Magnetic pulsations from 0.1 to 4.0 Hz and associated plasma properties in the Earth's subsolar magnetosheath and plasma depletion layer

Abstract

Dynamic spectral analysis covering frequencies 0.1 to 4.0 Hz has been applied to analyze Active Magnetospheric Particle Tracer Explorers CCE magnetic field data during periods of magnetospheric compressions when the spacecraft sampled the magnetosheath. Ion (H+, He2+) and electron data are used to identify regions, to monitor the upstream shock geometry, and to evaluate moments of the proton distribution. A total of 14 encounters representing 46 hours of observations are analyzed. For quasi‐perpendicular shock conditions, two distinctive band limited spectral types are found to characterize the majority of magnetosheath observations: (1) purely compressional fluctuations with F < 0.2 FH+ which are attributed to mirror waves (F and FH+ denote the wave frequency and proton gyrofrequency, respectively); and (2) 0.25–1.0 FH+ frequency purely transverse fluctuations that are often left hand polarized from 0.5–1.0 FH+ and are attributed to electromagnetic ion cyclotron (EMIC) waves. The plasma characteristics for these two pulsations are different. Mirror waves are associated with βp>5 and T⊥p/T∥p of 1.5 whereas EMIC waves occur for βp≤1 and T⊥p/T∥p = 2.5 ‐ 5 (the subscript p denotes quantities evaluated for protons). EMIC waves also occur for higher field strength and lower plasma densities than mirror waves, indicative of EMIC wave occurrence in the plasma depletion layer (PDL). The increased T⊥p/T∥p during EMIC waves relative to periods of mirror waves is due to a decrease in T∥p suggesting that the EMIC wave region has suffered a loss of particles in the parallel direction. A compilation of the temporal sequence of spectral structures shows that EMIC waves occur next to the magnetopause while mirror waves occur outside the region of EMIC waves. We conclude that there is a close relationship between EMIC waves in the sheath and the PDL. Both the density depletion and the increase in temperature anisotropy, Ap = T⊥p/T∥p ‐ 1, are attributed to extrusion of plasma along the draped field lines as plasma flows into the PDL. For quasi‐parallel upstream conditions, the magnetic field power spectra often exhibit no band limited structures but only broadband noise. The absence of EMIC wave emissions during quasi‐parallel conditions is consistent with predictions that the PDL does not form for radial IMF conditions.