Cusp latitude magnetic impulse events: 1. Occurrence statistics

Abstract

Magnetic impulse events were selected by a computer algorithm procedure from magnetic records obtained at the near cusp latitude conjugate stations Iqaluit, Northwest Territories, Canada, and South Pole Station, Antarctica. The algorithm was constructed to select large (≳ 50 nT in the vertical component of the magnetic field), short lived (6 to 12 min) events. These events were found to be highly localized in the 06 to 18 LT sector at the two stations. A strong minimum in occurrence was found during hour 11 LT. The field changes associated with these events can be interpreted as due to an approximately half‐cycle, odd‐mode, Alfvén wave along a near‐magnetopause flux tube. From the vertical magnetic deflections of the impulse events the directions of field‐aligned currents into the conjugate ionospheres were inferred. In the morning LT sector, field‐aligned currents were directed into the ionospheres, while in the afternoon LT sector, field‐aligned currents were directed out of the ionospheres. These findings are comparable with the statistical results for quasi‐stationary field‐aligned currents and suggest that at the times of these events, Iqaluit and South Pole are at a higher effective magnetic latitude. The average deflection in the vertical component for the events was measured to be ∼ 95 nT. From this the magnitude of the average field‐aligned currents was calculated to be J∥ ∼ 2 × 10−7 A/m².