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

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 that the radius of the plasmapause first reduces at the rate of -1.0 ∼ -0.5 R_E/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 -R_E/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.