Magnetospheric Multiscale Observations of Field-aligned Currents at the Magnetopause

Magnetospheric Multiscale (MMS) has demonstrated that the dayside magnetopause is usually highly structured, with multiple layers. Within these layers currents are observed to be flowing both parallel and perpendicular to the magnetic field. The parallel currents primarily act to bend or shear the magnetic field, while the perpendicular currents mainly change the field strength. We note that within the magnetopause MMS observes current densities that can be of the order of 100s of nAm-2. If these currents were to map to the ionosphere, then they would correspond to field-aligned current densities of the order of 100s of μAm-2, orders of magnitude larger than the observed low altitude field-aligned currents. A survey of several magnetopause crossings shows that the field-aligned currents on the magnetosphere side of the current layers can be flowing either parallel or anti-parallel to the magnetic field. On the magnetospheric side of the magnetopause current layers the plasma flows are generally in the -z direction (in GSM coordinates) for the events studied, suggesting that the observations are to the south of the reconnection site. If the currents were to correspond to Region-1 sense currents flowing into and out of the southern ionosphere, then the polarity of the currents would be ordered by local time, flowing parallel to the local magnetic field on the dusk-side of noon, and anti-parallel on the dawn-side of noon. But the current signatures are mixed, and sometimes the field-aligned currents reverse polarity within one crossing of the magnetopause. The issue is therefore how do we reconcile the local magnetopause field-aligned current signatures with what would be expected for the large-scale Region-1 currents that are typically associated with flow shears introduced through reconnection? Given the strength of the currents and their structure much of the current may be closed locally, with only a residual contributing to the large-scale currents.