Statistical Observations in Support of Bow Shock Current Closure to Earth's High‐Latitude Ionosphere During Non‐Zero IMF <i>B_y</i>

<p dir="ltr">The bow shock current (BSC) plays an important role in supplying the magnetosphere with solar</p><p dir="ltr">wind energy, in particular during times of low solar wind magnetosonic Mach numbers. Since the magnetic pile‐</p><p dir="ltr">up in the magnetosheath has to be maintained, the BSC cannot close locally, but must instead connect to</p><p dir="ltr">magnetospheric current systems. However, the details of this closure remain poorly understood. For east–west</p><p dir="ltr">interplanetary magnetic field (IMF) it has been hypothesized that the BSC partly closes to the high‐latitude</p><p dir="ltr">ionosphere, as field‐aligned currents (FACs) on open field lines, but there is still no statistical evidence of this.</p><p dir="ltr">In order to investigate this hypothesis, we use 9 years of Defense Meteorological Satellite Program (DMSP) data</p><p dir="ltr">to construct normalized FAC maps of the northern hemisphere polar cap. We sort them according to different</p><p dir="ltr">IMF clock angles, IMF magnitudes and magnetosonic Mach numbers. By separating opposite polarity FACs,</p><p dir="ltr">we show that, on average, a unipolar FAC exists in the dayside polar cap when the IMF By ≠ 0, regardless of the</p><p dir="ltr">sign of the IMF Bz. This current flows out of (into) the ionosphere in the northern hemisphere for IMF By > 0</p><p dir="ltr">(<0) and is thus of the correct polarity to connect to the north–south component of the BSC. Moreover, it is</p><p dir="ltr">strongest when the BSC flows predominantly in the north–south direction. These results constitute the first</p><p dir="ltr">statistical evidence in support of at least a partial closure of the BSC to the ionosphere during non‐zero IMF By.</p>