Proton gyro-harmonic (PGH) echoes stimulated by the radio sounder on the IMAGE 
satellite

D.L. Carpenter, T.F. Bell, D. Chen, D. Ng, I. Galkin, B. Reinisch


Within the Earth's plasmasphere between ~2000 and 20,000 km altitude, the Radio
Plasma Imager (RPI) instrument on the IMAGE satellite regularly detected
discrete echoes that arrived at delays equal to multiples of the local proton
gyro-period. Similar phenomena were observed at ionospheric altitudes during the
ISIS satellite era, and were interpreted as manifestations of a proton memory
process wherein the rf of a pulse is impressed on the protons in the vicinity of
an electric antenna and is then radiated back to the satellite after one or more
proton gyro-periods. PGH echoes on RPI were found: (1) at various frequencies
near and above 10 kHz in the whistler-mode domain; (2) above and near the
electron gyro-frequency in a nominally non-propagating domain; (3) near 300 kHz,
just below and within the Z-mode domain. Echoes in the whistler-mode domain were
particularly strong near 10 kHz, close to the local proton plasma frequency, and
on occasion as many as 20 additional echoes near 10 kHz were observed at
multiples of the proton gyro-period. A PGH echo form not previously found in the
ISIS-series data is a resonance centered at a frequency ~ 15% above the local
electron gyro-frequency. We suggest that energy to support the echo process
comes from the gradual collapse of the plasma sheath around the antenna in the
aftermath of a pulse and from frequency-dependent energization of thermal
protons during the pulse.

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2nd VERSIM Workshop on ELF/VLF Radio Phenomena: Generation, Propagation and 
Consequences in Observations, Theory and Modelling, Sodankyla, Lapland, Finland, 
26-30 September, 2006