Observation of continuum radiations from the Cluster fleet: first results from direction finding

Abstract

Abstract. The Cluster fleet offers the first possibility of comparing non-thermal terrestrial continuum radiation from similarly equipped nearby observation points. A very rich data set has already been acquired on the Cluster polar orbit, between 4 and 19 Earth radii geocentric distances, and preliminary analysis has been carried out on these emissions. We focus in this paper on direction finding performed from all four spacecraft as a means to locate the position of the sources of this continuum radiation. Directions are derived from spin modulation properties, under the usual analysis assumptions of the wave vector of the radiation lying in the plane containing the spin axis and the antenna position at electric field minimum. All the spin axes of the four Cluster spacecraft are aligned perpendicular to the ecliptic, thus the aligned spacecraft antenna spin planes provide redundant 2-D views of the propagation path of the radiation and source location. Convincing 2-D triangulation results have been obtained in the vicinity of the source region. In addition, the out of spin plane component of the wave vector reveals itself to a certain extent through directivity characteristics compared at different distances of the spin plane to the ecliptic. The four case events studied (two of them taken near apogee, the other two near perigee) have confirmed general properties derived from previous observations: trapping in the lower frequency range, radiation escaping into the magnetosheath region in the higher frequency range. All propagation directions are compatible with source positions in the plasmapause region, however, at a significant distance from the equator in one case. Our observations have also revealed new properties, like the importance of small-scale density irregularities in the local amplification of continuum radiation. We conclude that more detailed generation and propagation models are needed to fit the observations.

Remote sensing of a NTC radio source from a Cluster tilted spacecraft pair

“…6c), bears some resemblance with the time evolution of the type referred to as the continuum enhancement, which supported Gough's isoFce paradigm (Gough, 1982). Such events have been observed and studied following Gough's study (Kasaba et al, 1998;Décréau et al, 2004). The spectral signature of the continuum enhancement is characterized by banded features, with frequencies globally increasing over time intervals of one to several hours, reported to develop during substorms, after the start of an electron injection event.…”

“…The source position is then determined by the intersection of the ray path, traced back from a single spacecraft, with the relevant surface. The main difficulty encountered in several published applications of this strategy (Gough, 1982;Décréau et al, 2004) is the fact that the ray path orientation is known only in a plane. This difficulty disappears when the ray path orientation is known in 3-D, as shown by successful mapping of AKR radiation sources using DE-1 observations , or, more recently, of SKR radiation sources using Cassini observations (Cecconi et al, 2009).…”

“…Radio wave direction finding using the spin modulation of measured electric field intensity by a single receiving antenna has been used for many years (Kurth et al, 1975(Kurth et al, , 1981Gurnett, 1975;Etcheto et al, 1982;Gough, 1982;Steinberg et al, 1989;Morgan and Gurnett, 1991;Kasaba et al, 1998;Nagano et al, 2003;Décréau et al, 2004;Grimald et al, 2007). This tool, sometimes referred to as "spin-null method", uses a rotating directive antenna (Calvert, 1999).…”

A quantitative test of Jones NTC beaming theory using CLUSTER constellation

“…Assuming that this NTC radiation is electromagnetic and propagating in the O mode (quasi-circular polarisation), the spin modulation can be used to determine the projection of the direction of propagation onto the antenna spin plane (Calvert, 1985;Gurnett, 1975;Gough, 1982;Kasaba et al, 1998; Décréau et al, 2004). Figure 2 presents the frequency/time evolution of the two parameters which are derived from this process.…”

“…In other words, the source of the less modulated emissions appears to be located in the same general direction as the source of the highly modulated emission. When more than one satellite is available, the projections onto the spin planes of the various ray paths can be used to estimate source positions by triangulation (Gough, 1982;Décréau et al, 2004). If the source is stable with time, a single spacecraft can provide the information using ray paths at different times (Morgan and Gurnett, 1991).…”