Relations Between vz and Bx Components in Solar Wind and their Effect on Substorm Onset

Abstract

We analyze two substorm onset lists, produced by different methods, and show that the (Bx·vz) product of the solar wind (SW) velocity and interplanetary magnetic field (IMF) components for two thirds of all substorm onsets has the same sign as IMF Bz. The explanation we suggest is the efficient displacement of the magnetospheric plasma sheet due to IMF Bx and SW flow vz, which both force the plasma sheet moving in one direction if the sign of (Bx·vz) correlates with the sign Bz. The displacement of the current sheet, in its turn, increases the asymmetry of the magnetotail and can alter the threshold of substorm instabilities. We study the SW and IMF data for the 15‐year period (which comprises two substorm lists periods and the whole solar cycle) and reveal the similar asymmetry in the SW, so that the sign of (Bx·vz) coincides with the sign of IMF Bz during about two thirds of all the time. This disproportion can be explained if we admit that about 66% of IMF Bz component is transported to the Earth’s orbit by the Alfvén waves with antisunward velocities.Plain Language SummaryWe explore the interplanetary magnetic field configuration together with the direction of the solar wind flow velocity to demonstrate that the plasma sheet motion, which is caused by both magnetic field component Bx and solar wind flow component vz, may affect the substorm onset. The obtained statistical result shows that the number of substorms, which broke in a situation when IMF Bz has the same sign as (Bx·vz) (favorable for the plasma sheet displacement), is 30–50% larger than the number of other substorms. The same relation was obtained for two different substorm onset databases, one obtained from Imager For Magnetopause‐to‐Aurora Global Exploration Far Ultraviolet observations and the other from ground‐based AL index.Key PointsUsing two substorm lists, we show that for two thirds of all substorm onsets, the (Bx·vz) product in the solar wind has the same sign as the IMF BzWhen the signs of the product (Bx·vz) and the IMF Bz coincide, the asymmetry (bending) of the magnetotail current sheet increasesSigns correlation of (Bx·vz) and Bz in the SW (a feature of Alfvénic waves propagating from the Sun) is observed during two thirds of all the time