Pi2 pulsations with second harmonic: CRRES observations in the plasmasphere

Abstract

Pi2 pulsations observed on the ground at low latitudes often exhibit multiple spectral peaks, and this property has been attributed to the harmonics of plasmaspheric cavity mode resonance. We show that multifrequency Pi2 pulsations simultaneously observed from the Combined Release and Radiation Effects Satellite (CRRES) and at Kakioka (L = 1.26) can be explained by the cavity mode. The pulsations occurred in a 40-min time span on 20 January 1991. CRRES was within the plasmasphere at L = 6.1, on the dipole equator, and very near midnight. Kakioka was also very close to midnight. The pulsations consisted of two spectral components, ∼7 mHz (f1, fundamental) and ∼12 mHz (f2, second harmonic). At CRRES, the f1 oscillation was observed in the azimuthal component, Eφ, of the electric field, while the f2 oscillation was observed in the compressional component, Bz, of the magnetic field. At Kakioka, by contrast, both the fundamental and second harmonic oscillations were detected in the horizontal component, H. The spectral separation between Eφ and B z implies that the spacecraft was located near the node/antinode of cavity mode resonance excited at multiple harmonics. Further support for this interpretation is provided from analyses of the phase of Eφ and Bz. Copyright 2003 by the American Geophysical Union.