Plasma Waves in the Plasmapause Region: Recent Results

J. L. Green, S. F. Fung, S. Boardsen, R. F. Benson, and B. W. Reinisch

Over the last several decades a number of space-born plasma wave instruments 
have observed a characteristic emission peak that has frequently been used to 
determine the local plasma electron density in the plasmapause region. The 
emission has sometimes been referred to as the upper-hybrid resonance or uhr 
(where fuhr**2 = fp**2 + fg**2, with fg being the electron gyrofrequency and fp 
the electron plasma frequency). The Radio Plasma Imager (RPI) on the Imager for 
Magnetopause-to-Aurora Global Exploration (IMAGE) satellite makes observations 
of natural emissions but also performs radio sounding, which creates both 
long-range and short-range echoes. The short-range echoes are from local 
resonances that sometimes can yield accurate information on both in situ 
electron density and magnetic field strength, which in turn can be used to 
determine the true fuhr for comparison against the commonly observed emission 
peak. From this technique it has been found that the peak of the emission is 
near but not typically at the uhr frequency. 

This emission peak, actually occurring in the upper hybrid wave band, has also 
been observed to be near the source of the low-frequency portion of the 
non-thermal continuum (~10 to 100 kHz). The generation of the continuum is 
believed to occur when fuhr = (n +1/2)fg at the plasmapause. The source region 
of the non-thermal continuum at higher frequencies (~100 to greater than 800 kHz), 
also known as the kilometric continuum (KC), was unknown until recently. The KC, 
observed by wave instruments on both IMAGE and Geotail, has been found to be 
associated with plasmaspheric bite-out structures as observed by the IMAGE 
Extreme Ultraviolet (EUV) instrument. In addition, the KC source appears to lie 
deep within the bite-out at the plasmapause near the magnetic equator and has 
been observed to coincide with an intensification of the emission near the fuhr, 
much like its lower frequency counterpart. The resulting KC emission cone, 
assumed to be corotating with the plasmasphere, is observed to be within 10 
degrees of the magnetic equator, and has a typical magnetic longitudinal extent 
of ~42 degrees. The KC emission has been observed at all local times. 

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To be presented at the 2002 Spring A.G.U. Meeting, 
Washington, D.C., U.S.A., 28-31 May 2002