Implications of NRL/ATM Solar Flare Observations on Flare Theories

Abstract

During the Skylab mission, many solar flares were observed with the NRL XUV spectroheliogram in the wavelength region from 150 to 650 A. Because of its high spatial resolution ( ~ 2″) the three-dimensional structures of the flare emission regions characterized by temperatures from 104 K to 20 × 106 K can be resolved. Thus the spatial relationship between the relatively cool plasma and the hot plasma components of a flare, and the associated magnetic field structure can be inferred. For example, the Fe XXIV plasma (T ≃ 20 × 106 K) observed near the soft X-ray maximum during the 2B (M2) disk flare of 1973, July 15 is elongated perpendicular to the neutral line shown on the photospheric magnetogram, while lines of lower ionization temperatures such as He ii–Fe xvi show the familiar double ribbon structures on either side of the neutral line (Widing and Cheng, 1974). The disk flare of 1973, September 5 shows the same spatial structures. The flare was observed at 18 31 UT at the X-ray maximum, and shows that the hot Fe xxiv cloud is located spatially between the ribbons of the He II, Fe xiv–Fe xvi emissions. As the flare cools at 18 37 UT, the Fe xxiv cloud disappears while the gap is filled with emissions from ions of lower ionization potentials, and exhibit loop structure. Many other flares also show similar spatial distributions in the XUV emissions. The linear dimensions of the Fe xxiv plasmas as measured from the photospheric plates range from 7000 km to 14000 km, and the heights of associated loops range from 10000 to 30000 km. From the spatial distributions of the XUV emissions of the many flares, and the comparisons with the magnetograms, we concluded that the magnetic field configuration for the flares we observed are simple bipolar magnetic flux loops with the hot flare plasma located near the top and the cool plasma component on the footprints of the loop.