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Statistical study of emissions near fp and 2fp in the dayside and nightside auroral region and polar cap
Journal article   Open access   Peer reviewed

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

J. D Menietti, I. H Cairns, C. W Piker and T. F Averkamp
Journal of geophysical research. Space physics, Vol.103(A7), pp.14925-14938
1998
DOI: 10.1029/98JA00887
url
https://doi.org/10.1029/98JA00887View
Published (Version of record) Open Access

Abstract

In this study we examine a large database of relatively intense plasma waves observed at frequencies near the plasma frequency ƒp and near 2ƒp (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 2ƒp 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 2ƒp. 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 ƒp/ƒg) the ratio of E/cB can be quite large for the z mode (E/cB ∼ 10) near ƒp, as it can be for the whistler mode near ƒp. 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.
 Earth, ocean, space Electrostatic and electromagnetic waves Exact sciences and technology External geophysics Physics of the magnetosphere

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