Ground signatures of convecting reconnected flux tubes

Abstract

The ground magnetic and ionospheric signatures of reconnected flux tubes have been modeled using an expanding and rotating elliptical field‐aligned current system. Assuming a constant total current of 2 × 105 A, initial ground field strengths of ∼100 nT are shown to decay rapidly with distance from the current system and with current system expansion. The associated ionospheric electric field estimates show that the flux tube footprint convects initially eastward or westward with velocity decreasing from 3 to 1 km s−1 and then poleward decreasing from 0.5 to 0.2 km s−1. The signature detected by a ground magnetometer array will be a 0.5 to 1‐cycle oscillation with period of 8 min as the current system convects through the ionosphere above. These signatures are characterized by an initially rapid increase in field strength, with maximum amplitudes being reached within the first 5 min, followed by gradual decay. This modeling emphasizes that the temporal variation of the position of a recording magnetometer with respect to the ground image of the ionospheric footprint determines the form of the magnetic signature. The formalism should be useful in modeling flux tube behavior at high latitudes and may be relevant to dayside auroral observations.