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Abstract:
A new quality index is proposed for quantitatively assessing the level of Alfvénicity of the flow in the magnetopause and other quasi one‐dimensional plasma/field structures. Perfect Alfvénicity is expressed by the so‐called Walén relation, which describes the agreement of the observed jump in velocity across a suitable portion of the magnetopause and the corresponding jump in Alfvén velocity. In the past, approximate agreement with this relation has been used as an important indicator of ongoing magnetic reconnection. The new index incorporates the effects of deviations in direction as well as in magnitude of the two vectors. Its properties and utility are illustrated by application to a large set of dayside magnetopause crossings by the Magnetospheric Multiscale spacecraft MMS 1. Time series for two of these events are also presented and discussed.
