Features of ion trajectories during ''taillike'' to ''dipolelike'' magnetic transitions are examined by means of three-dimensional particle codes. It is demonstrated that the large electric fields induced by the ''collapse'' of the geomagnetic tail result in enhanced earthward convection of the midtail (~10 RE) magnetospheric populations. It is shown that the particle motions is controlled, to a major extent, by drifts that are negligible in steady state, namely, (1) in the direction parallel to the magnetic field, a centrifugal acceleration related to the rapid E¿B transport, which can account for creation of new high-altitude mirror points, (2) perpendicularly to the magnetic field, a drift of polarization which possibly yields ''frozen-in'' violation and transient breaking of the first adiabatic invariant. In this latter case the simulations emphasize that the guiding center validity is at stake depending upon particle mass and/or charge state and allow us to distinguish three types of ion behavior (adiabatic,nonadiabatic and gyrophase dependent, nonadiabatic and gyrophase independent) according to injection depth in the plasma sheet. Also, systematic orbit calculations show that the low-altitude populations are most affected by the ''collapse'' of the geomagnetic tail and reveal possible dramatic (from hundreds of eV up to hundreds of keV) accelerations of the particles. These trajectory results are of relevance to explain various storm time signatures at geosynchronous altitudes and provide a populating mechanism for the ring current region. ¿ American Geophysical Union 1990