Explicit IMF $B_y$-dependence in high-latitude geomagnetic activity

TL;DR

This study demonstrates that the sign of the IMF $B_y$ component significantly influences high-latitude geomagnetic activity, with notable suppression effects depending on hemisphere, season, and Earth's dipole orientation.

Contribution

It explicitly quantifies the independent effect of IMF $B_y$ sign on geomagnetic activity, revealing asymmetries not captured by traditional coupling functions.

Findings

Geomagnetic activity is 40-50% suppressed for certain $B_y$ signs in respective hemispheres.

On average, NH geomagnetic activity is 12% weaker for $B_y<0$ than for $B_y>0$.

The $B_y$-effect predominantly impacts the westward electrojet, especially during NH winter.

Abstract

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 $B_y$ component. However, recent studies have suggested that the sign of $B_y$ is an additional, independent driver of high-latitude geomagnetic activity, leading to higher (weaker) geomagnetic activity in Northern Hemisphere (NH) winter for $B_y\! >\! 0$ ($B_y\! <\! 0$). In this paper we quantify the size of this explicit $B_y$-effect with respect to the solar wind coupling function, both for Northern and Southern high-latitude geomagnetic activity. We show that high-latitude geomagnetic activity is significantly (by about 40-50\%) suppressed for $B_y\!<\!0$ in NH winter and for $B_y\!>\!0$ in SH winter. When averaged over all months, high-latitude geomagnetic activity in NH is about 12\% weaker for $B_y\!<\!0$ than for $B_y\!>\!0$. The $B_y$-effect affects the westward electrojet strongly but hardly at all the eastward electrojet. We also show that the suppression of the westward electrojet in NH during $B_y\!<\!0$ maximizes when the Earth's dipole axis points towards the night sector, i.e., when the auroral region is maximally in darkness.