ULF waves at very high latitudes

Abstract

Observations in the ULF frequency range (from mHz to Hz) at very high latitudes, at the poleward auroral boundary and beyond, make it possible to characterize the turbulent energy transfer from the solar wind into dayside regions of the magnetosphere and the magnetotail. The regions near the cusp are characterized by elevated levels of ULF wave activity. There was hope that several categories of these waves (Pc1-2, Pc3-4, Pc5) could be used as ground-based indicators of location of the cusp or related regions. However, recent studies have revealed wave sources distinct from the cusp proper, often in the low-latitude boundary layer (LLBL) or slightly deeper in the magnetosphere, and for unstructured Pc1-2 activity, in the plasma mantle. The polar cap, on the other hand, has often been assumed to be a quiet region, with any wave power observed entering only from neighboring regions. Studies using arrays of stations in Antarctica have now shown the presence of ULF phenomena unique to the polar cap in Pc3-4 and Pc5-6 bands. A class of long-period variations (4-20 min) that are coherent throughout the polar cap and are decoupled from auroral and cusp ULF activity has been revealed. New types of waves and transients in all nominal ULF bands have been found in the magnetotail, but their ground images have not been thoroughly examined yet. The high-latitude regions of Earth's magnetosphere, both the cusp/LLBL/mantle and polar cap, are thus even more complex than we had thought, generating ULF waves in regions and by processes that we had not anticipated and do not yet fully understand.