Determination of the substorm initiation region from a major conjunction interval of THEMIS satellites

We investigate in detail the time history of substorm disturbances in the magnetotail observed during a major tail conjunction of Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites on 29 January 2008, 0700–0900 UT. During this interval, all THEMIS satellites were closely aligned along the tail axis near midnight and were bracketed in local time by GOES 11 and 12. The radial distance covered ranges from the geosynchronous altitude to ∼30 R E in the tail. This interval consists of three activations detected by the THEMIS satellites with good ground all‐sky‐camera observations of auroral activity. The first activation is a small substorm with spatially limited disturbance in the tail. The onset arc was equatorward of an undisturbed arc. The second activation is a moderate size substorm with the onset arc also being equatorward of an undisturbed arc. The third activation is an intensification of the substorm with its onset indicated by the second activation. The active auroral arc for this intensification was near the poleward boundary of the auroral oval. Analysis of these observations indicates that the first activation is a small substorm initiated in the near‐Earth plasma sheet and does not involve magnetic reconnection of open magnetic field lines. Magnetic reconnection on closed field lines can be ruled out for this substorm because it cannot generate the observed high‐speed plasma flow. The second and third activations are part of a moderate size substorm initiated also in the near‐Earth plasma sheet, with a subsequent substorm intensification involving activity initiated tailward of ∼30 R E . Overall, the time history of substorm activity for these two substorms is consistent with the near‐Earth initiation model.