[1] We present observations of Pc 1 waves ($0.6 Hz) that occurred shortly after a strong (>20 nPa) compression of Earth's magnetosphere at 1321 UT, 18 March 2002. Intense Pc 1 waves were observed at several high-latitude ground stations in Antarctica and Greenland from 1321 UT to beyond 1445 UT. Two wave bursts were recorded at the Polar satellite at 1338 and 1343-1344 UT as it passed outbound in the Southern Hemisphere at 1154 local time (SM magnetic latitude of À22°and near L = 7.5) in good magnetic conjunction with the Antarctic. The pressure increase created a significant population of protons between a few hundred eV and several keV, whose fluxes were mostly perpendicular to B. These protons seem to have replaced the quiescent stream of protons (presumably convected from the plasma sheet) that existed before this increase. There was also a nearly two-order-of-magnitude increase in the population of thermal/suprathermal (0.32-410 eV) protons. The generation of ion cyclotron waves is expected to limit the proton temperature anisotropy A, defined as T ? /T k À 1. The ion cyclotron instability driven by the observed hot ion temperature anisotropy is studied using two models, with and without the presence of cold background plasma. Peaks in the calculated instability as a function of time show excellent agreement with the times of the Polar wave bursts, which were measured a few tens of seconds after maxima in the instability calculation. The time delay is consistent with the propagation time to the spacecraft from a source nearer to the equatorial plane. The hot proton population at Polar appears to be driven back to stability by a sudden increase in very field-aligned protons having energies less than the hot perpendicular population, suggesting a different source for the two populations. These observations confirm the importance of both the energization and/or increase in population of protons transverse to B in the several keV range (possibly betatron acceleration as a result of the pressure pulse), and the presence of greatly increased fluxes of lower energy protons (100s of eV to a few keV), predominantly aligned along B, in determining whether the particle population is unstable at a given time.Citation: Arnoldy, R. L., et al. (2005), Pc 1 waves and associated unstable distributions of magnetospheric protons observed during a solar wind pressure pulse,

To study the magnetopause on both MHD and kinetic scales, we have analyzed two Time History of Events and Macroscale Interactions during Substorms/Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon' s Interaction with the Sun magnetopause crossings under steady slow-solar wind and minimum magnetic shear conditions. These events approximate a ground state of the magnetospheric boundary with minimum influences from large-solar wind disturbances and magnetic reconnection. Our observations reveal evidence for the Kelvin-Helmholtz instability, the quasi-periodicity of magnetopause surface waves accompanied by highly asymmetrical plasma signatures between the inbound (from magnetosheath to low-latitude boundary layer (LLBL)) and the outbound (from LLBL to magnetosheath) magnetopause crossings. Stronger plasma and magnetic gradients were observed during the outbound crossings but more gradual and volatile variations at higher frequencies during the inbounds. The scale lengths of the magnetic and plasma gradients were comparable or less than the proton gyroradius. Enhancements of lower hybrid waves occurred at the locations of strong gradients or variations. We interpreted the collocations of the lower hybrid waves and plasma gradients and their variations in terms of (1) lower hybrid instabilities that directly convert solar wind flow energy into lower hybrid waves and other wave modes in the LLBL, or (2) Kelvin-Helmholtz instability and magnetic reconnection which produce the conditions for the lower hybrid instabilities to grow. The rate of ion diffusion across the magnetopause caused by the lower hybrid instability is marginally sufficient to populate the LLBL. The diffusion coefficient of O + is~30 times larger than that of H + . The lower hybrid waves could contribute to the energy dissipation at plasma gradients in magnetopause surface wave structures and limit Kelvin-Helmholtz instability growth further downstream.