CRRES electric field study of the radial mode structure of Pi2 pulsations

The radial mode structure of Pi2 pulsations in the inner magnetosphere (L < 7) and its relation to the plasmapause are studied using data acquired by the Combined Release and Radiation Effects Satellite (CRRES) between August 1990 and September 1991. Low‐latitude Pi2 pulsations detected on the ground at Kakioka (L = 1.25) are used as the reference signal to determine the relative amplitude and phase of the electric field oscillations detected at CRRES. The plasmapause is identified using electron density inferred from the plasma wave spectra observed on CRRES. Pi2 events at CRRES are defined to be 10‐min intervals of high coherence between oscillations in the Kakioka horizontal northward magnetic field (H) and CRRES dusk‐to‐dawn electric field (Eφ) components within the Pi2 band (6–25 mHz). The Eφ component represents the poloidal oscillation of the geomagnetic field lines for satellite local times near midnight. Fifty‐five high‐coherence Eφ‐H Pi2 events occurred when both CRRES and Kakioka were within 3 hours of magnetic midnight. For these events CRRES was on L shells ranging from 2 to 6.5 and was either in the plasmasphere or in the close vicinity of the plasmapause, providing evidence for the plasmaspheric origin of low‐latitude Pi2 pulsations. The amplitude of Eφ varied significantly but there is an indication of a maximum near L = 4. The phase of Eφ (relative to Kakioka H) remained near −90° at all distances. These properties are consistent with the radial structure of the fundamental cavity mode oscillations confined in the plasmasphere. For some events observed at L > 3.5 it was also possible to determine the amplitude and phase of the compressional component Bz at CRRES. In contrast to Eφ, the phase of Bz (relative to H) was clustered both at ∼180° and ∼0 for events occurring near the plasmapause. This observation still is consistent with the cavity mode according to a numerical simulation using a dipole magnetic field and a realistic plasmapause plasma density structure, which indicates that the node of Bz is located near the plasmapause. Depending on the satellite position relative to the node, the phase can be either –180° or 0. A negative correlation is found between the Pi2 frequency and the distance of the plasmapause, which is additional support for the cavity mode origin of low‐latitude Pi2 pulsations.