ELECTROMAGNETIC ION-CYCLOTRON WAVES STIMULATED BY MODEST MAGNETOSPHERIC COMPRESSIONS

The relationship between sudden impulses and enhancements or onset of pc 1 at high-latitude ground stations is well established. It has been proposed that increased bot proton temperature anisotropy resulting from convection during magnetospheric compression is responsible for the enhancement in Pc 1 emissions via generation of electromagnetic ion cyclotron (EMIC) waves in the dayside outer equatorial magnetosphere (e.g., Olson and Lee, 1983). We use AMPTE/CCE magnetic field and particle data to test this suggestion. Compression-induced Pc 1 events are very common in the AMPTE/CCE data set: among isolated sudden (<5 min) magnetic field increases of 10 nT or greater in the 8-16 MLT/L > 6 region, 47% are associated with EMIC wave onset at the satellite. Compressions energize particles via earthward convection, so the association of local Pc 1 onsets with compressions may be either a temporal effect or merely the result of inward transport to the spacecraft of plasma previously unstable to EMIC waves. EMIC waves occur 10-20% of the time in the outer magnetosphere, which is considerably less than the occurrence rate of Pc 1 with compressions, suggesting that convection alone does not account for compression Pc 1 onset. To test whether the plasma is more unstable to EMIC waves after compressions than before, the proton distribution (1-300 keV) measured at the peak of the compression is used together with an image dipole model to calculate the distribution that would result from adiabatic expansion back to the precompression state. Comparison of the EMIC wave growth rates from the measured compression and extrapolated expansion distributions provide an indication of the compression-induced enhancement of EMIC wave activity. Differences in wave growth of several orders of magnitude are found, indicating significant increases in EMIC wave activity. The results suggest that enhancements in dynamic pressure pump the energetic proton distributions in the outer magnetosphere, driving EMIC waves. Waves are expected to be generated most readily close to the magnetopause, and transient pressure pulses may be associated with bursts of EMIC waves, which would be observed on the ground in association with iono-spheric transient signatures.