Unraveling the Role of IMF Bx in Driving Geomagnetic Activity

In this work, we question the existing opinion that the interplanetary magnetic field (IMF) Sun‐Earth (Bx) component is of minor importance for the geomagnetic activity and show that both Bx and By, being two principal components of the Parker's spiral, are equally important factors. We show that the asymmetric behavior of activity indices around solstices can be explained as a combination of symmetric (sign‐independent) effect of IMF By and asymmetric (sign‐dependent) effect of Bx, the latter interpreted as a result of the geodipole tilt influence on the magnetotail stability and nightside reconnection. We also show that, due to the 7.5° inclination of the Earth orbital plane to the solar equatorial plane, the geoeffective IMF Bz component may increase or decrease, depending on the IMF sector polarity and season. Plain Language Summary Interaction of the solar wind with the Earth's magnetic field drives the magnetospheric activity and space weather. Magnetosphere's response to the external driving can be forecasted, once the incoming solar wind state and its dynamics are known. Of great importance for space weather forecasting is the interplanetary magnetic field (IMF). When its North‐South component (Bz) turns southward, the magnetosphere energy accumulates and the geomagnetic activity increases. The other two IMF components that lie in the equatorial plane also affect the magnetosphere, but there exist different opinions on their role. In this paper we show that both Sun‐Earth (Bx) and East‐West (By) components are equally important. The latter component mostly affects the magnetic field dynamics on the dayside, such that its increase (regardless of orientation) enhances the activity. IMF Bx component affects the magnetosphere in a more complex way: its impact is small at equinoxes but increases at solstices. Due to the global spiral‐shaped IMF geometry, Bx and By are strongly correlated and, hence, act simultaneously. Therefore, to evaluate the IMF impact on the magnetosphere, one should properly take into account the magnitude/orientation of all its components, combined with inclination of the Earth's dipole axis to the terminator plane. Key Points AL‐index dynamics is controlled by both X and Y components of interplanetary magnetic field, with and without sign dependence, respectively Interplanetary magnetic field (IMF) Bx is efficient only for nonzero values of geodipole tilt Ψ; AL‐index increases/decreases if Ψ and Bx have the same/opposite signs O