On the source of Pc1-2 waves in the plasma mantle

Abstract

[1] Several ground-based observational studies have reported unstructured Pc1-2 waves near and poleward of the dayside cusp. Dyrud et al. (1997) were the first to suggest that these waves originated in the high-altitude plasma mantle. We report here on simultaneous field and particle observations from the Polar satellite and from ground magnetometer data that confirm the presence of these "mantle" waves and characterize the ion distributions associated with them. Unstructured Pc1-2 waves were found to occur during 45% of the available passes of Polar across the mantle during the interval March-May 2000, during which time the orbit of Polar was near local noon as it crossed these latitudes. Poynting vector calculations using Polar electric and magnetic field data show a mixture of upward and downward energy flux in the cusp but consistently downward flux when mantle waves are observed. This is consistent with earlier observations that ground-based magnetometers can detect such mantle waves but not the more intense wave activity observed by spacecraft in the middle- and low-altitude cusp. In situ particle observations from the Hydra instrument appear to confirm the generation mechanism suggested by Dyrud et al. that cusp particles mirroring at low altitudes and then traveling outward in the plasma mantle poleward of the cusp are the source of the free energy for these waves. Quantitative estimates of ion cyclotron instability of the observed ion distributions suggest they are stable at the ≤9 RE altitudes traversed by Polar but may be reasonably inferred to be unstable in the exterior mantle, a location consistent with their observed frequency. Copyright 2005 by the American Geophysical Union.