Solar Flares and Concurrent Phenomena in the Solar Atmosphere

Abstract

The observation of solar flares in visual light is mostly made in the light of hydrogen-α, which shows only a small part of the interesting phenomena which accompany the flare. We discuss observations of coronal and prominence phenomena accompanying the flares, and their connection with non-optical phenomena. At the moment of onset of a large flare a number of phenomena occur. These include (1) a shock wave, (2) a radio burst both in decimeter and later in decameter frequencies, (3) ionospheric fade-outs and absorptions, (4) a great increase in the density and temperature of the corona and (5) a brightening of an area of the surface as observed in hydrogen-alpha (Balmer-α). These phenomena, particularly (2), (3), and (4), are shown to be connected with the production above the flare of a very hot coronal region, with a temperature about 4 million degrees, and with a high energy non-thermal component with fast electrons with mean energies around 2 keV. The density of the coronal condensation is between 3 × 10¹⁰ and 3 × 10¹¹ hydrogen atoms per cubic centimeter. A little after the flare cosmic rays and low energy protons are noted at the earth in some cases. A day or two later, the particles which produce a geomagnetic storm arrive. These various effects are discussed, and several flares are examined in detail. We see that before attempting to understand the reasons for the occurrence of flares, or to predict them, it is important to find out just exactly what a solar flare is.

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