Comment on “Unraveling the Role of IMF B_x in Driving Geomagnetic Activity” by Kubyshkina et al

Abstract

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 (By): the AL index is stronger for By > 0 than By < 0 during negative dipole tilt (Northern Hemisphere winter). Kubyshkina et al. (2023, https://doi.org/10.1029/2022ja031275) argued that this By dependence of the AL index is not really caused by IMF By but by the component along the Sun‐Earth line (x axis)—namely IMF Bx, which is strongly anticorrelated with By. Here we provide strong evidence that the By dependence of the AL index is indeed much stronger than the suggested Bx 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 Bx dependence is contradicted by observations. However, we cannot completely rule out its existence.

Interplanetary Magnetic Field By Effects on the Strength and Latitude of Field‐Aligned Currents in Different Magnetic Local Time Sectors

“…Laundal et al (2018) used the solar wind and IMF parameters averaged over the 20 min preceding the corresponding Swarm/CHAMP measurements. Holappa et al (2023) and Kubyshkina et al (2023) adopted 1 hr averaged IMF components spanning from 1981 to 2019 to investigate the IMF B x effect on AL index. To investigate the effect of delay times on the influence of IMF B y on FACs, we changed the time window size in 10-min intervals from 10 min up to 1 hr.…”

“…Holappa et al. (2023) and Kubyshkina et al. (2023) adopted 1 hr averaged IMF components spanning from 1981 to 2019 to investigate the IMF B x effect on AL index.…”

Effect of interplanetary magnetic field <i>B</i>_<i>x</i> on the polar electrojets as observed by CHAMP and Swarm satellites

“…As E m is primarily associated with the amplitudes of IMF B y and B z , it remains weak during times when IMF B x prevails. (Laundal et al, 2018b) or less than 2 nT (Holappa et al, 2023) in order to eliminate the influence of IMF B y . In our study, the variation in IMF B y falls within these specified ranges.…”