CRRES observation of Pi2 pulsations: Wave mode inside and outside the plasmasphere

Abstract

The relationship between the plasma density and the mode of Pi2‐band (40–150 s) waves in the magnetosphere is studied using measurements made from the Combined Release and Radiation Effects Satellite (CRRES). We focus on wave events that occurred from 1100 to 1500 UT on February 17, 1991, on a single CRRES orbit. During this interval the low‐latitude ground station Kakioka (L = 1.2) observed a series of Pi2 pulsations within 2 hours of the local time of CRRES. When CRRES was in the plasmasphere, the poloidal components (Ey, Bx, Bz) at the spacecraft had high coherence with the Pi2 pulsations seen at Kakioka, where Ey, Bx, and Bz are, respectively, the azimuthal component of the electric field and the radial and compressional components of the magnetic field. This relationship lasted until the spacecraft completely moved into the plasmatrough. Waves were present in the plasmatrough, but they did not maintain high coherence with the ground Pi2. For a plasmaspheric event, Ey and Bz oscillated nearly in quadrature, in support of a radially standing wave mode. These results confirm previous AMPTE/CCE magnetic field studies that suggested that fast‐mode waves in the inner magnetosphere are the source of low‐latitude Pi2 pulsations. In addition, the CRRES results strongly suggest that the outer limit of the “inner magnetosphere” is the plasmapause. The observed wave properties are discussed in light of possible Pi2 mechanisms, including plasmaspheric cavity modes and braking of periodic bursty bulk flows.