Small- and meso-scale field-aligned auroral current structures, their spatial and temporal characteristics deduced by Swarm constellation

Abstract. Magnetic field recordings by the Swarm A and C spacecraft during the Counter Rotation Orbit phase are used for checking the stationarity of auroral region small-and meso-scale field-aligned currents (FAC). The varying separation between the spacecraft in along- and cross-track direction during this constellation phase allow for determining the spatial and temporal correlation lengths for FAC structures of different along-track wavelengths. We make use of the cross‐correlation analysis to check the agreement of the magnetic signatures at the two spacecraft. When the cross-correlation coefficient exceeds 0.75 at a time lag that equals the along-track time difference, the event is identified as stationary. It is found that meso-scale FACs of along-track wavelength >100 km are primarily stable for more than 40 s and over cross-track separations of 20 km. An important reason for their deselection is the latitudinal motion of the current system. Conversely, stable small-scale FACs (10–75 km wavelength) are found primarily only in a very limited space, up to about 12 km in cross-track and ~18 s in along-track time difference. This class of small-scale FACs is the typical one found commonly in the cusp region and near the midnight sector. Not all the FACs within this limited spatial and temporal regime are stable. In particular for those with high current density occurring during enhanced solar wind input we do not find equivalent signatures at the accompanying satellite. They seem to represent narrow solitary Alfvén wave features.