Multipoint analysis of a bursty bulk flow event on April 11, 1985

Abstract

In an attempt to place short‐lived, high‐speed magnetotail flows termed bursty bulk flow events (BBFs) in the context of substorm phenomenology we analyze one such event that took place on April 11, 1985, using data from several spacecraft and many ground stations. The substorm onset, which took place at 0127 UT, had a meridian 2 hours of local time east of AMPTE/IRM. The satellite did not detect high‐speed flows at that time. A high‐latitude (∼70° corrected geomagnetic) substorm intensification took place at 0202 UT centered ∼0.5 hour of local time west of the AMPTE/IRM meridian. The ISEE 2 satellite at the magnetotail lobe and the LANL 019 satellite at geosynchronous altitude were both at the same meridian as AMPTE/IRM at the time. The 0202 UT substorm intensification was associated with (1) a dipolarization at the ISEE 2 satellite at 0200:30 UT, (2) a BBF oiiset at AMPTE/IRM at 0202 UT accompanied by an intense dipolarization consistent with current wedge formation, (3) an energetic particle injection at geosynchronous altitude that took place at 0204 UT. The plasma acceleration region associated with this substorm intensification was estimated to be ∼ 8 RE tailward of AMPTE/IRM. Thus, during this activity the BBF event was due to an observed tail collapse Earthward of X ∼ −26 RE. 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 RE2 in the Y–Z direction. Similarly, the Earthward magnetic flux transport rate measured at AMPTE/IRM during the BBF event can result in a potential drop comparable to the expected transpolar cap potential if the BBF event has a size of 1–2 RE in the Y direction. The large amounts of flux transport measured past the satellite necessitate the existence of lobe flux reconnection tailward of AMPTE/IRM. The above results assume the validity of the frozen‐in condition over the ∼10‐min duration of the BBF event. Although activity continued in the ionosphere and the ring current for well over 1.5 hours after the 0202 UT substorm intensification, most of the earthward energy and magnetic flux transport past IRM had ceased ∼10 min after the BBF onset. We propose that the fast flows transport and pile up magnetic flux through a very narrow (a few RE in Y extent) flow channel in the midtail to the edge of an expanding dipolarization front in the near‐Earth region. After the plasma sheet dipolarizes at a given location enhanced flux transport ceases, resulting in an apparent short (10‐min timescale) duration of the fast flows. Unlike the near‐Earth plasma sheet, which dipolarizes across many hours of local time, the midtail plasma sheet may exhibit longitudinally localized dipolarization. This may explain the often observed lack of one‐to‐one correlation between midtail activity and substorms.