Characteristic Properties of Chromospheric Flares

Abstract

Characteristic properties of flares, derived from visual and photographic records extending over nine years, are briefly summarized. While the areas of the flare filaments provide a rough index of importance , the development curves , obtained by plotting effective line-widths of the Hα emission against time, provide a simple measure of the intensity changes and give a fuller quantitative basis for the classification of flares. The geophysical (ultra-violet) effects of flares can be closely correlated with the line-width curves. They also indicate that the peak of the flare emission is sometimes of such short duration and high intensity that it may correctly be described as a flash of radiation, a result which lends strong support to the discharge mechanism of flare formation advocated by Giovanelli. Flares occur close to sunspots (within a radius of 10 5 km.) and most frequently between the two main spots of a bi-polar group. The emission patches are relatively stationary both in the horizontal and vertical directions, but evidence is given for variability of height as between one flare and another. High-velocity absorption filaments are ejected from more than 50 per cent of the Class 2 and Class 3 flares near the times of their maximum brightness. A detailed study of this process has been carried out visually in Hα light and by photographic photometry from calibrated spectrograms. Following Newton, the flare filaments are identified with the larger limb surges (discovered at McMath-Hulbert Observatory and well shown in the Menzel films) and it is proved that they must differ fundamentally from other prominence types. From a discussion of emission line intensities it is concluded that flares of comparable Hα intensity produce similar, though by no means identical, spectra. The He I lines of helium and the D lines of sodium exhibit the most outstanding anomalies, reasons for which are suggested. Line profiles of Hα during flares have been plotted, showing that a great expansion of the emission line wings occurs (possibly as a result of Stark effect) as soon as the central intensity surpasses approximately 150 per cent of the continuous spectrum. These profiles also confirm the absence of appreciable Doppler shifts in the emission lines, a result which is consistent with all the earlier visual measures made with the spectrohelioscope. An analysis of over 500 measures of the wave-lengths of the red and blue wings of the Hα emission line, made with two different instruments at Sherborne and Edinburgh respectively, confirm the existence of an asymmetry (first described in 1943) in the widened emission wings. Absorption of light from the blue wing is considered to be the cause of asymmetry; the hypothesis is advanced that this arises from H atoms which have been previously excited to the 2-quantum state in sufficient numbers by the enhanced Lα radiation from the flare, the atoms being thereby accelerated radially away from the flare region near the time of peak emission intensity. This effect (clearly distinguishable from the flare surges which return to the chromosphere) in all probability represents the initial departure from the Sun of the aurora-generating particles at a time before they have reached their terminal velocity.