The interplanetary magnetic field By‐dependent field‐aligned current in the dayside polar cap under quiet conditions

Spatial distribution and temporal variation of the interplanetary magnetic field (IMF) By‐dependent cusp region field‐aligned currents (FACs) during quiet periods were studied by use of magnetic data observed by Magsat. The analysis was made for 11 events (each event lasts more than one and a half days) when the IMF By component was steadily large and Bz was relatively small (|Bz|<|By|). Results of the analysis of total 62 half‐day periods for the IMF By‐dependent cusp region FAC are summarized as follows: (1) The IMF By‐dependent cusp region FAC is located at around 86°–87° invariant latitude local noon, which is more poleward than the location of the IMF Bz‐dependent cusp region FAC. (2) The current density of this FAC is greater than previous studies (≥4 µA/m² for IMF By = 6 nT). (3) There are two time scales for the IMF By‐dependent cusp region FAC to appear: the initial rise of the current is on a short time scale, ∼10 min, and it is followed by a gradual increase on a time scale of several hours to a half day. (4) The seasonal change of this FAC is greater than that of the nightside region 1 or region 2 FACs. (5) The IMF Bz‐dependent cusp region FAC is not well observed around the cusp when the IMF By‐dependent cusp region FAC is intense.