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A fortuitous configuration of five Time History of Events and Macroscale Interactions during Substorms probes indicated that on 15 February 2008 between 04:00 and 11:00 UT a B-Z dip with radial size of about 2.5 Earth radii (R-E) and azimuthal size that could be as large as 10 R-E was present for over 3 hr in the plasma sheet at X-GSM approximate to -11 R-E. Ballooning/InterChange Instability heads were observed at the tailward side of the dip with partial derivative B-Z/partial derivative X approximate to -10 nT/R-E. The Ballooning/InterChange Instability heads appeared to drift azimuthally toward dawn, in accord with particle-in-cell simulations of a charged current sheet. The signatures of the latter, for example, a finite average E-Z directed toward the center of the plasma sheet, are verified by the Time History of Events and Macroscale Interactions during Substorms data.

Plain Language Summary On the antisunward side of the Earth, the terrestrial magnetic field lines are stretched out to form an elongated structure called the magnetotail. At the center of this structure the magnetic field strength normally decreases continually with increasing distance away from the Earth. However, periodically, the magnetotail is disrupted in a process called a substorm; the cause of this disruption remains controversial. A fortuitous configuration of five space probes is used to investigate a possible mechanism for this disruption. The probes observed the formation of a substantial region where the magnetic field strength possessed a minimum (rather than always decreasing). By comparing with the results of plasma computer simulations, it is shown that this region should generate an instability that produces dawnward propagating clumps of more dipolar field lines, in agreement with the probe observations.