Electric fields and plasma structure in coronal magnetic loops

Abstract

Solar flare loop spectra are analyzed by means of recently developed density diagnostics to determine if the Stark effect observed in the Balmer emission lines is due to pressure broadening or the presence of macroscopic electric fields. The database covers five postflare loops and three active prominences. The diagnostics include the density sensitivity of the intensity ratios of strong Na I D, Mg I b, and the Mg I lines relative to the Sr II lines. A total of 43 spectra were obtained in the 3500-6000 A interval, with most lines falling in the 3600-4500 A region. A prism at the entrance to the instrument permitted extracting additional data on the structure of Stark-broadener Balmer lines. A dc electric field was detected, with an energy level of 100 V/cm, oriented transverse to the loop magnetic field vector. Electron densities of 1-10 trillion/cu cm were estimated for the cool structures within loops and 100 billion/cu cm in active prominences. The densities indicated that a pressure deficit may be present in the loop structures in the corona, suggesting an inward directed gas pressure force in cool loop structures.