Proton cyclotron echoes and a new resonance observed by the Radio Plasma Imager instrument on the IMAGE satellite

Details Proton cyclotron echoes and a new resonance observed by the Radio Plasma Imager instrument on the IMAGE satellite Proton cyclotron echoes and a new resonance observed by the Radio Plasma Imager instrument on the IMAGE satellite

Aug 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007jgra..11208208c&link_type=abstract

Journal of Geophysical Research, Volume 112, Issue A8, CiteID A08208

Physics

Plasma Physics

3

Space Plasma Physics: Spacecraft Sheaths, Wakes, Charging, Space Plasma Physics: Plasma Energization, Space Plasma Physics: Active Perturbation Experiments, Electromagnetics: Antennas, Electromagnetics: Plasmas

Scientific paper

At various altitudes in the plasmasphere, sounder pulses from the Radio Plasma Imager (RPI) instrument on the IMAGE satellite can couple strongly to protons, a process revealed in echo time delay versus frequency forms that arrive at multiples of the local proton cyclotron period t p . Lower-altitude (<4000 km) versions of two of these proton cyclotron (PC) forms were previously observed in the topside ionosphere. A new resonance, apparently confined to altitudes above ~7000 km, was observed at a frequency ~15% above the electron cyclotron frequency f ce . We believe that PC echoes and the new resonance are driven by a variety of mechanisms, but only exceptionally strong echoes in the whistler-mode domain are discussed in detail. Those echoes indicate that peak excitation of the protons occurs as a transient event at the beginning of each rf pulse. We infer that there is spatial bunching of accelerated protons during the initial formation of an electron sheath around the positive-voltage antenna element. The gyrating protons then produce a series of electrostatic pulses at multiples of t p . The most efficient proton excitation is expected to occur at frequencies near and below the proton plasma frequency f pp . In contrast to these echoes, the discrete PC echoes above f ce and near f Z show evidence of thermal-mode wave propagation at the rf frequency of the sounder pulses, while the new resonance above f ce suggests the existence of a ringing phenomenon in the plasma that is unique to altitudes above ~7000 km.

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