Diagnostics of magnetospheric electron density and irregularities at altitudes <5000 km using whistler and Z mode echoes from radio sounding on the IMAGE satellite

Abstract

[1] When the Radio Plasma Imager (RPI) on the IMAGE satellite operates in the inner plasmasphere and at moderate to low altitudes over the polar regions, pulses emitted at the low end of its 3-kHz to 3-MHz sounding frequency range can propagate in the whistler mode and/or in the Z mode. During soundings with both 25.6-ms pulses and 3.2-ms pulses, whistler mode echoes have been observed in (1) ''discrete,'' lightning whistlerlike forms and (2) diffuse, widely time spread forms suggestive of mode coupling at the boundaries of density irregularities. Discrete echoes have been observed at altitudes less than %5000 km both inside the plasmasphere and over the auroral and polar regions, being most common inside the plasmasphere. Diffuse echoes have also been observed at altitudes less than 5000 km, being most common poleward of the plasmasphere. Either discrete or diffuse echoes or both have been detected during one or more soundings on at least half of all IMAGE orbits. In regions poleward of the plasmasphere, diffuse Z mode echoes of a kind reported by Carpenter et al. (2003) were found to accompany both discrete and diffuse whistler mode echoes 90% of the time and were also present during 90% of the soundings when no whistler mode echoes were detected. It is proposed that the observed discrete whistler mode echoes are a consequence of RPI signal reflections at the bottom side of the ionosphere and that diffuse whistler mode echoes are a result of scattering of RPI signals by geomagnetic field-aligned electron density irregularities located within 2000 km earthward of the satellite and in directions close to that of the field line passing through IMAGE. Diffuse Z mode echoes are believed to be due to scattering of RPI signals from electron density irregularities within 3000 km of the satellite, particularly those in the generally cross-B direction. Consistent with previous works, our results indicate that the magnetosphere at high latitudes is highly structured, with electron density irregularities that exist over cross-B scales ranging from 10 m to 100 km and that profoundly affect whistler mode propagation. It is demonstrated that both kinds of whistler mode echoes as well as diffuse Z mode echoes have potential for local and remote diagnostics of electron density distributions and structures. , et al. (2004), Diagnostics of magnetospheric electron density and irregularities at altitudes <5000 km using whistler and Z mode echoes from radio sounding on the IMAGE satellite,

Determination of the averaged charge-to-mass ratio of the heavy charged constituents of a magnetoplasma using whistler wave measurements

“…Then, near ω pi , the variation of |α x | around unity will not be smooth if µ is not small -what is typical of the Earth magnetosphere at altitudes below 4000 km (Sonwalkar et al, 2004). Actually, the large axis of the magnetic field polarization ellipse performs a rotation of π/2 when the frequency crosses the local ion plasma frequency ω pi ; the ellipse becomes a circle exactly at ω pi .…”

Active Wave Experiments in Space Plasmas: The Z Mode

“…1.17 are also observed on IMAGE. They are called diffuse W-mode echoes [Sonwalkar et al,62]. In regions poleward of the plasmasphere, diffuse Z-mode echoes of the kind illustrated in Fig.…”

“…In an investigation of IMAGE/RPI data in the inner plasmasphere and at moderate to low altitudes over the polar regions, Sonwalkar et al [62] found diffuse Z-mode echoes often accompanied whistler (W)-mode echoes. An example from their study is shown in Fig.…”

On the dispersion law of low-frequency electron whistler waves in a multi-ion plasma

“…This condition is indispensable, particularly for the near-Earth plasma at altitudes where the transition occurs from the lightest ions' to the more heavy ions' prevalence, i.e. typically at altitudes of few thousands kilometers above the Earth (Sonwalkar et al, 2004).…”

“…In an inhomogeneous magnetospheric plasma, the sources of whistler emissions can be located far from the region where the wave emissions effectively reveal their properties and are registered by remote receivers. Then the GO approach is conventionally used to solve the problem of the spectral characteristics' evolution of the emissions during their distant propagation from their sources; it was also used effectively in the recent active radio sounding projects similar to that one realized on board the IMAGE satellite (Burch, 2000;Reinish, 2000;Sonwalkar et al, 2004;Carpenter, 2004). Moreover, the GO technique was also applied to fit the parameters of space density model distributions using the Direction Finding Data of groundbased transmitter signals registered on board the "Ikebono" satellite (Sawada et al, 1993).…”