Ultraviolet and EUV studies of selected structures in the solar corona

Abstract

New UV and EUV observations of off-limb and upper solar corona made by the UltraViolet Coronagraph Spectrometer (UVCS) and the Coronal Diagnostic Spectrometer (CDS) on SOHO (Solar and Heliospheric Observatory) are presented in this thesis. These data were used to establish the physical properties, such as temperature, density and element abundances, of different structures of the off-limb and upper corona.

For this project, the UVCS team provided UV spectra of the extended solar corona, which included lines of primary interest for the characterization of this region and the solar wind. CDS data consisted of NIS (Normal Incidence Spectrometer) EUV spectra of the low corona. The data included several density-sensitive and temperature-sensitive lines which enabled diagnostic studies to be made.

Absolute element abundances were derived from UVCS data; relative element abundances were derived from CDS data. The Differential Emission Measure (DEM) technique was used (for the first time on UVCS data) as the primary method for abundance study. Electron temperatures along the line of sight were derived applying the line ratio and the DEM techniques to both CDS and UVCS data. From the CDS data, electron densities along the line of sight were derived using the line ratio technique. For the UVCS data, the density was inferred using the 0 VI radiative and collisional components of the line.

One equatorial and two mid-latitude streamers were observed with the two instruments, in the low corona and at about 1.6 R®. Electron density and temperature profiles with the solar distance were derived. They appeared to satisfy the hydrostatic equilibrium condition. The equatorial streamer appeared to be cooler than the mid-latitude one. Moreover, while the former tended to isothermal values already in the low corona, the latter was multithermal at the base and isothermal at 1.6 R®. The derived composition at 1.6 R®. showed a depletion with respect to the photospheric values for almost all the elements. The Fe/O ratio found here was consistent with values found in the solar wind. Moreover, a depletion in element composition with increasing solar distance was also found.

While observing off-limb in the south polar coronal hole, CDS observations of a macrospicule were obtained. For the first time the background emission was isolated from that of the macrospicule itself, and a diagnostic study of both regions was made. The ambient coronal hole appeared to have a density of = 2 x 108 cm -3 that was about half the density found in the streamer base studied in this thesis. Multithermal components were found at the coronal hole base. The macrospicule spectrum indicated chromospheric and transition region temperatures, and fluctuations in density along the feature were registered. In particular, the data showed the presence of an outward-moving cloud of cool material. Its time evolution and velocity were studied. The maximum (initial) outflow velocity was about 80km s-1 , consistent with the outflow velocity measured in interplume regions.

An off-limb hot loop system was observed with CDS at mid-latitude. Its top and base were selected for diagnostic studies. Some newly observed hot lines were identified. The loop system appeared to be composed of multi-thermal plasma. The highest temperature registered was log T=6.4 at the top of the loop. A depletion of element abundances which mainly involves the plasma at the top of the loop was found.

A coronal hole bounded by a loop system was observed with CDS. Electron density appeared up to a factor two higher in the boundary than in the coronal hole, and the loop system appeared multi-thermal. The ambient coronal hole temperature appeared to be enhanced by the proximity of this loop arcade. Abundances were derived in both regions, and the coronal hole boundary showed slight depletions relative to photospheric values.

New coordinated observations with CDS and UVCS were made during one of the SOlO-Ulysses quadratures. Preliminary results regarding temperatures and densities of the core and boundary regions of streamers were obtained from UVCS data.