Abstract
AbstractMagnetic reconnection at the magnetopause occurs with a large density asymmetry and for a large range of magnetic shears. In these conditions, a motion of the X line has been predicted in the direction of the electron diamagnetic drift. When this motion is super Alfvenic, reconnection should be suppressed. We analysed a large data set of Double Star TC‐1 dayside magnetopause crossings, which includes reconnection and nonreconnection events. Moreover, it also includes several events during which TC‐1 is near the X line. With these close events, we verified the diamagnetic suppression condition with local observations near the X line. Moreover, with the same close events, we also studied the motion of the X line along the magnetopause. It is found that, when reconnection is not suppressed, the X line moves northward or southward according to the orientation of the guide field, which is related to the interplanetary magnetic field BY component, in agreement with the diamagnetic drift.
Highlights
Magnetic reconnection between the interplanetary magnetic field (IMF) and the geomagnetic field is the main process that allows the transfer of solar wind mass, energy, and momentum into the Earth’s magnetosphere
The diamagnetic drift of ions and electrons causes a motion of the X line along the dayside magnetopause, which is proportional to the local pressure gradient and to the intensity of the guide field at the X line [Swisdak et al, 2003]
In order to investigate the effects of diamagnetic drift, we analysed a large data set of Double Star TC-1 magnetopause crossings (207) which includes both reconnection events and magnetopause crossings without reconnection signatures
Summary
Magnetic reconnection between the interplanetary magnetic field (IMF) and the geomagnetic field is the main process that allows the transfer of solar wind mass, energy, and momentum into the Earth’s magnetosphere. It is found that reconnection at the dayside magnetopause can occur when the local magnetic shear angle is quite low (90° or less), i.e., in presence of a strong guide field [Scurry et al, 1994; Phan et al, 1996; Trenchi et al, 2008] In these low shear conditions, according to the simulations of Swisdak et al [2003], the X line should experience a motion along the magnetopause due to the diamagnetic drift of ions and electrons. The main result of our paper is that by considering the latter subset, we are able to demonstrate statistically that the motion of the X line along the magnetopause is controlled by the orientation of the guide field This verifies a second prediction made by Swisdak et al [2010]; in their simulation, when the local Δβ and θ are in the nonsuppressed regime, the X line moves in the direction of the diamagnetic drift of electrons. Since the pressure gradient at the dayside magnetopause is directed outward, this results in the motion of the X line being controlled by the orientation of the guide field, i.e., by the BY GSM component of the IMF
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