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

[1] 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 tp. 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 fce. 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 tp. The most efficient proton excitation is expected to occur at frequencies near and below the proton plasma frequency fpp. In contrast to these echoes, the discrete PC echoes above fce and near fZ show evidence of thermal-mode wave propagation at the rf frequency of the sounder pulses, while the new resonance above fce suggests the existence of a ringing phenomenon in the plasma that is unique to altitudes above 7000 km.