Solat-Terrestrial Environment Laboratory, Nagoya University, Toyokawa 442, Japan

Although the magnetic field intensity in the lobe of the magnetotail is usually in the range of 7-10 nT at distances beyond 150 R_E as observed by ISEE 3 and GEOTAIL, it becomes at times more than 30 nT. The magnetic field of as large as 53 nT was observed in the distant tail lobe by GEOTAIL at -182 R_E on March 9, 1993, when a moderate magnetic storm was in progress. From the scan of daily plots of magnetic field data during the two year period of the GEOTAIL observations, intervals were chosen when the lobe field strength exceeding 20 nT was registered. Large field events are found to mostly occur during a growing stage of the ring current. Evens observed beyond 100 R_E downstream from the earth are associated with intermittent and temporal excursions of the spacecraft into the magnetotail. The characteristic time scale of magnetospheric period is approximately 20 minutes. These entries into the magnetotail are usually preceded by a strong southward component of sheath magnetic field. Since magnetic storms are associated with enhanced solar wind flux and IMF conditions, reconfiguration of the magnetotail due to increases of the total pressure in the sheath and directional changes of the sheath plasma flow should control the probability of spacecraft residence in the magnetotail. It is confirmed that the pressure balance approximately holds at the tail magnetopause under enhanced solar wind conditions by using high resolution plasma data. Observations of large lobe field events suggest that additional magnetic fluxes caused by the merging process near the earth during magnetic storms constrain the magnetotail radius from decreasing the nominal value significantly, although the magnetotail is in a compressed state due to the increase in exterior pressure.

J. Geomag. Geoelectr., 48, 561-575, 1996