STATISTICAL STUDY OF EMISSIONS NEAR F(P) AND 2F(P) IN THE DAYSIDE ANDNIGHTSIDE AURORAL REGION AND POLAR-CAP

In this study we examine a large database of relatively intense plasma waves observed at frequencies near the plasma frequency f(p), and nea r 2f(p) (the PF and H components, respectively) by the DE 1 spacecraft . These data are from a period of approximately 2.5 years, correspondi ng to over 2600 orbits, and specifically include observations in the a moral regions and over the polar caps. The data show PF and H emission s 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 observ ed with the Ap and Dst indices. Polarization data available Rom a subs et 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 p asses. It was not possible to discern the polarization for the remaini ng similar to 80 cases. This preponderance of z-mode PF emission is un expected on the basis of previous observations in the amoral regions. The polarization of the H emissions near 2fp was z mode on all II pass es in which definitive measurements were possible. The PF emissions ar e observed to have a magnetic component (typically with E/cB similar t o 20), but no magnetic components were detected for the weak H emissio ns near 2f(p),. We believe that the enhanced PF emissions are sometime s primarily z mode, sometimes primary whistler mode, and sometimes a r elatively equal mixture of whistler and z-mode emission. Cold plasma t heory shows that under certain conditions (nearly field-aligned propag ation for small ratios of f(p)/f(g)) the ratio of E/cB can be quite la rge for the z mode (E/cB similar to 10) near f(p),, as it can be for t he whistler mode near fp. Thus both the z mode and whistler mode can e xplain the E/cB observations in some cases. Our current thinking is th at both the PF and H emissions are produced in the z mode by direct li near instabilities involving streaming nonthermal electrons, probably with significant temperature anisotropies and a beam distribution. Pre liminary examination of plasma wave data from the Polar satellite reve al similar observations of H component waves on a number of polar cap passes at different altitudes from DE I. These data have yet to be ana lyzed in detail but show both similarities to and differences from the DE I observations, perhaps due to the different spacecraft locations.