Physical Structure of a Coronal Streamer in the Closed‐Field Region as Observed from UVCS/ SOHO and SXT/ Yohkoh

Abstract

We analyze a coronal helmet streamer observed on 1996 July 25 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 R_ȯ from SXT/Yohkoh observations. At this height, the streamer temperature is about log T (K) = 6.28 +/- 0.05, and the electron density is about log n_e(cm^-3) = 8.09 +/- 0.26, while at 1.5 R_ȯ a temperature of log T (K) = 6.2 and a 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 R_ȯ. 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 and gravitational settling effects. We then 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 1996 July 22 to July 27, the physical properties of the streamer are nearly constant. We conclude that we may be observing the same loop system over 6 days.