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M.L. Goodman (Catholic University and NASA/Goddard)
A mechanism for chromospheric network heating, and a necessary and sufficient condition for its onset are presented. The heating mechanism consists of resistive dissipation of proton Pedersen currents, which flow orthogonal to the magnetic field in weakly ionized chromospheric plasma. The currents are driven by a convection electric field generated by velocity oscillations of linear magnetoacoustic waves with periods greater than about 5 minutes. Heating occurs in thin magnetic flux tubes, and begins lower in the chromosphere in flux tubes with higher photospheric field strength. The lower chromosphere, which emits most of the chromospheric radiation, is heated by flux tubes with photospheric field strengths in the range of 700 - 1500 G. A typical diameter and field strength for a heated flux tube in the lower chromosphere are 10 km and 100 G. The condition for the onset of this heating mechanism is that the proton cyclotron frequency equal the proton-hydrogen collision frequency. When this occurs, heating by dissipation of heavy ion Pedersen currents begins to raise the gas temperature, and hydrogen ionization increases exponentially with temperature according to the Saha equation. The Pedersen current density rapidly increases as the proton number density rapidly reaches and exceeds the heavy ion number density, resulting in an increase in heating rate by an order of magnitude over a height increase of only one pressure scale height. During this process the protons rapidly dominate the Pedersen current. This heating mechanism and condition for its onset apply to all solar type stars: stars with a convection zone and associated dynamo action causing the formation of photospheric convection cells with strong magnetic field concentrations at their boundaries.
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