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 degrees 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.