Photospheric Source of White-Light Flare Energy

Abstract

Additional heating of the photosphere and lower chromosphere to temperatures of around 10⁴ K with the generation of a flare continuum (white-light flares) has been studied. Additional heating is associated with the dissipation of the ring electric currents that arise in the penumbra of sunspots or in the magnetic-loop footpoints with a pulsed increase in the rate of photospheric convection. A criterion for additional heating, which depends on the photospheric plasma velocity and the vertical component of the magnetic field, is obtained. For the penumbra region with a vertical component of the magnetic field of ${{B}_{z}} = {{10}^{2}}\,{\text{G}}$, heating can occur at a photospheric convection velocity of ${{V}_{r}} \geqslant {{10}^{5}}\,{\text{cm }}\,{{{\text{s}}}^{{ - 1}}}$, which exceeds the typical convection velocity of $\approx$ 3 $~ \times \,{{10}^{4}}\,\,{\text{cm}}\,\,{{{\text{s}}}^{{ - 1}}}$. An increase in the photospheric plasma velocity and radiation growth in the continuum can be associated with an increase in the magnetic-field gradient near the neutral line as magnetic regions with opposite polarities of the vertical component of the magnetic field approach each other. For magnetic loop footpoints, an increase in velocity can be initiated by the development of the Rayleigh–Taylor instability. For the white-light flare on April 24, 1981, which is characterized by known values of the temperature, electron density, density of neutral hydrogen atoms, and observed luminosity, the emissivity in the flare core is estimated, and the thickness of the flare layer on the line of sight is determined.