Abstract
The plasmasphere is a high-density region of cold plasma around the earth residing on closed field lines that supports both electromagnetic and electrostatic emissions. The electrostatic upper hybrid emission is found through out the plasmasphere and has been used extensively by a number of researchers as a method to obtain in situ plasma densities. Recent sounder observations by the RPI instrument on IMAGE clearly show that the upper hybrid emission is really a band of emission in which the peak is with 5% of the local plasma frequency (fp) for large fp/fg (> 4) but can be as much as 20% of fp for smaller fp/fg ratios. In the region of the plasmapause the (n+1/2)fg electrostatic emissions greatly intensify at the magnetic equator. This is also the site of the electromagnetic non-thermal continuum radiation (NTC) which is believed to generated when (n+1/2)fg = fuhr over the frequency range of 5 kHz to 800 kHz. The higher frequency component of NTC, also referred to as kilometric continuum, has been found to be generated in notch structures in the plasmasphere. At lower frequencies, the plasmaspheric whistler mode spectrum consists of four major emissions: electromagnetic (EM) equatorial radiation, plasmaspheric hiss, chorus, and emissions from ground-based transmitters. Plasma wave maps (spatial distribution of average wave intensities) from the IMAGE and DE-1 plasma wave instruments will be used to study the origin of these emissions. Observations of EM equatorial emissions in the low-frequency spectrum (10 - 300 Hz) show that the most intense region is in or near the magnetic equator in the afternoon sector and that during times of negative IMF Bz that maximum intensity moves from L values of 3 to less than 2. These observations are consistent with particle-wave interactions in or near the magnetic equator. Chorus emissions (300 Hz - 12.5 kHz) are observed in the outer portions of the plasmasphere and are associated with disturbed times and are believed to be responsible for energetic electron precipitation on closed field lines in the subauroral regions. Plasmaspheric hiss (300 Hz - 3.3 kHz) shows the peak-intensity regions both at high latitudes near L= 4 and in the magnetic equator in the L = 2 to 3 range. The longitudinal distribution of the average intensity of plasmaspheric hiss peaks near late afternoon with the minimum near early morning local times. Although the generation of plasmaspheric hiss is controversial the observations presented here points to lightning, which has a similar local-time distribution, as a major contributor to the hiss emission spectrum. At frequencies from ~10 - 50 kHz whistler mode emissions from ground-based transmitters are also observed. For these emissions the maximum intensity shifts almost exclusively to the local evening with enhancements along all L shells from 1.8 to 3. Indications are that the cyclotron resonance also operates in this frequency range.