Structure and Coalescence of Magnetopause Flux Ropes and Their Dependence on IMF Clock Angle: Three-Dimensional Global Hybrid Simulations

<p>Flux ropes are ubiquitous at Earth’s magnetopause and play important roles in energy transport between the solar wind and Earth’s magnetosphere. In this paper, structure and coalescence of the magnetopause flux ropes formed by multiple X line reconnection in cases with different southward interplanetary magnetic field (IMF) clock angles are investigated by using three-dimensional global hybrid simulations. As the IMF clock angle decreases from 180°, the axial direction of the flux ropes becomes tilted relative to the equatorial plane, the length of the flux ropes gradually increases, and core field within flux ropes is formed by the increase in the guide field. The flux ropes are formed mostly near the subsolar point and then move poleward towards cusps. The flux ropes can eventually enter the cusps, during which their helical structure collapses, their core field weakens gradually, and their axial length decreases. When the IMF clock angle is large (i.e., the IMF is predominantly southward), the flux ropes can coalesce and form new ones with larger diameter. The coalescence between flux ropes can occur both near the subsolar point when they are newly formed and away from the subsolar point (e.g., in the southern hemisphere) when they move towards cusps. However, when the IMF clock angle is small (≤ 135° ), we do not find coalescence between flux ropes.</p>

Explicit IMF By-dependence in geomagnetic activity

<p>The interaction of the solar wind with the Earth’s magnetic field produces geomagnetic activity, which is critically dependent on the orientation of the interplanetary magnetic field (IMF). Most solar wind coupling functions quantify this dependence on the IMF orientation with the so-called IMF clock angle in a way, which is symmetric with respect to the sign of the B<sub>y</sub> component. However, recent studies have shown that IMF B<sub>y</sub> is an additional, independent driver of high-latitude geomagnetic activity, leading to higher (weaker) geomagnetic activity in Northern Hemisphere (NH) winter for B<sub>y</sub> > 0 (B<sub>y</sub> < 0). For NH summer the dependence on the B<sub>y</sub> sign is reversed. We quantify the size of this explicit B<sub>y</sub>-effect with respect to the solar wind coupling function, both for northern and southern high-latitude geomagnetic activity. We show that for a given value of solar wind coupling function, geomagnetic activity is about 40% stronger for B<sub>y</sub> > 0 than for B<sub>y</sub> < 0 in NH winter. We also discuss recent advances in the physical understanding of the B<sub>y</sub>-effect. Our results highlight the importance of the IMF B<sub>y</sub>-component for space weather and must be taken into account in future space weather modeling.</p>