Anders Ohma

The coupling of the interplanetary magnetic field (IMF) with the Earth's magnetic field can result in spectacular auroral displays, including substorms. An important feature of substorms is the westward electrojet that is routinely monitored by the AL index, based on ground magnetometer measurements. Recent research shows that, during periods of significant dipole tilt, the value of the AL index is strongly modulated by the dawn‐dusk (or y ) component of the IMF ( B y ): the AL index is stronger for B y > 0 than B y < 0 during negative dipole tilt (Northern Hemisphere winter). Kubyshkina et al. (2023, https://doi.org/10.1029/2022ja031275 ) argued that this B y dependence of the AL index is not really caused by IMF B y but by the component along the Sun‐Earth line ( x axis)—namely IMF B x , which is strongly anticorrelated with B y . Here we provide strong evidence that the B y dependence of the AL index is indeed much stronger than the suggested B x dependence. In fact, the analysis of Kubyshkina et al. (2023, https://doi.org/10.1029/2022ja031275 ) provided similar evidence. We also note that the physical mechanism proposed by Kubyshkina et al. (2023, https://doi.org/10.1029/2022ja031275 ) to explain the suggested B x dependence is contradicted by observations. However, we cannot completely rule out its existence.