# Explicit IMF $B_y$-effect maximizes at subauroral latitudes (Dedicated to the memory of Eigil Friis-Christensen)

## TL;DR

This study demonstrates that the IMF $B_y$ component significantly influences geomagnetic activity, with effects peaking at subauroral latitudes and during CMEs, emphasizing its importance in space weather modeling.

## Contribution

It provides the first comprehensive analysis of how IMF $B_y$ affects geomagnetic activity across latitudes and during different solar wind conditions.

## Key findings

- Geomagnetic activity is 20% stronger for $B_y>0$ at 60° CGM latitude.
- During CMEs, the $B_y$-effect increases to about 40% at 57° CGM latitude.
- IMF $B_y$ is a significant independent driver of geomagnetic activity.

## Abstract

The most important parameter in the coupling between solar wind and geomagnetic activity is the $B_z$-component of the interplanetary magnetic field (IMF). However, recent studies have shown that IMF $B_y$ is an additional, independent driver of geomagnetic activity. We use here local geomagnetic indices from a large network of magnetic stations to study how IMF $B_y$ affects geomagnetic activity at different latitudes for all solar wind and, separately during coronal mass ejections (CMEs). We show that geomagnetic activity, for all solar wind, is 20% stronger for $B_y>0$ than for $B_y<0$ at subauroral latitudes of about $60^{\circ}$ corrected geomagnetic (CGM) latitude. During CMEs, the $B_y$-effect is larger, about 40%, at slightly lower latitudes of about $57^{\circ}$ (CGM) latitude. These results highlight the importance of the IMF $B_y$-component for space weather at different latitudes and must be taken into account in space weather modeling.