Abstract
Abstract. A number of recent studies have shown that the upstream Mach number may play a significant role in the energy transfer between the solar wind and the magnetosphere. Magnetohydrodynamic (MHD) simulation results of the magnetosphere-ionosphere system are presented that show the transition from nominal solar wind and interplanetary magnetic field driving to extremely strong driving. One of the predominant features of the magnetosphere that becomes apparent during low Mach number conditions is the formation of Alfvén wings above and below the magnetosphere. Alfvén wing are cavities of low flow, and have been observed at Io and Ganymede, both of which reside in regions of sub-Alfvénic flow. It is shown that Alfvén wings exist even during nominal Mach number time periods – the wings fold over to form what has been classically viewed as the magnetotail. The regions of low flow within the Alfvén wing limit the electric field applied across the ionosphere, hence causing the ionospheric cross polar cap potential to be dependent upon the Mach number, and in turn, causing the saturation of the potential.
Highlights
In the past few years, only a hand-full of extremely strong solar wind driven events have occurred
The purpose of this study is to examine the transition of the magnetospheric domain from being driven by a nominal Mach number solar wind to being driven by a low Mach number solar wind
During time periods in which the interplanetary magnetic field increases and the solar wind density decreases, as is the case in magnetic clouds, the Alfven Mach number can decrease to very small values though
Summary
In the past few years, only a hand-full of extremely strong solar wind driven events have occurred. Inside the CME proper, the magnetic field became very strong, while the density became very low, typ-. During this event, the solar wind became subAlfvenic (i.e. a Mach number of less than one). The Alfven Mach number is one of the strong controlling factors of the behavior of a plasma embedded in a magnetic field. In the typical solar wind, the Alfven Mach number is 8 or above. In the middle of a CME, the Mach number can become quite reduced, reaching values of 2 or less. It is reasonable to investigate how the magnetosphere reacts under these types of flow conditions, and to understand if some of the features observed during major storms are due to the Mach number, instead of internal mechanisms
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