ALBERT

Description: Although the magnetic field intensity in the lobe of the magnetotail is usually in the range of 7-10 nT at distances beyond 150 RE 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 RE on March 9, 1993, when a moderate magnetic storm was in progress. Prom 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. Events observed beyond 100 RE 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.

Description: On March 29, 1993, The Geotail spacecraft was 134RE downstream from the earth and within 12 RE of the axis of the magnetotail whose orientation was calculated from the solar wind direction measured by IMP 8. The IMF had a magnitude near 12 nT and remained steady and roughly 50° northward for 9 hours. Geotail moved back and forth between two regions, an interior region with characteristics similar to the tail and an exterior region with characteristics similar to the magnetosheath. The flow in the dense rapidly flowing exterior region was lilted 8° toward the tail axis rather than parallel to a cylindrical tail. Calculated boundary normals were primarily in the solar magnetospheric Y direction but with a small X component consistent with a slightly flaring or cylindrical tail. The unusual flow direction in the exterior region along with the presence of slower, less dense, tailward flowing plasma on taillike field lines inside the boundary suggest that the exterior flow has a component across the boundary and into the tail. These data support accumulating evidence from MHD simulations that suggest that the magnetotail assumes a very different configuration when the IMF remains very northward for periods of at least several hours. In this unusual configuration, field lines which would normally extend into the distant tail instead close on the flanks and nearer to the earth, creating a distant tail with less open flux and a reduced Y dimension. © 1996, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.

Description: The large scale dynamics of the tail during substorms are examined using multi-satellite measurements made between 18 and 22:00 UT on August 22,1983. We use data from ISEE1/2, 1977-007, IMP-8, and ISEE-3 located in the solar wind or tail at distances along the Sun-Earth line ranging from+11 to -205REDuring the period studied, which corresponded to a southward directed IMF, successive plasma injections were detected at 6.6RE first during a general increase of the magnetic field in the lobe. The magnetic field increase is attributed to an enhanced reconnection rate, as evidenced by the close relationship between the tail lobe magnetic energy density and the available Poynting energy in the solar wind, for near constant solar wind pressure. A Traveling Compression Region (flux rope) was detected in close association with the first energetic particle injection at geostationary orbit. After the first injections, the series of injections continued and the last ones are shown to be linked with a tailward propagating cross-tail current disruption detected onboard IMP-8 at ~35R£- This event is seen in the distant tail at 205RE onboard ISEE-3 as an increase of the tail radius; the far magnetbsphere engulfed the ISEE-3 satellite which was located in the solar wind prior to the event. The characteristic time delay between cross-tail current disruption onset occurring in the near-Earth plasma sheet and the enhancement of the tail diameter at 205RE is on the order of 40 minutes. Several hours before this event, a similar case was detected in which partial cross-tail current disruption in the mid-tail was also related to a far tail expansion. In both cases, the observed signatures are in agreement with the concept of the ejection of plasmoids from the magnetospheric tail with velocities comparable to that of the solar wind. These results strongly suggest that traveling cross-tail current disruption and plasmoid ejection from the far tail are the signature of the same propagating plasma process. © 1996, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.