Remote sensing of the plasmapause during substorms: Geotail observation of nonthermal continuum enhancement

Abstract

Abstract. The continuum enhancement, another type of terrestrial nonthermal continuum radiation, has been frequently observed by the Geotail spacecraft. This radiation is a short-lived enhancement generated at the plasmapause from the midnight to dawnside sector. Simultaneous Geotail and Wind observation shows that usual nonthermal continuum radiation (the normal continuum) generated at the dayside plasmapause appears to follow the continuum enhancement. This suggests that both radiations are generated by a series of electrons injected at the midnight sector associated with the same substorm. The continuum enhancement regularly consists of "fast" and "main" components. The fast component has faster rising rate of average frequency (+50,•+100 kHz/h) and shorter duration of 0.5-1 hours. The main component has slower rising rate of average frequency (+10,•+20 kHz/h) and longer duration of 1-3 hours. We suggest that the former is generated by the lower-energy electrons at the midnight plasmapause, while the latter is generated by the higher-energy electrons at the dawnside plasmapause. However, the harmonic structure of the continuum enhancement indicates the radius of the source on the plasmapause close to the magnetic equatorial plane. We find thag the radius of the plasmapause first reduces at the rate of-1.0 ,• -0.5 RE/h in the first 1 hour after the substorm onset. This shows a typical scale of the peeling off of plasma from the outer plasmasphere associated with individual substorms. When geomagnetic activity is low we also find the reversal expansion of the plasmapause at the rate of +0.1 ,• +0.5 RE/h in the next 1 hour. The expansion rate is faster than the value expected from the upwelling from the upper ionosphere. This suggests that fast compression and recovery processes should also affect the radial motion of the plasmapause.

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 error angle ε varies with ellipse configuration, as expected, and can reach unacceptably large values, up to 90 • . This result had been foreseen by Jones (1982Jones ( , 1983.…”

“…Most NTC enhancement events reported so far display characteristics that can be compared to what is observed here. According to Kasaba et al (1998), who analysed a significant number of such events observed from GEOTAIL satellite (48 events during years 1994-1995), the radiation frequency mainly ranged from 10 to 80 kHz, thus including the frequencies encountered on 18 May 2008. During this specific event, however, while the central frequency of the event increased, the frequency distance between bands (Fig.…”

“…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).…”

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

“…First discovered about 30 years ago (Gurnett, 1975, and references therein) they have been observed from all geo-spatial missions suitably equipped, in particular on ISEE, GEOS, DE I, GEOTAIL, WIND (Morgan and Gurnett, 1991, and references therein; Kasaba et al, 1998, and references therein). The appearance of continuum radiation in the observations (e.g.…”