Explanation of Pi2 pulsations in auroral zone: Azimuthal propagation of ionospheric loop currents

The modes of Pi2 oscillations in space and on the ground, particularly in the auroral zone, have been discussed individually. The former is associated with the cavity oscillations either ideally trapped or partially trapped in the magnetosphere, and the latter is associated with the transient oscillations of the currents in the ionosphere. In this report, we show that ground polarization patterns (ellipticity and major axis orientation) of Pi2 pulsations in the nighttime sector from auroral zone to midlatitudes can be explained by the wave polarizations in the magnetosphere transmitted by the fundamental and the third harmonic deformations of the geomagnetic field lines. These ground polarization patterns are, however, understood alternatively as propagating large‐scale ionospheric loop currents in auroral zone expanding ~1000 km longitudinally. One pattern propagates eastward in the dawn sector and the other westward in the dusk sector. The propagating loop currents for the generation of the ground polarizations were consistent with the vortical motion of auroras observed by all‐sky imager in association with the Pi2 pulsations. Propagation of the ionospheric loop currents and vortical motion of auroras are a consequence of the Pi2‐associated magnetosphere and ionosphere coupling through the Alfvén waves.