Plasma properties above coronal active regions inferred from SOHO/UVCS and radio spectrograph observations

Abstract

Information on coronal plasma was inferred from the joint analysis of type  radio bursts and SOHO UltraViolet Coronagraph Spectrometer (UVCS) observations. The data sample comprises 37 metric type  radio bursts observed by ground- based radio spectrographs in 1999, during the rising phase of the solar cycle. The coronal electron densities ne were estimated with the diagnostic provided by UVCS observations of the O  doublet line intensities. Upper limits to the coronal Alfven speed above active regions were inferred from the derived shock speeds by requiring that the disturbances propagate at least faster than the local characteristic speed. Information on the magnetic field strength B and plasma β in the middle corona were finally extracted from the coronal Alfven speed upper limits. A major improvement with respect to previous studies that used type  radio bursts to derive plasma properties, is that density profiles of the pre-shock plasma obtained a few hours before the passage of the shock were used instead of general electron density models taken from literature. Our main result is expressed by the inequality B(r) ≤ (0.6 ± 0.3)(r − 1) −1.2 G, that is valid in the range 1.5 ≤ r ≤ 2.3 solar radii, and is consistent with previous estimates of the magnetic field above coronal active regions.