Y. Kasaba, H. Matsumoto, K. Hashimoto, R. R. Anderson,
J.-l. Bougeret, M. L. Kaiser, X. Y. Wu, I. Nagano.
J. Geophys. Res., in press, 1997.
Usual nonthermal continuum radiation (classical continuum) has long duration and generated from the dawnside to the dayside plasmapause. In this study, we investigate another type of nonthermal continuum radiation, continuum enhancement, observed by the GEOTAIL spacecraft from November 1994 to December 1995. This radiation is short-lived enhancement of nonthermal continuum radiation generated at the nightside plasmapause. Its spectrum can indicate the plasma density and the radial distance at the source on the plasmapause during substorms.
We sometimes find that the classical continuum in the dayside zone is observed following the continuum enhancement in the nightside zone. This indicates that both are generated by a series of injected electrons associated with the same substorm. Typical interval between the onset of both radiations is about 1 hour. This is consistent with the time lag expected from dawnward motion of the source associated with gradient and curvature drift of injected electrons.
We also find that some of the continuum enhancement consist of 'fast' and 'main' components which have different duration and rising rate in frequency. We suggest that the former is first generated by the lower- energy electrons at the plasmapause in the local midnight zone, while the latter is later generated by the higher-energy electrons at the plasmapause in the more dawnward zone.
On the other hand, we find that the radial distance estimated from the continuum enhancement decreases with -1.0 ~ -0.5 RE/h for the first 1 hour after the onset of each substorms. This suggests that decrease of the radius of the plasmapause is mainly due to rapid and short decreases just after the onset of each substorms. When geomagnetic activity is low, we also find that the radial distance increases with +0.1 ~ +0.5 RE/h for the next 1 hour after the fast reduction of the plasmasphere. Observed increase is too fast and can not be explained only by up-welling of plasma from the sub- auroral F-layer. This suggests that short variation of the plasmapause is caused not only by the peeling off but also by the compression and recovery process of the plasmasphere.
Contact Information