Statistical study of emissions near <formula>fp and <formula>2fp in the dayside and nightside auroral region and polar cap

Details Statistical study of emissions near <formula>fp and <formula>2fp in the dayside and nightside auroral region and polar cap Statistical study of emissions near <formula>fp and <formula>2fp in the dayside and nightside auroral region and polar cap

Jul 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998jgr...10314925m&link_type=abstract

Journal of Geophysical Research, Volume 103, Issue A7, p. 14925-14938

Physics

10

Magnetospheric Physics: Plasma Waves And Instabilities

Scientific paper

In this study we examine a large database of relatively intense plasma waves observed at frequencies near the plasma frequency fp and near 2fp (the PF and H components, respectively) by the DE 1 spacecraft. These data are from a period of approximately 2.5 years, corresponding to over 2600 orbits, and specifically include observations in the auroral regions and over the polar caps. The data show PF and H emissions during approximately 100 passes, or approximately 4% of passes. The PF and H emissions are observed exclusively at latitudes corresponding to the polar cap and auroral regions, with a clear preponderance for the polar cap. The emissions are observed at all magnetic local times but with a concentration near local midnight. No association is observed with the Ap and Dst indices. Polarization data available from a subset of these passes show that PF emissions have a polarization that is consistent with whistler mode on 10 passes and with the z mode on 25 passes. It was not possible to discern the polarization for the remaining ~80 cases. This preponderance of z-mode PF emission is unexpected on the basis of previous observations in the auroral regions. The polarization of the H emissions near 2fp was z mode on all 11 passes in which definitive measurements were possible. The PF emissions are observed to have a magnetic component (typically with E/cB~20), but no magnetic components were detected for the weak H emissions near 2fp. We believe that the enhanced PF emissions are sometimes primarily z mode, sometimes primary whistler mode, and sometimes a relatively equal mixture of whistler and z-mode emission. Cold plasma theory shows that under certain conditions (nearly field-aligned propagation for small ratios of fp/fg) the ratio of E/cB can be quite large for the z mode (E/cB~10) near fp, as it can be for the whistler mode near fp. Thus both the z mode and whistler mode can explain the E/cB observations in some cases. Our current thinking is that both the PF and H emissions are produced in the z mode by direct linear instabilities involving streaming nonthermal electrons, probably with significant temperature anisotropies and a beam distribution. Preliminary examination of plasma wave data from the Polar satellite reveal similar observations of H component waves on a number of polar cap passes at different altitudes from DE 1. These data have yet to be analyzed in detail but show both similarities to and differences from the DE 1 observations, perhaps due to the different spacecraft locations.

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