| Abstrakt: |
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. Plain Language Summary: The most important driver controlling magnetospheric activity, such as displays of aurora and disturbances in the terrestrial magnetic field is the north‐south directed z component of the magnetic field in solar wind. Recent research has shown that during winter and summer solstices the magnetospheric activity is significantly modulated also by the dawn‐dusk y component, which is statistically correlated with the Sun‐Earth directed x component, due to the well‐known geometry of the magnetic field in solar wind. It is therefore challenging to separate the effects of y and x components. Here we show that magnetospheric activity is strongly modulated by the y component, but not by the x component, contrary to Kubyshkina et al. (2023, https://doi.org/10.1029/2022ja031275), who argued that the opposite is true. Key Points: The interplanetary magnetic field (IMF) Bx component does not significantly modulate the AL index as proposed by Kubyshkina et al. (2023, https://doi.org/10.1029/2022ja031275)The AL index is strongly modulated by IMF By during periods of large dipole tilt [ABSTRACT FROM AUTHOR] |
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