Other
Scientific paper
Apr 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000jgr...105.7441p&link_type=abstract
Journal of Geophysical Research, Volume 105, Issue A4, p. 7441-7458
Other
15
Ionosphere: Auroral Ionosphere, Ionosphere: Electric Fields, Ionosphere: Plasma Waves And Instabilities, Ionosphere: Wave/Particle Interactions
Scientific paper
It is commonly believed that Alfvénic waves observed in the ionosphere originated at trans ionospheric altitudes. However, recent observations of waves localized over skin depth scales, which are prone to Landau damping, and upward going Poynting flux suggest an ionospheric source. This theoretical study establishes the generation of electromagnetic waves at subcyclotron frequencies by localized static electric fields of the type commonly observed in the auroral ionosphere. The problem is formulated in terms of an eigenvalue system of equations which can describe all cold plasma normal modes, ion acoustic waves, and the various mode couplings induced by nonuniform electric fields. It is found that the velocity shear associated with the background electric field can significantly affect the observable properties of Alfvén waves. In particular, Alfvén waves can be destabilized when the magnitude of the velocity shear frequency exceeds the ion cyclotron frequency. Velocity shear can also significantly modify the ratio of E/B for these waves. The analysis further reveals electromagnetic Kelvin-Helmholtz instabilities, which are non-Alfvénic, with lower velocity shear thresholds. We model conditions encountered by several rockets and satellites, for example, FAST, Freja, AMICIST, AT2, to analyze wave properties and find that for typical ionospheric parameters the Kelvin-Helmholtz instabilities can generate electromagnetic waves with broadbanded spectra and other physical characteristics similar to observations. It is also shown that these waves are capable of resonant interaction with electrons over localized regions and hence may be important to understanding the generation of suprathermal electron bursts.
Ganguli Gurudas
Penano Joseph R.
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