Magnetic reconnection, buoyancy, and flapping motions in magnetotail explosions

Abstract

A key process in the interaction of magnetospheres with the solar wind is the explosive release of energy stored in the magnetotail. Based on observational evidence, magnetic reconnection is widely believed to be responsible. However, the very possibility of spontaneous reconnection in collisionless magnetotail plasmas has been questioned in kinetic theory for more than three decades. In addition, in situ observations by multispacecraft missions (e.g., THEMIS) reveal the development of buoyancy and flapping motions coexisting with reconnection. Never before have kinetic simulations reproduced all three primary modes in realistic 2‐D configurations with a finite normal magnetic field. Moreover, 3‐D simulations with closed boundaries suggest that the tail activity is dominated by buoyancy‐driven instabilities, whereas reconnection is a secondary effect strongly localized in the dawn‐dusk direction. In this paper, we use massively parallel 3‐D fully kinetic simulations with open boundaries to show that sufficiently far from the planet explosive processes in the tail are dominated by reconnection motions. These motions occur in the form of spontaneously generated dipolarization fronts accompanied by changes in magnetic topology which extend in the dawn‐dusk direction over the size of the simulation box, suggesting that reconnection onset causes a macroscale reconfiguration of the real magnetotail. In our simulations, buoyancy and flapping motions significantly disturb the primary dipolarization front but neither destroy it nor change the near 2‐D picture of the front evolution critically. Consistent with recent multiprobe observations, dipolarization fronts are also found to be the main regions of energy conversion in the magnetotail.

Kinetic Dissipation Around a Dipolarization Front

“…Tailward of the DF a new X line forms because of the flux starvation effect (e.g., Bessho & Bhattacharjee, 2014;Pritchett, 2015;Sitnov et al, 2013) as is seen from Figure 1a showing the distribution of the equatorial magnetic field B z (x, z = 0) at 0i t = 32. They were identified earlier (Sitnov et al, 2014) as signatures of the long-wavelength (∼ 2 √ 0i 0e ) modification (Daughton, 2003) of the lower-hybrid drift instability (LHDI) of thin CSs with zero B z component (Huba et al, 1977), which is also similar in case of nonzero B z to the kinetic ballooning/interchange instability (Pritchett & Coroniti, 2010). It reveals in particular the formation of the EDR as identified by the enhancement of j ⋅ E ′ near the X line .…”

Current sheet flapping in the near-Earth magnetotail: peculiarities of propagation and parallel currents

“…Several theoretical models predict the substantial variations in B z with x in oscillating CSs Pritchett and Coroniti, 2010;Sitnov et al, 2014). The large distances between Cluster spacecraft allow us to investigate the homogeneity of the flapping wave within spatial scale around 10 4 km (i.e.…”

Evolution of generalized two‐dimensional magnetotail equilibria in ideal and resistive MHD

“…This is clear from equations and showing that the stabilizing term under the integral (the last one) is inversely proportional to the equilibrium flux tube volume; i.e., the flux tube has to be sufficiently long to reduce the stabilizing term and allow an instability (this is equivalent to saying that ). The previous kinetic simulations [ Sitnov et al ., , ; Bessho and Bhattacharjee , ] effectively resolved this problem by using simulations with open boundaries. While computational constraints in MHD simulations are not as stringent, expanding the simulation box is still challenging.…”

“…Since the analysis of Sitnov and Schindler [] showed that the kinetic stabilization parameter can be reduced in equilibria with a magnetic flux accumulation leading to a potential for instability, it is plausible to contemplate that it may not be sufficiently stabilizing in ideal MHD either. In addition, kinetic simulations [ Sitnov and Swisdak , ; Sitnov et al ., , ; Bessho and Bhattacharjee , ] reveal interesting plasma dynamics, e.g., the formation of a DF, prior to a change in topology (magnetic reconnection per se). Although those dynamics are not necessarily ideal, the fact that they take place before reconnection motivates us to investigate the same equilibrium configurations in the MHD approximation.…”