCS/SOHO measurements of H I Ly-$\alpha$ and O VI 103.2 nm intensities in the solar corona have been made from 1996 to the present, thus spanning the rising phase of cycle 23. At solar minimum, the corona consisted of large coronal holes at the poles and quiescent streamers at the equator. During the ascending phase of the cycle, the corona presented high latitude streamers. Finally, recent observations as the Sun approached solar maximum have shown the presence of coronal holes at the equator and streamers at the poles. Our observations provide descriptions of the various coronal structures that existed over the rising phase of the solar cycle. We compare spectral line intensities of quiescent equatorial streamers that occurred at solar minimum to those of high latitude and polar streamers observed toward solar maximum. We also compare spectral line intensities of solar minimum polar coronal holes to those of equatorial coronal holes present at solar maximum. We discuss how these results are related to the plasma properties. \par \noindent (Co-authors: {\it M. P. Miralles, A. Panasyuk, L. Strachan, L. D. Gardner, R. Suleiman, S. R. Cranmer, M. Romoli, J. L. Kohl}.)


IAU General Assembly, Kyoto, Japan, August 1997

Observations of plume and interplume regions with UVCS/SOHO

G. Poletto and the UVCS team

to be presented at "Physics of the Sun and Heliosphere in the Era of Space Probes - Scientific Highlights of Soho, Ulysses, and Yohkoh", I.A.U. General Assembly, Kyoto, 26-27 August, 1997,

Observations of plumes and interplume regions acquired with SOHO-UVCS at heliocentric heights between 1.5 and 2.3 R_sun are preeented. Analysis of this data allow us to derive information on the beahvior of plasma in the structured and unstructured corona. we present here some examples of the intensity vs. height profile in Ly-alpha and OVI lines, possibly ascribable to Doppler dimming effects, and give an ewstimate of coronal densities and flow speeds both in plumes and interplumes. We also compare our results with simulations from available models.