Dynamics of the magnetospheric mid-tail induced by substorms; a multisatellite study

Abstract

The dynamics of the mid-tail region during substorms is examined using multi-satellite measurements. This study shows that partial cross-tail current disruptions detected at radial distances of the order of 20 to 30 earth radii in the anti-solar direction, linked with a gross-scale decrease of the magnetic field of the tail, correspond to substorm onsets. The period which preceedes the onset is associated with a lobe magnetic field increase which is either due to an enhanced reconnection, as evidenced by concurrent southward turnings of the interplanetary magnetic field, or/and to an increase of the solar wind pressure. These two effects are shown to act together to the thinning of the plasma sheet which leads to substorm onset. It is also shown that multiple substorms can occur during an overall increase sequence of the magnetic field of the tail, which can be interpreted in the context of the control of the lobe magnetic field by the solar wind properties.