Magnetotail Configuration Under Northward IMF Conditions

Abstract

Due to the sparsity of space probes, it is still not clear on how the magnetic structure of the magnetotail looks like and how it evolves when the interplanetary magnetic field (IMF) directs northward. This simulation study uses two different global magnetosphere magnetohydrodynamics (MHD) models to simulate two northward IMF events and study the evolution of the magnetotail. Both models show that the magnetotail may form a structure that is composed of a dawnside tail lobe and a duskside tail lobe, under northward IMF conditions with significant By, instead of a northern tail lobe and a southern tail lobe under southward IMF conditions. In this magnetic configuration, a tail lobe extends a domain from northern (southern) cusp to southern (northern) IMF. The larger the magnitude of IMF clock angle, the longer and wider the magnetotail. Such magnetic configuration suggests that magnetotail reconnection is possible to occur when the dawnside tail lobe contacts with the duskside tail lobe and thus a substorm is also possible to occur under northward IMF conditions with significant By.

3D GUMICS Simulations of Northward IMF Magnetotail Structure

“…ARTEMIS data and the OpenGGCM global MHD model were used to study the conditions of northward IMF events and the resulting magnetotail structures. Their results were consistent with the topological features observed within their previous study Li et al (2021), as well as the alpha-like configurations shown in this study. Like KJ04, Li et al (2022) interpreted their results as consistent with the Chang et al (1998) model and described their formation of a closed field line region as being due to field lines that were trapped between new and old lobe regions (shown in their Figure 12).…”

“…It is not clear, from an observational point of view, how closely this resembles the real magnetosphere under northward IMF conditions (when the magnetotail's structure will be influenced by earlier periods of southward IMF and the pre-existing high-latitude field lines are open). However, we note that the structure in Figure 1a resembles that expected for reconnection with initially closed high-latitude field lines (Cowley, 1981(Cowley, , 1983Crooker, 1992;Fuselier et al, 2015;Milan et al, 2022) and also that observed in other simulations that are initialized with northward IMF conditions (e.g., Li et al, 2021Li et al, , 2022.…”

“…We see similarities and differences between our results and the conclusions from Li et al. (2021, 2022), which we make direct comparisons to throughout this paper.…”

“…They found that this only occurred in one of the two models, despite running both models with the same initialization conditions and solar wind parameters. We see similarities and differences between our results and the conclusions from Li et al (2021Li et al ( , 2022, which we make direct comparisons to throughout this paper. et al (2005) proposed a mechanism to explain how TPAs can form, based on a period of bursty nightside tail reconnection during northward IMF.…”

“…We do note that there is a key difference between our simulation results and observational interpretations, which is that the magnetotail reconnection occurs due to the "alpha-like" configuration of the magnetotail, which arises from the northward IMF initialization of the simulation, but bears similarities to dawn-dusk lobes which have been argued to form during northward IMF configurations by previous studies(Fuselier et al, 2018;Li et al, 2021Li et al, , 2022. A consequence of this "alpha-like" configuration is that the two twisted open field line regions are brought into contact by the rotation of the magnetotail.…”