Closure of F Region Dynamo Currents: Revisiting CHAMP Magnetic Field Data

Abstract

AbstractIn the equatorial F region exist upward/downward electric currents, which are generally attributed to dynamo action of thermospheric zonal wind. Despite the long history of their observations since the 1970s, the return path has not been thoroughly investigated. In this study, we revisit the magnetic field data of the Challenging Mini‐satellite Payload (CHAMP) to address the statistical distribution of the return currents. In addition to the F region dynamo currents near the dip equator, we have identified off‐equatorial bands with the reverse polarity. Both equatorial and off‐equatorial bands flip signs around 1600 magnetic local time (MLT) and are the weakest during June solstice. These similarities suggest that the off‐equatorial currents are tied to the equatorial F region dynamo and provide the return paths. The off‐equatorial return currents have the following characteristics. First, they are mostly confined within ±20° in magnetic latitude (MLAT) at CHAMP altitudes, which corresponds to <1,300‐km apex height. Second, the peak locations of the equatorial dynamo and the off‐equatorial return currents are zonally displaced from each other in terms of MLT and longitude. It implies that zonal currents in the topside F region participate in the current closure. Third, the return currents exhibit multiple zonal bands (beyond |MLAT| > 20°) near dusk during combined equinoxes, whose origin is currently unknown.