MHD SIMULATIONS OF THE TRANSITION OF MAGNETIC RECONNECTION FROM CLOSED TO OPEN-FIELD LINES

Two- and three-dimensional resistive MHD simulations are used to study the behavior of a dynamic magnetotail model when reconnection proceed s from closed to open field lines. For this purpose, the initial, quas i-static magnetotail configuration is set up for various lobe density values. Initiated by the increase or occurrence of anomalous resistivi ty, all simulations then exhibit the growth of an unstable tail mode, which leads to neutral line and plasmoid formation. The reconnection p rocess starts in a plasma sheet region of high mass density and progre sses to open field lines with much lower plasma density and much lower plasma beta. At the time of transition, drastic changes of typical dy namical parameters occur, involving strongly enhanced electric fields and reconnection rates and the generation of field-aligned currents. F urther, typical substorm signatures such as the dipolarization of the magnetic field in the inner regions of the magnetotail appear on very short timescales, comparable to those found in spacecraft observations . It is argued that this behavior should indeed become important durin g substorm evolution, in support of substorm models based on this tran sition.