A statistical study of banded magnetospheric emissions

Satellite wave instruments commonly detect banded magnetospheric emissions between harmonics of the electron gyrofrequency f(ce) in the outer magnetos phere. 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 the A ctive Magnetospheric Particle Tracer Experiment/Ion Release Module (AMPTE/I RM), normalizing these peaks to f(ce) as determined by the onboard magnetom eter, and plotting them against the ratio of the plasma to gyrofrequency f( pe)/f(ce) where fp, was determined from the onboard particle or wave instru ment. A histogram of the spectal peaks normalized by f(ce) reveals well-def ined peaks between harmonics of f(ce). In this histogram, as well as in ind ividual spectra, the higher-frequency peaks tend to occur closer to the ele ctron gyroharmonic than do the lower-frequency peaks, In about 15% of the s pectra sampled, more than one peak was observed between gyroharmonics. The statistics also suggest that the banded emissions shift downward in normali zed frequency with increasing f(pe)/f(ce) although this observation is subj ect to the considerable uncertainty in the measured f(pe). These trends may be consistent with several wave generation mechanisms such as excitation o f electron Bernstein modes destabilized by non-Maxwellian magnetospheric el ectron distributions.