Satellite wave instruments commonly detect banded magnetospheric emissions, called ``n+1/2" waves, between harmonics of the electron gyrofrequency () in the outer magnetosphere. To complement an earlier statistical study on the amplitude of these emissions, we investigate the statistical properties of their frequencies by identifying the spectral peaks in 193 sample spectra observed with AMPTE/IRM, normalizing these peaks to as determined by the on-board magnetometer, and plotting them against the ratio of the plasma to gyrofrequency () where was determined from the on-board particle or wave instrument. A histogram of the spectal peaks normalized by reveals well defined peaks between harmonics of . In this histogram, as well as in individual spectra, the higher frequency peaks tend to occur closer to the electron gyroharmonic than the lower frequency peaks. In about 15% of the spectra sampled, more than one peak was observed between gyroharmonics. The statistics also suggest that the banded emissions shift downward in normalized frequency with increasing , although this observation is subject to the considerable uncertainty in the measured . These trends may be consistent with several wave generation mechanisms such as excitation of electron Bernstein modes destabilized by non-Maxwellian magnetospheric electron distributions. The presence of well-defined peaks in the statistical distribution of the normalized emission frequencies suggests that either the emissions correspond to specific plasma conditions or that the normalized frequencies are relatively insensitive to plasma conditions. The presence of the well-defined peaks and the relative paucity of multiple peaks suggest that propagation effects may not be significant in most of the cases in this sample. This investigation clarifies some of the directions of experimental and theoretical work needed to better identify these magnetospheric wave emissions and their generation mechanisms.
AGU Index Terms: 2772 Plasma waves and instabilities; 7871 Waves and instabilities; 6984 Waves in plasma
Keywords/Free Terms: electron bernstein waves, magnetospheric emissions
JGR-Space 1998JA900050
Vol. 104
, No. A1
, p. 293