ELECTRON ACCELERATION AND HEATING IN SOLAR-FLARES - INTERPRETATION OF H-ALPHA SIGNATURES

Vector magnetogram, Halpha, and hard X-ray observations of flares are reviewed which show that nonthermal electron signatures in Halpha are never cospatial with regions of maximum current density for the small number of flares analyzed, but lie to the sides of these regions. By considering electron acceleration and transport requirements, four conditions are found that must be fulfilled to observe nonthermal electron signatures in Halpha: (1) The plasma beta beta less-than-or-equal-to 0.3 in the acceleration region. (2) The energy flux of electrons above 20 keV is greater than 10(10) erg cm-2 s-1. (3) The column density N less-than-or-equal-to 10(20) cm-2 between the electron source and the chromosphere. (4) The coronal pressure in the flux tube connecting to the Halpha layer p less-than-or-equal-to 100 dyne cm-2. Condition 2 can be most easily met in the initial stages of flares. In contrast, the only condition for a high-pressure Halpha signature is p greater-than-or-equal-to 1000 dyne cm-2. Which is most easily met in a region of maximum current density or heating and far enough into the flare for significant heating to have occurred. Thus, high-pressure signatures should be expected to occur more frequently than nonthermal electron signatures and to occur generally later in time.