Structure of small-scale field-aligned currents at middle and low latitudes having lower atmospheric origin

The CHAMP magnetic data indicate that small amplitude (1-5 nT) magnetic fluctuations with period around a few tens of seconds along the orbit exist globally and almost all the time. Characteristics of the magnetic fluctuations including seasonal dependence having geographical characteristics strongly suggest that they are the small-scale spatial structure of field-aligned currents with lower atmospheric origin (Nakanishi et al., 2014). We suppose that gravity waves generated by lower atmospheric disturbances propagate to the ionosphere and drive the E-layer dynamo. The currents in the ionosphere divert along the magnetic field into the other hemisphere and make a closed circuit.

To confirm the above scenario and to find the scale of the current circuit in longitudinal direction, we use the magnetic data observed by the SWARM satellites. By analysis of the magnetic data observed by the SWARM satellites, the magnetic fluctuations as recorded earlier by CHAMP are confirmed to have the same characteristics i.e., the magnetic fluctuation is perpendicular to the geomagnetic field; the amplitude on the dayside is much larger than that on the nightside; towards the dip equator the period tends to get longer.

Because the three Swarm satellites have various spatial relations in 3-D space between their orbits, we could easily confirm that the objective magnetic fluctuations are not temporal but spatial structures. The longitudinal scale seems to be of the order of 100 km. We shall show the above results and some other characteristics of the current circuit and discuss whether or not our suggested model fits the observed characteristics.