Physics – Plasma Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsm53b1393m&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #SM53B-1393
Physics
Plasma Physics
[7836] Space Plasma Physics / Mhd Waves And Instabilities
Scientific paper
It is generally accepted that some of the main sources of coherent ULF waves at high (polar) latitudes are from either compressional waves entering the magnetosphere from the solar wind, or from boundary instabilities such as the Kelvin-Helmholtz instability or over-reflection modes. While these sources may couple energy to field line resonances in the Pc5 range, the origin of higher frequency Pc3 signals, which are frequently seen at the ground, is not clear. We performed a detailed study of the properties and propagation characteristics of Pc3-4 events seen at local daytime with high coherence across 56-76 deg magnetic latitude. Most of these events seem to originate from waves generated in the upstream solar wind but appeared to propagate across the ground poleward and westward. About 24-30% of the events are higher harmonics of field line resonances. The observations suggest the a mechanism involving mode coupling and field-guided propagation, in which fast mode waves in the Pc3-4 range entering near the subsolar point propagate Earthward along the Tamao minimum travel time path, and due to the inhomogeneity of the magnetosphere couple to the field-guided Alfven mode. At certain latitudes, standing oscillations are established at harmonics of the local resonant frequency, while at other latitudes travelling waves convey energy to low altitudes. The expected L dependence of wave power and travel time agree well with observed amplitude and phase profiles.
Howard Tim A.
Menk Frederick W.
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