MULTIPOINT ANALYSIS OF A BURSTY BULK FLOW EVENT ON APRIL 11, 1985

In an attempt to place short-lived, high-speed magnetotail flows terme d bursty bulk flow events (BBFs) in the context of substorm phenomenol ogy we analyze one such event that took place on April 11, 1985, using data from several spacecraft and many ground stations. The substorm o nset, which took place at 0127 UT, had a meridian 2 hours of local tim e east of AMPTE/IRM. The satellite did not detect high-speed flows at that time. A high-latitude (similar to 70 degrees corrected geomagneti c) substorm intensification took place at 0202 UT centered similar to 0.5 hour of local time west of the AMPTE/IRM meridian. The ISEE 2 sate llite at the magnetotail lobe and the LANL 019 satellite at geosynchro nous altitude were both at the same meridian as AMPTE/IRM at the time. The 0202 UT substorm intensification was associated with (1) a dipola rization at the ISEE 2 satellite at 0200:30 UT, (2) a BBF onset at AMP TE/IRM at 0202 UT accompanied by an intense dipolarization consistent with current wedge formation; (3) an energetic particle injection at g eosynchronous altitude that took place at 0204 UT. The plasma accelera tion region associated with this substorm intensification was estimate d to be similar to 8 R(E) tailward of AMPTE/IRM. Thus, during this act ivity the BBF event was due to an observed tail collapse Earthward of X similar to-26 R(E). The Earthward energy transport measured at AMPTE /IRM can account for the expected magnetospheric power consumption if the BBF has a cross-sectional area of only 1-2 R(E)(2) in the Y-Z dire ction. E Similarly, the Earthward magnetic flux transport rate measure d at AMPTE/IRM during the BBF event can result in a potential drop com parable to the expected transpolar cap potential if the BBF event has a size of 1-2 R(E) in the Y direction. The large amounts of flux trans port measured past the satellite necessitate the existence of lobe flu x reconnection tailward of AMPTE/IRM. The above results assume the val idity of the frozen-in condition over the similar to 10-min duration o f the BBF event. Although activity continued in the ionosphere and the ring current for well over 1.5 hours after the 0202 UT substorm inten sification, most of the earthward energy and magnetic flux transport p ast IRM had ceased similar to 10 min after the BBF onset. We propose t hat the fast flows transport and pile up magnetic flux through a very narrow (a few R(E) in Y extent) flow channel in the midtail to the edg e of an expanding dipolarization front in the near-Earth region. After the plasma sheet dipolarizes at a given location enhanced flux transp ort ceases, resulting in an apparent short (10-min timescale) duration of the fast flows. Unlike the near-Earth plasma sheet, which dipolari zes across many hours of local time, the midtail plasma sheet may exhi bit longitudinally localized dipolarization. This may explain the ofte n observed lack of one-to-one correlation between midtail activity and substorms.