ALBERT

Description: [1]  From a survey of Polar plasma waves conducted over the interval 1 April, 1996 to 4 April, 1997 (during solar minimum) at and inside the plasmasphere, magnetosonic waves were detected at all local times with a slight preference of occurrence in the midnight-postmidnight sector at L = 3 to 4. The waves occurred primarily during heightened geomagnetic (AE) activity. Wave occurrence (and intensities) peaked at ~ ±5° of the magnetic equator, with half-maxima at ~ ±10°. For other wave events, magnetosonic waves were also detected as far from the equator as +20° and -60° MLAT, but at lower intensities. An extreme magnetosonic wave intensity event of amplitude B w  = ~ ± 1 nT and E w  = ~ ± 25 mV/m was detected during the survey period. The event occurred near local midnight (0022 MLT), at the magnetic equator (MLAT = -0.5°), at the plasmapause (L = 3.5), and during an intense substorm/convection event (AE = 624 nT; SYM-H = -33 nT). If more stringent requirements (| MLAT| ≤ 5° and AE 〉 300 nT) are imposed, the wave occurrence rate approaches ~50% for the 23 to 00 MLT bin at L = 3 to 4. This strong local time anisotropy in the location of magnetosonic wave occurrence rate supports the idea of generation by protons injected from the plasmasheet into the midnight sector magnetosphere by substorm electric fields. Magnetosonic waves were also detected near late morning (1031 MLT) during relative geomagnetic quiet (low AE). We mention that one possible generation mechanism is a recovering/expanding plasmasphere engulfing preexisting energetic ions, which in turn leads to an ion instability. The wave magnetic component oscillations are aligned along B 0 , the ambient magnetic field direction, and the electric component oscillations are orthogonal to B 0 , indicating linear polarization. The magnetosonic wave amplitudes decreased at locations further from the magnetic equator, while transverse whistler mode wave amplitudes increased. We argue that intense magnetosonic waves are always present somewhere in the magnetosphere during strong substorm/convection events. We thus suggest that modelers use dynamic particle tracing codes and the maximum (rather than average) wave amplitudes to simulate wave-particle interactions.