Reconnection and Viscous Control of Dayside Field-Aligned Currents for Northward Interplanetary Magnetic Field

Abstract

We study the morphology and magnitude of dayside field-aligned currents (FACs) for northward interplanetary magnetic field (IMF) using observations from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). For near-zero IMF clock angles the FACs form a quadrupolar pattern centered at local noon, which is associated with the reverse convection cells driven by lobe reconnection. The poleward pair of FACs, known as the northward-$B_Z$ or NBZ FAC system, comprises upward and downward FAC pre- and post-noon; and the equatorward pair have the opposite polarity. The magnitude of the NBZ FACs is modulated by dipole tilt and phase of the solar cycle, the former controlling the solar zenith angle in the vicinity of the FACs, and the latter controlling the solar radio flux at 10.7 cm (F10.7), both of which contribute to the conductance of the ionosphere. The NBZ FACs are also modulated by the magnitude of the IMF (or the Z-component of the IMF for near-zero clock angle), which we presume controls the lobe reconnection rate. The NBZ FACs do not respond to the X-component of the IMF, solar wind speed, nor solar wind density, so we presume that these do not affect the lobe reconnection rate. High solar wind speed leads to the appearance of region 1 and 2 FACs at auroral latitudes, which we suggest are associated with a viscous interaction between the solar wind and the magnetosphere.