The magnetosphere of the Earth under sub-Alfvénic solar wind conditions

On 24 and 25 May 2002, the solar wind density was so low (0.1 /cc), that the flow became sub‐Alfvénic for intervals that lasted as long as 4h (the Alfvén Mach number was as low as 0.4). The magnetosphere changed dramatically and (according to simulations and theory) became very asymmetric: the bow shock disappeared and two Alfvén wings formed on the flanks of the magnetosphere (the wings were 600 Re long, the deceleration 30% in one wing and 60% in the other). Geotail’s data suggest that it crossed one of these wings multiple times. The magnetosphere was geomagnetically extremely quiet, showed no substorm activity and almost no auroral activity. Simulations show that the closed field line region was very symmetric, extending to 20 Re on the dayside and on the nightside. The open field lines became highly asymmetric: the field lines emanating from the Northern Hemisphere all pointed along the dawn Alfvén wing (around 8:00 LT), the field lines from the Southern Hemisphere all pointed along the other wing (around 22:00 LT). Between 28 November 1963 and 27 September 2015, there were 16 recorded sub‐Alfvénic solar wind intervals, lasting for more than 1h and caused by low solar wind density. Considering the uneven data coverage, these events occur, on average, every 2.2 years.edition: 1status: publishe