A method to determine the emission measure and hence the effective extension of solar flares is described. The conditions for the Balmer continuum to be visible in disk flares is also considered. The concept of the effective mass is introduced and it is shown that this mass is a little less than that of a plasma filling the whole spectroscopic volume and including thin and dense filaments and the intervals between them as well. In this connection it is necessary to have high energy electron fluxes. It is assumed that these electrons maintian a high temperature in the interval volume; being a source of powerful X-rays, such hot intervals can heat cool filaments and they in their turn reradiate in the visible and ultraviolet regions of the flare spectrum. Taking one flare as an example, we give curves of the emission measure, effective mass and numbers of hydrogen atoms in the second quantum state versus time. These curves agree qualitatively with the light curve characteristics for most of the flares.
According to modern concepts solar flares are structural features consisting of many thin and dense filaments. One of the methods to give evidence of such filamentary structures is the study of Balmer continuum in flares. This idea was first proposed by Svestka (1963). This problem was also studied by Kurochka (1970). It became necessary to explain the physics of the formation of such thin and dense filaments. A first attempt was made by Guseinov et al. (1971). Unlike selected limb flares, in disk flares the Balmer continuum has never been observed. Therefore one cannot estimate its emission measure nZ~L and hence the effective extension L for a known electron density n e. From the non-existence of the Balmer continuum we can only determine the upper limit of the emission measure and thus with a known ne value only the upper limit of the effective extension L.
In our method for determining the emission measure the ionization-recombination equilibrium is taken into account for every individual level (Polupan and Yakovkin, 1965). We also assume that the Balmer continuum may be discovered in disk flares only when its intensity Iz2 amounts to >~2% of the intensRy of the continuous spectrum of the solar disk i.e. I~2 ~> 5.4 x 1012 erg cm-3 s-1 ster-1.
1. The Emission Measure
First of all we shall consider the balance between photorecombination and photoionization. Neglecting induced radiation we determine the photorecombination coefficient as follows: