Other
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufmsa51a0768m&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #SA51A-0768
Other
2431 Ionosphere/Magnetosphere Interactions (2736), 2471 Plasma Waves And Instabilities, 2481 Topside Ionosphere
Scientific paper
The conventional models of SAR arcs' glow usually employ a Maxwellian distribution of thermal electrons with the electron temperature elevated due to, presumably, electron heat flux from the plasmasphere. It turns out that these basic assumptions are not always valid. Indeed, classical thermal conduction used in these models is physically unreal due to large electron mean free path in the topside ionosphere. As a result, the distribution function becomes sharply anisotropic in the tail energy region ("thermal runaway"). Runaway electrons carry heat by convection. The convection term exceeds the thermal conduction at altitudes above 1000 km even in moderately perturbed conditions. Furthermore, steep gradients of the electron temperature characteristic of the perturbed topside ionosphere can cause an instability as is found in laser-heated plasma corona. The heat flux instability will additionally modify heat flow (anomalous heat conduction). Furthermore, excitation of vibrational levels of molecular nitrogen may be significant for fast electrons in the energy range of 2-4 eV even in the F-region. As a result, the number of fast (tail) particles responsible for red line excitation decreases compared to Maxwellian and so does the red line excitation rate. This effect is significant when the electron temperature exceeds 2500 K and gets stronger when the electron density decreases. The above conditions are characteristic of the SAR arc events. Thus, it is necessary to account for the kinetic effects in SAR arc models that have to significantly, up to a factor of 4, vary the density and composition of the thermosphere in order to match modeling and observational results. On the other hand, this is valid only if the process of electron heating is purely collisional. If effective ''collisions'' due to resonant interaction of electrons with plasma waves were significant, not only would the EDF be heated more efficiently, but it would also acquire an enhanced electron tail (anomalous Joule heating). Intense ion acoustic waves were observed in some part of the region of SAR arcs. Thus, it is likely that kinetic and turbulent effects dominate different spatial and time intervals in the course of SAR arcs' evolution.
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