By‐dependent convection patterns during northward interplanetary magnetic field

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Abstract
Observations of Birkeland currents, electric fields, and auroral forms in the dayside polar regions during periods of northward interplanetary magnetic field (IMF) can be unified to fit into consistent convection patterns ordered principally by the IMF By component. During periods of weak IMF By, two polar convection cells are symmetrically located on either side of the noon‐midnight meridian, producing sunward convection over the pole, as previously reported. As the IMF By becomes significantly positive (negative), the dawn (dusk) convection cell expands across the northern polar cap, whereas the other cell shrinks. In the southern polar cap it is the dusk (dawn) cell which expands. During strong By this cell expansion gives the appearance of a single convection cell in the polar region. At the “collapsed” cell a large convective flow gradient is developed where reversal of the northward Bz (NBZ) Birkeland current system, antisunward polar ionospheric current, and the most intense polar cap electric fields are statistically observed. We suggest that this convection gradient region is also associated with sun‐aligned arcs and the transpolar arc of the theta aurora, which are observed in the polar regions during northward IMF. The convection patterns proposed here are consistent with the antiparallel merging model in which the IMF merges with the geomagnetic field lines in the tail lobe during northward IMF intervals. The resulting convection in the tail brings plasma from the plasma sheet to the tail. This process divides the open field lines of the lobes and produces an area of closed field lines in the polar cap associated with the transpolar arc of the theta aurora.

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