Small‐scale field‐aligned plasmaspheric density structures inferred from the Radio Plasma Imager on IMAGE

Abstract

[1] Among the objectives of the Radio Plasma Imager (RPI) on IMAGE is the observation of the Earth's plasmasphere from the satellite's polar orbit, with apogee %8 R E geocentric distance and perigee near 1200 km altitude. This objective is here pursued by (1) remote sounding from high-altitude regions outside the main plasmasphere, (2) sounding within the plasmasphere, and (3) in situ passive measurements of natural wave activity. During sounding of the plasmasphere from both outside and inside the plasmapause, RPI echoes that follow non-field-aligned ray paths are usually not the discrete traces on range-versus-frequency records (plasmagrams) that are predicted by raytracing simulations in smooth magnetospheric density models. Instead, such RPI echoes exhibit various amounts of spreading, from %0.5 R E to %2 R E in virtual range (range assuming free-space speed of light propagation). The range spreading is attributed to scattering from, partial reflection from, and propagation along geomagnetic field-aligned electron density irregularities. There exists a substantial body of theoretical work on mechanisms that might explain the appearance of such irregularities both within and beyond the plasmasphere. That the spread-producing irregularities are field-aligned is suggested by the efficiency with which RPI excites discrete echoes that propagate along the geomagnetic field, sometimes into both hemispheres. The spatial distributions and scale sizes of the spread-producing irregularities remain to be investigated. The RPI echo data, however, coupled with earlier evidence from topside sounders and whistler mode instruments, suggest that they can have cross-field scale sizes within a range from %200 m to over 10 km and electron densities within %10% of background. RPI is found capable of detecting plasmapause locations from distances of %3 R E or more. When minimal signal integration is used, the location and range of density values of a steep plasmapause can be determined from distances of order 1 R E , and echoes can at times be returned from points extending inward from the plasmapause to locations where the electron density reaches %3000 cm À3 , which is usually at L < 3.

Multi-spacecraft observations of small-scale fluctuations in density and fields in plasmaspheric plumes

“…Such spontaneously induced currents may be related to the interchange motion (e.g., Southwood and Kivelson, 1989). The possibility of the interchange motion around the plasmasphere has been suggested in several articles (Lemaire, 1974;Southwood et al, 2001;Carpenter et al, 2002;McFadden et al, 2008). The interchange motion has also been reported at outer planets (e.g., Kivelson et al, 1997;André et al, 2007).…”

“…Simultaneous observations of plumes are made by EUV (Extreme UltraViolet) instrument onboard IMAGE (Imager for Magnetopause-to-Aurora Global Exploration) satellite. Field aligned density structures have been reported by Carpenter et al (2002). They used plasmagram data from IMAGE RPI (Radio Plasma Imager).…”

Density structures inside the plasmasphere: Cluster observations

“…Density structures had also been inferred from high-resolution magnetometer data obtained during the Earth swing-by of Cassini (Southwood et al, 2001) by interpreting the magnetic field strength variations as signatures of diamagnetic plasma elements produced by the interchange instability (Lemaire, 1975). More recently, smallscale plasmaspheric density structures have been reported by the Radio Plasma Imager (RPI) on board IMAGE (Carpenter et al, 2002), and in the dusk sector by WHISPER on board Cluster (Décréau et al, submitted, 2004 1 ). Large-scale density structures have also been observed in the past and called "detached plasma elements" or "plasmaspheric tails" (Chappell et al, 1970b;Horwitz et al, 1990).…”

Ducted propagation of chorus waves: Cluster observations

“…In their model the conversion of whistler mode waves into lowerhybrid waves leads to increased pitch angle scattering and precipitation of radiation belt particles, resulting in a density enhancement in the ionosphere that forms field aligned density irregularities due to the upward diffusion of cold plasma. An overview of various theoretical mechanisms for the formation of field aligned density irregularities can be found in Carpenter et al (2002). Implicit experimental evidence for the existence of ducts in the region of chorus waves have been reported on a number of occasions, for example Carpenter et al (2002) analysed data from Radio Plasma Imager experiment on the IMAGE satellite.…”