Global manifestations of a substorm onset observed by a multi-satellite and ground station network

With a favorable constellation of spacecraft and ground stations, a study is made on the global manifestations of a substorm onset. The onset occurred simultaneously and conjugately in both hemispheres, confirmed by observations of the auroral breakup from IMAGE FUV-WIC and a sudden intensification of a westward electrojet from ground-based magnetometers. Concurrently with the onset, field-aligned and Hall currents in the auroral ionosphere are observed by CHAMP, which are consistent with the signature of a Harang discontinuity. Immediately after the onset a magnetic field dipolarization is clearly observed by Double Star TC-1, located near the central magnetotail and subsequently, by the Cluster quartet. The observations can be explained by a dawnward propagation of the substorm current wedge at a speed of about 300 km/s

Cluster observations of currents in the plasma sheet during reconnection

We present Cluster PEACE observations of parallel electron currents near a reversal of accelerated ion flows which indicated that the spacecraft were in the vicinity of an active reconnection X-line (XL). Moments calculated from the PEACE electron spectrometer 3D 4s resolution data are analysed. We surveyed the electron current structure to reveal their dependence on distance from the neutral sheet (NS). The electron density and the magnetic field component parallel to the lobe magnetic field were selected as proxies of the distance to the NS. We found that earthward from the XL the electron parallel current switches direction: tailward closer to the NS and earthward nearer to the lobe. On the interface between the tailward and earthward currents we found a narrow layer of strong earthward current. At the same place the largest transverse magnetic disturbances were detected. The observed current structure is consistent with the collisionless reconnection model. However, tailward of the XL no such structure was evident in the data

The magnetic structure of an earthward-moving flux rope observed by Cluster in the near-tail

We investigate the magnetic structure of a small earthward-moving flux rope observed by Cluster in the near-Earth plasma sheet through application of the Grad-Shafranov (GS) technique to reconstruct the transverse magnetic field distribution perpendicular to the flux rope axis at X=−14.75 RE. We find that the principal axis of the flux rope lies approximately along the dawn-dusk direction and that the diameter of the flux rope is about 1.5 RE. There is a strong duskward core magnetic field in the center of the flux rope. According to the AE index, there is no obvious substorm associated with the magnetic flux rope. Recent studies indicate that the formation of the flux rope in the plasma sheet can be understood in terms of simultaneous reconnection at multiple X-line points in the near-tail. The distribution of the transverse magnetic field on the cross section is the asymmetric circles, which requires that the reconnections at multiple X-line points occur. So our results also provide additional evidence for the occurrence of multiple-X line reconnection in the magnetotail

The magnetic structure of an earthward-moving flux rope observed by Cluster in the near-tail

International audienceWe investigate the magnetic structure of a small earthward-moving flux rope observed by Cluster in the near-Earth plasma sheet through application of the Grad-Shafranov (GS) technique to reconstruct the transverse magnetic field distribution perpendicular to the flux rope axis at X=-14.75 RE. We find that the principal axis of the flux rope lies approximately along the dawn-dusk direction and that the diameter of the flux rope is about 1.5 RE. There is a strong duskward core magnetic field in the center of the flux rope. According to the AE index, there is no obvious substorm associated with the magnetic flux rope. Recent studies indicate that the formation of the flux rope in the plasma sheet can be understood in terms of simultaneous reconnection at multiple X-line points in the near-tail. The distribution of the transverse magnetic field on the cross section is the asymmetric circles, which requires that the reconnections at multiple X-line points occur. So our results also provide additional evidence for the occurrence of multiple-X line reconnection in the magnetotail