Simulations of the Earth's magnetosphere embedded in sub-Alfvenic solar wind on 24 and 25 May 2002

During 24 and 25 May 2002, the solar wind conditions at Earth's orbit were very unusual: the density was extremely low (below 0.1/cc) and, as a result, the flow was subfast and sub-Alfvenic (the Alfven Mach number was as low as 0.4 in the rest frame of the Earth). Consequently, the Earth's bow shock disappeared and two Alfven wings formed on the flanks of the magnetosphere. These two long structures (estimated extension of 600R(E) for this event) affect the incoming plasma as follows: the velocity is reduced and the magnetic field rotates. In the present study, global magnetohydrodynamical simulations of the magnetosphere are performed for such upstream solar wind conditions. The simulations show how the magnetosphere configuration dramatically changes when the sub-Alfvenic solar wind reaches the magnetosphere: the dayside magnetopause expands up to 20R(E), and on the nightside the position of the last closed magnetic field line diminishes to 20R(E). As a result the closed magnetic field line region becomes very symmetric. The open field line configuration also changes such that field lines emanating from the Northern Hemisphere all point in the direction of the dawn Alfven wing (around 8:00LT), while the field lines from the Southern Hemisphere all point in direction of the other wing (around 22:00LT). During the formation of the Alfven wings, the tail lobes completely disappeared and the auroral activity greatly diminished, i.e., the magnetosphere becomes geomagnetically quiet.