Modeling Solar Flare Conduction Fronts. II. Inhomogeneous Plasmas and Ambipolar Electric Fields

Details Modeling Solar Flare Conduction Fronts. II. Inhomogeneous Plasmas and Ambipolar Electric Fields Modeling Solar Flare Conduction Fronts. II. Inhomogeneous Plasmas and Ambipolar Electric Fields

Nov 1990

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990apj...364..302m&link_type=abstract

Astrophysical Journal v.364, p.302

Astronomy and Astrophysics

Astronomy

7

Hydromagnetics, Shock Waves, Sun: Corona, Sun: Flares, Sun: Radio Radiation, Sun: X-Rays

Scientific paper

The expansion of superheated electron populations plays a fundamental role in understanding thermal processes during solar flares. For a homogeneous plasma, the electrons can free-stream away from the source region, resulting in the rapid cooling of the source. We show that the particle dynamics changes dramatically if the heated plasma is at low altitudes and expands upward into the more tenuous plasma at higher altitudes. Two important applications are the radio-frequency heating of the corona ("coronal heating case") and the collisional heating of the chromosphere by precipitating energetic electrons ("chromospheric heating case"). In both cases, the overlying plasma has a density that is too low to supply a balancing return current to the expanding hot electrons. As a result, an ambipolar electric field develops that tends to confine the energetic electrons behind a front that propagates outward at about the sound speed.
In the coronal heating case, the thermal velocity of the ambient ions is assumed to be much less than the sound speed of the expanding hot electrons; as a result, an electrostatic shock and an associated pressure wave develop and propagate through the system. The ambipolar electric field in the shock front strongly accelerates the ions outward in bulk. Both light and heavy ions are then heated by ion-acoustic and ion-ion streaming instabilities.
In the chromospheric heating case, the ambient ion thermal velocity is comparable to the sound speed, and thus ions can propagate through the ambipolar electric field with little deceleration. As a result, a shock does not develop, and ion heating by ion-sound and ion-ion waves is suppressed. We compare the ion distributions from the two cases with those inferred from soft X-ray line emissions.

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