Height structure of thermal hard x-ray sources on the sun

The height distribution of hard X-ray bremsstrahlung is predicted for dissipative thermal models, involving rapid heating of many small (tearing mode) islands near the top of a magnetic arch. Emission at low energies () originates mainly at high altitudes in the heated kernels themselves while high energy emission comes from the Maxwellian tail electrons escaping to the chromosphere.For a power-law distribution of kernel production temperatures, the ratio of high to low altitude emissions should vary as ^–2. Recent stereo occultation results (Kane et al., 1979) are consistent with this prediction for a typical size of primary dissipation kernel 10 km/(n/10¹¹) for kernel density n(cm^–3). The prediction should also prove a useful diagnostic for SMM data.However the small kernel size required to explain the weakness of the coronal emission demands the heating of 8 × 10³⁶ electrons per second to above 5 keV temperatures. This is comparable to the acceleration rate above 5 keV needed in a thick target model, so that the thermal model has little energetic advantage in this event.