Flux transfer events: Motion and signatures

Abstract

We present results from a 2.5 dimensional hybrid code simulation for the evolution of flux transfer events (FTEs) during intervals of due southward interplanetary magnetic field (IMF) orientation. The structures invariably form between pairs of reconnection lines, often remaining nearly stationary on the subsolar magnetopause before their motion begins. Although a few structures move sunward, ultimately coalescing with others, most eventually accelerate antisunward, reaching velocities many times greater than the sound speed in the magnetosheath but only about one Alfvén speed greater than the ambient magnetosheath flow. At these velocities, slow mode wakes (but not shocks) marked by density enhancements and magnetic field strength decreases extend outward from the structures into the magnetosheath. Upon encountering the cusps, the structures decelerate, undergo reconnection, and are destroyed.

Introduction

Transient (∼1–2 min) events are common in the immediate vicinity of the Earth's dayside magnetopause. Those exhibiting magnetic field strength enhancements and symmetric bipolar magnetic field signatures normal to the nominal magnetopause are termed flux transfer events or FTEs (Russell and Elphic, 1978). However, FTEs can also display depressed or crater-like magnetic field strength variations (Labelle et al., 1987) and/or asymmetric bipolar magnetic field signatures normal to the nominal magnetopause (Fear et al., 2010). Because they tend to occur for southward interplanetary magnetic field orientations and strongly sheared magnetosheath and magnetospheric magnetic field configurations (Berchem and Russell, 1984, Rijnbeek et al., 1984) and frequently exhibit accelerated mixtures of streaming magnetosheath and magnetospheric plasmas with densities intermediate between those of either region (Paschmann et al., 1982, Daly et al., 1984), FTEs observed near local noon are generally interpreted in terms of bursty reconnection. If FTEs contribute significantly to (Lockwood et al., 1990) or dominate (Lockwood et al., 1995) the overall solar wind–magnetosphere interaction, then studies of FTEs may tell us much about when, where, and how reconnection occurs as a function of solar wind conditions.

For example, case and statistical studies of observed (Korotova et al., 2009), calculated (Dunlop et al., 2005), or inferred (Rijnbeek et al., 1984, Berchem and Russell, 1984) structure velocities can reveal the location(s) of the reconnection line. In the absence of readily available high spatial and temporal resolution results from global numerical simulations, several researchers have used the analytical Cooling et al. (2001) model for the motion of reconnected magnetosheath and magnetospheric magnetic field lines to demonstrate that the locations where FTEs are observed are consistent with their formation along postulated subsolar reconnection lines (e.g., Dunlop et al., 2005, Fear et al., 2005, Wild et al., 2005, Wild et al., 2007). In this model, FTEs move at the sum of the local magnetosheath plasma and Alfvén velocities throughout their existence, i.e. they always move at the Alfvén velocity through the ambient magnetosheath flow. The model predicts structure speeds within a factor of 2 and directions to within 30° of those inferred from multispacecraft timing in 78% of cases in which structure dimensions exceed 5000 km (Fear et al., 2007).

The speed at which structures move through the ambient media is also an important factor in determining the signatures that they produce. Farrugia et al. (1988) showed that an extended cylinder moving through an incompressible magnetohydrodynamic plasma generates transient magnetic field strength enhancements and bipolar magnetic field signatures normal to the nominal magnetopause, as frequently observed. However, when Mach numbers approach or exceed unity compressible solutions must be employed. Fig. 1 illustrates the solution regimes predicted for a cylindrical structure moving through a compressible plasma when the ambient magnetic field lies perpendicular to the structure axis (Sonnerup et al., 1992). Supersonic and superAlfvénic structures generate fast mode shocks marked by magnetic field strength and density enhancements, while subsonic and subAlfvénic structures moving faster than the slow mode speed generate slow mode shocks marked by density enhancements but magnetic field strength decreases. Structures moving at velocities much slower than the slow mode velocity, superAlfvénic but subsonic velocities, or supersonic but subAlfvénic velocities generate magnetic field strength and density perturbations, but no shocks.

When the sum of the pressure gradient and magnetic curvature forces acting upon structures is finite, they accelerate. With the advent of multispacecraft missions, it has become possible to calculate this rate of acceleration. Fear et al. (2009) demonstrated that the rate of acceleration was negligible in a case study of multipoint THEMIS observations near the flanks of the magnetosphere.

As space plasma physics transitions towards an environment in which results from global simulations based on first-principle physics become increasingly available, there is a need to compare the predictions of numerical simulations with those of the analytical models. Omidi and Sibeck (2007) presented results from a 2.5 dimensional global hybrid code simulation for the interaction of the solar wind with the magnetosphere indicating the frequent occurrence of FTEs on the dayside magnetopause. They identified one large structure that generated a slow mode bow wave as it accelerated towards the northern cusp and reached a speed approaching one Alfvén velocity faster than the ambient magnetosheath flow. This paper presents more detailed observations of the same structure and examines the predictions of the hybrid code model for more structures in the same run, addressing the locations where they are generated, their velocity and acceleration, and the slow mode bow waves that they generate. It places the new simulation results within the context of previously reported observations, simulations, and theoretical models.