REMOTE-SENSING OF THE PLASMAPAUSE DURING SUBSTORMS - GEOTAIL OBSERVATION OF NONTHERMAL CONTINUUM ENHANCEMENT

The continuum enhancement, another type of terrestrial nonthermal cont inuum radiation, has been frequently observed by the Geotail spacecraf t. This radiation is a short-lived enhancement generated at the plasma pause from the midnight to dawnside sector. Simultaneous Geotail and W ind observation shows that usual nonthermal continuum radiation (the n ormal continuum) generated at the dayside plasmapause appears to follo w the continuum enhancement. This suggests that both radiations are ge nerated by a series of electrons injected at the midnight sector assoc iated with the same substorm. The continuum enhancement regularly cons ists of ''fast'' and ''main'' components. The fast component has faste r rising rate of average frequency (+50 similar to +100 kHz/h) and sho rter duration of 0.5-1 hours. The main component has slower rising rat e of average frequency (+10 similar to +20 kHz/h) and longer duration of 1-3 hours. We suggest that the former is generated by the lower-ene rgy electrons at the midnight plasmapause, while the latter is generat ed by the higher-energy electrons at the dawnside plasmapause. However , the harmonic structure of the continuum enhancement indicates the ra dius of the source on the plasmapause close to the magnetic equatorial plane. We find that the radius of the plasmapause first reduces at th e rate of -1.0 similar to -0.5 R-E/h in the first 1 hour after the sub storm onset. This shows a typical scale of the peeling off of plasma f rom 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 similar to +0.5 R-E/h in the next 1 h our. The expansion rate is faster than the value expected from the upw elling from the upper ionosphere. This suggests that fast compression and recovery processes should also affect the radial motion of the pla smapause.