Coordinated cluster and ground-based instrument observations of transient changes in the magnetopause boundary layer during an interval of predominantly northward in the magnetopause boundary layer during an interval of predominantly northward IMF: relation to the reconnection pulses and FTE signatures

Abstract

We study a series of transient entries into the low-latitude boundary layer (LLBL) of all four Cluster craft during an outbound pass through the midafternoon magnetopause ([XGSM, YGSM, ZGSM] ≈ [2, 7, 9] RE). The events take place during an interval of northward IMF, as seen in data from the ACE satelliteand lagged by a propagation delay of 75 min that is well defined by two separate studies: (1) of the magnetospheric variations prior to the northward turning (Lockwood et al., 2001, this issue) and (2) of the field clock angle seen by Cluster after it had emerged into the magnetosheath (Opgenoorth et al., 2001, thisissue). With an additional lag of 16.5 min, the transient LLBL events correlate well with swings of the IMF clock angle (in GSM) to near 90°. Most of this additional lag is explained by ground-based observations, which reveal signatures of transient reconnection in the pre-noon sector that then take 10-15min to propagate eastward to 15 MLT, where they are observed by Cluster. The eastward phase speed of these signatures agrees very well with the motion deduced by cross-correlation of the signatures seen on the four Cluster craft.The evidence that these events are reconnection pulses includes: transienterosion of the noon 630 nm (cusp/cleft) aurora to lower latitudes; transient and travelling enhancements of the flow into the polar cap, imaged by the AMIE technique; and poleward-moving events moving into the polar cap, seen by the EISCAT Svalbard Radar (ESR). A pass of the DMSP-F15 satellite reveals that the open field lines near noon have been opened for some time: the more recently opened field lines were found nearer dusk where the flow transient and the poleward-moving event intersected the satellite pass. The events at Clusterhave ion and electron characteristics predicted and observed by Lockwood and Hapgood (1998) for a Flux Transfer Events (FTE), with allowance for magnetospheric ion reflection off Alfvénic disturbances in the magnetopause reconnection layer. Like FTEs, the events are about 1RE in their direction of motion and show a rise in the magnetic field strength but, unlike FTEs, in general they show no pressure excess in their core and hence no characteristic bipolar signature in the boundary-normal component. However, most of the events were3 observed where the magnetic field was southward, i.e. on the edge of the interior magnetic cusp, or when the field was parallel to the magnetic equatorial plane. Only when the satellite begins to emerge into the exterior boundary (where the field was northward), do the events start to show a pressure excess in their core and the consequent bipolar signature. We identify the events as the first observations of flux transfer events at middle altitudes.

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