Observation of field-aligned and ionospheric currents during space weather month, September 1999 Original Research Article

September 1999 was a magnetically active month, with several geomagnetic storms occurring in sequence which influenced the current pattern of the high-latitude ionosphere. The DMI Greenland magnetometers recorded variations of the ionospheric Hall currents while Ørsted, a Danish low-altitude Earth-orbiting satellite, measured the magnetic effect of field-aligned currents (FAC). We discuss a particular dayside event which occurred a few hours after the peak of a moderate storm on Sept. 15, 1999. Ørsted proceeded polebound on a trajectory closely aligned with the Greenland west coast magnetometer chain, and the Sondrestrom Incoherent Scatter Radar was in operation scanning the ionosphere almost parallel to the Østed trajectory. Coincident solar wind (SW) and interplanetary magnetic field (IMF) data were obtained from the ACE spacecraft and ballistically propagated to a nominal subsolar bow shock location at 12 Re. A synoptic view of the data suggests that the dayside field-aligned and ionospheric current system responded quickly (within 5–10 min) to variations of the northward component of the IMF (IMF- Bz). Specifically, the latitude of the boundary between westward and eastward high-latitude Hall currents as well as the latitudes of the maxima of the eastward and westward currents followed closely IMF-Bz variations.