Comprehensive Analysis of the Magnetic Signatures of Small‐Scale Traveling Ionospheric Disturbances, as Observed by Swarm

Based on Swarm satellite data from 2015 through 2018, we present the mean characteristics of

magnetic fifield flfluctuations at midlatitudes and low latitudes. It is the fifirst comprehensive study focusing on

small‐scale variations (<10 km). Events are observed on about 35% of the orbits. The highest occurrence

rates are detected after sunset, in the East Asian/Australian sector, and during months around June solstice.

Low occurrence rates are found at low magnetic latitudes (below ±10° quasi‐dipole latitude), in the region

of the South Atlantic Anomaly, and during equinox seasons. All these occurrence features compare well

with those of medium‐scale traveling ionospheric disturbances. We therefore term our small‐scale events

small‐scale traveling ionospheric disturbances (SSTIDs). SSTIDs exhibit high fifield‐aligned current (FAC)

densities connected to narrow current sheets with meridional width of typically 4 km. The intense FACs of

several μA/m2 flflow typically between the hemispheres. Return currents are distributed over larger scales

and thus have smaller amplitudes. Peak current densities get larger toward lower latitudes. There are two

groups of events, around morning‐noontime and evening‐night, which are separated by demarcation lines

near 04 and 15 magnetic local time. The magnetic amplitudes of the small‐scale flfluctuations are larger in

sunlight than in darkness, indicating larger total currents in the loops. But the FAC peak current densities

are larger in darkness, inferring a stronger squeezing of the current sheet under low‐conductivity conditions.

We suggest that our SSTIDs are an evolutional state of medium‐scale traveling ionospheric disturbances.