How the IMF B_y induces a B_y component in the closed magnetosphere and how it leads to asymmetric currents and convection patterns in the two hemispheres

Abstract

We used the Lyon‐Fedder‐Mobarry global magnetohydrodynamics model to study the effects of the interplanetary magnetic field (IMF) By component on the coupling between the solar wind and magnetosphere‐ionosphere system. When the IMF reconnects with the terrestrial magnetic field with IMF By≠0, flux transport is asymmetrically distributed between the two hemispheres. We describe how By is induced in the closed magnetosphere on both the dayside and nightside and present the governing equations. The magnetosphere imposes asymmetric forces on the ionosphere, and the effects on the ionospheric flow are characterized by distorted convection cell patterns, often referred to as “banana” and “orange” cell patterns. The flux asymmetrically added to the lobes results in a nonuniform induced By in the closed magnetosphere. By including the dynamics of the system, we introduce a mechanism that predicts asymmetric Birkeland currents at conjugate foot points. Asymmetric Birkeland currents are created as a consequence of y directed tension contained in the return flow. Associated with these currents, we expect fast localized ionospheric azimuthal flows present in one hemisphere but not necessarily in the other. We also present current density measurements from Active Magnetosphere and Planetary Electrodynamics Response Experiment that are consistent with this picture. We argue that the induced By produces asymmetrical Birkeland currents as a consequence of asymmetric stress balance between the hemispheres. Such an asymmetry will also lead to asymmetrical foot points and asymmetries in the azimuthal flow in the ionosphere. These phenomena should therefore be treated in a unified way.

Magnetospheric response and reconfiguration times following IMF By reversals

“…The ratio indicates that for their step function IMF B y transition the final reconfiguration time should be close to 30 min. For this reason the conclusion in Tenfjord et al [] that the IMF B y induces a B y component on closed field lines on times scales of tens of minutes is in agreement with the empirical results of this study. Tenfjord et al [] found a shorter reconfiguration time on the dayside.…”

“…In this scenario, the response on closed field lines is expected to be considerably slower compared to the more “direct” shear flows inducing B y and asymmetric foot points. This differs from the mechanism explained in Tenfjord et al [] in that it relies on tail reconnection in order to produce B y on closed field lines.…”

“…Tenfjord et al [] modeled the response of the IMF B y changing from zero to 10 nT as a simple step function. The authors showed that after 25 min, a magnetospheric B y component was established at L = 11 R E on the nightside.…”

“…These flows are directed in opposite directions in the two hemispheres. Following this idea, first suggested by Khurana et al [], and adapted by Liou and Newell [], Tenfjord et al [] argued that these flows also affect closed field lines already present in the magnetosphere and therefore induce a B y component on closed field lines.…”

“…Magnetospheric B y perturbations can be caused by any combination of the following mechanisms: IMF‐induced B y : asymmetric loading of magnetic flux in the lobes [ Tenfjord et al , ]. This is illustrated in Figure a, looking from the magnetotail toward the Sun for a positive IMF B y .…”