CRRES satellite observations associated with low‐latitude Pi2 pulsations

Abstract

We studied the electric and magnetic field variations and their relation to the ambient plasma density observed from the Combined Release and Radiation Effects Satellite (CRRES) during two Pi2 events observed on the ground at the low‐latitude (L = 1.2) station Kakioka. At the time of the Pi2s, CRRES was moving outward at L ∼ 5, magnetic local time (MLT) of ∼2200, and magnetic latitude of ∼4°. The satellite was in an apparently detached dense plasmaspheric plasma for the first Pi2 but in a low‐density region for the second Pi2. The first Pi2 had a period of 120 s at Kakioka (MLT ∼ 2000), and the satellite observed a nearly identical oscillation in the (approximately) azimuthal component of the electric field and in the radial and compressional components of the magnetic field. At CRRES the compressional magnetic field and the azimuthal electric field oscillated nearly in quadrature in the decaying phase of the pulsation, which suggests a radially trapped fast mode wave. However, comparison of this observation with numerical models of the plasmaspheric cavity mode is difficult because the plasma density did not have a simple plasmapause structure. The second Pi2 observed at Kakioka exhibited a period of 180 s and was similar to the azimuthal electric field oscillation seen at CRRES but quite different from the compressional magnetic perturbation at the satellite. For this event it is not clear whether the electric field oscillation represents a fast mode wave that was directly related to the ground Pi2 or a local shear Alfvén wave that was not directly related to the Pi2. If the first interpretation is the case, the midlatitude Pi2 is not necessarily confined within the plasmasphere by fast mode trapping or cavity resonance. These examples indicate that electric field oscillations measured by equatorial satellites are a sensitive and possibly better indicator of the onset of substorms and that if the plasma density measurements are also available, the electric field data can be useful for identifying the wave mode of Pi2 pulsations.