The Magnetosphere of the Earth under Sub‐Alfvénic Solar Wind Conditions as Observed on 24 and 25 May 2002

On the 24th and 25th of May 2002, the solar wind density at 1 AU was so low (lower than 0.1 /cc) that the flow became sub-Alfvénic for intervals that lasted as long as four hours (the Alfvén Mach number was as low as 0.4). The Earth magnetosphere dramatically changed: the bow-shock disappeared and two Alfvén wings formed on the flanks of the magnetosphere. These Alfvén wings are two structures on both the East and West side of the Earth's magnetosphere, where the solar wind plasma was decelerated (the deceleration was 30% in one wing and 60% in the other) and the magnetic field direction changed. The Alfvén wings reached an extension of 600 Earth radii. We present observations of the Geotail spacecraft, which are consistent with Geotail crossing one of these Alfvén wings multiple times. During this event, the magnetosphere was geomagnetically extremely quiet, showed no substorm activity and almost no auroral activity. Global MHD numerical simulations show that the closed field line region was very symmetric, extending to 20 Earth radii on the day-side and on the night-side. Whereas the open field lines became highly asymmetric: the field lines emanating from the northern hemisphere all pointed along the dawn Alfvén wing, the field lines from the southern hemisphere all pointed along the other wing. Since November 28, 1963, there were 16 recorded sub-Alfvénic solar wind intervals, lasting for more than one hour and caused by low solar wind density. Considering the uneven data coverage, these events occur, on average, every 2.2 years.