Classification of IMAGE/RPI‐stimulated plasma resonances for the accurate determination of magnetospheric electron density and magnetic field values

The Radio Plasma Imager (RPI) on the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) satellite stimulates short‐range plasma‐wave echoes and plasma emissions, both known as local plasma resonances, that are detected on plasmagrams (virtual range versus frequency presentations of the amplitude‐modulated received signals). These resonances are used to provide the local electron density N e and magnetic field strength ∣ B ∣ that are required for accurate inversions of the RPI reflection traces into magnetospheric N e profiles. Examples are presented that have been used for this purpose and also that yield N e gradients within a single plasmagram in the vicinity of the plasmapause and accurate (near apogee) N e measurements during the 31 March 2001 magnetic storm. The RPI‐stimulated resonances are the magnetospheric analog of plasma resonances stimulated by ionospheric topside sounders at the harmonics of the electron cyclotron frequency f ce , the electron plasma frequency f pe , the upper‐hybrid frequency f uh (where f uh 2 = f pe 2 + f ce 2 ), between nf ce both above and below f pe known as Qn and Dn resonances, respectively, and also at other frequencies. While they are observed to have an inherent bandwidth of 300 Hz or less, the effective detection bandwidth for strong resonances is nearly 2 kHz. The Qn resonances are often observed with time durations exceeding the 178 ms limit of the RPI operating programs commonly used for resonance detection. The f uh resonance is also observed with a long time duration even when it is in the plasma domain where it is normally weaker, i.e., when f uh > 2f ce . A strong resonance at f pe is often but not always observed. In earlier investigations, the Dn resonances had been related to natural magnetospheric plasma‐wave emissions and to sounder‐stimulated plasma‐wave emissions in Jupiter's Io plasma torus. The present RPI observations represent the first evidence for the stimulation of these resonances by a sounder deep in the terrestrial magnetosphere. These observations suggest the possible widespread occurrence of N e field‐aligned irregularities (FAI) or the ease of sounder‐stimulated FAI based on one Dn generation mechanism involving eigen modes of cylindrical plasma oscillations which have been associated with FAI. The RPI observations provide additional support to earlier suggestions that the Qn and Dn resonances have components of natural origin. The capability of simultaneous reception on three mutually orthogonal dipole receiving antennas often aids in the identification of spectral features. The RPI capability to generate magnetospheric reflection traces, leading to well‐defined wave cutoff frequencies at the satellite, provides independent Ne determinations and additional spectral‐identification confidence. Combining these capabilities with new analysis techniques that produce three‐antenna plasmagrams normalized by f ce and amplitude plots based on averages over different range‐bin intervals, N e and ∣ B ∣ can often be accurately determined from the plasma‐resonance spectra to within uncertainties of the order of 1% and 0.1%, respectively, when RPI sounds using frequency steps equal to the 300 Hz receiver bandwidth. Such accuracy in magnetospheric N e determination, even when N e ∼ 1 cm −3 , is difficult to attain by other techniques. It can only be obtained by RPI with proper spectral‐identification. For example, identification uncertainties between f pe and f uh can lead to 20% uncertainties in N e ; even larger uncertainties can result when the N e determinations are based solely on the Qn resonances and f ce when non‐Maxwellian electron velocity distributions are present. Except for such frequency deviations of the Qn resonances, the main controlling factor of the plasma‐resonance spectra appears to be, as in the ionosphere, the plasma parameter f pe /f ce .