Timescales of Asymmetries in Magnetospheric Dynamics Induced by the IMF By Component

Abstract

Abstract

Recent studies have shown that global geomagnetic activity and particle precipitation areenhanced, when the dawn‐dusk (By) component of the interplanetary magnetic field (IMF) and Earth's magneticdipole tilt angle Ψ have opposite signs. This IMF By effect has been demonstrated so far only in statisticalstudies, which do not reveal its timescales or dynamical evolution. In this paper we study the response ofelectron precipitation, field‐aligned currents (FACs) and auroral electrojets during events where the IMF rotatesfrom northward or southward orientation to dawn/dusk orientation. The events are collected from years 1995–2022 separately for the winter (Ψ < 10°) and summer (Ψ > 10°) seasons, resulting into 573–730 events foreach dipole tilt and rotation category. Our results show that the IMF By‐effect in the nightside FACs and in thewestward electrojet takes 2 hr to develop and 3 hr to maximize, regardless of whether the dusk‐ or dawnward IMF turning occur from a predominantly northward or southward IMF sector. On the dayside, the IMF By‐effectwas found to be faster with timescales less than 1 hr, especially in the summer hemisphere. The observed 2–3 hrtimescales in the nightside suggests that an asymmetry build‐up in the magnetosphere by convecting openmagnetic field lines is required to manifest the IMF By‐effect. We show that the amount of open flux in themagnetosphere responds rapidly to IMF rotations and that the open flux is larger for the opposite signs of theIMF By and Ψ, suggesting a physical mechanism modulating the dayside reconnection rate.