Small-scale and mesoscale field-aligned auroral current structures: their spatial and temporal characteristics deduced by the Swarm constellation

Abstract

Magnetic field recordings by the Swarm A and C spacecraft during the counter-rotating orbit phase are used for checking the stationarity of auroral region small-scale and mesoscale field-aligned currents (FACs). The varying separations between the spacecraft in the 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 mesoscale FACs of along-track wavelengths >100 km are primarily stable for more than 40 s and over cross-track separations exceeding 20 km. A prime reason for their occasional deselection is the latitudinal motion of the current system. Conversely, stationary small-scale FACs (10-50 km scale sizes) are typically found to have azimuthal sizes of only 12 km. Their temporal stability is limited to about 18 s. This class of small-scale FACs is commonly found in the cusp region and prenoon sector at 75-80° magnetic latitude. In the midnight sector these FACs are weaker and appear at lower latitudes. The small-scale FACs are assumed to be associated with kinetic Alfvén waves. Another class of kilometer-scale FACs exists, which cannot be resolved well by our dataset but seems to influence our analysis.