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

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.