Evidence and theory for trapped electrons in guide field magnetotail reconnection

Abstract

[1] A previous analysis of electron distributions measured in situ by the Wind spacecraft has revealed that electrons were trapped in the electromagnetic geometry of the reconnection event encountered in the deep magnetotail. In this paper we develop a detailed theory that can account for the main anisotropic features of the electron distributions associated with trapping in reconnection. The analysis shows that electron trapping in electric fields is generic in reconnection, as it is required in order to maintain the condition of quasineutrality. In addition to the spacecraft data, evidence of trapping in numerical simulations is also presented. Trapping is effective in eliminating free-streaming electrons along magnetic fields and thereby reduces parallel electron currents. Its importance for fast reconnection is discussed and emphasized by observations in a laboratory plasma.

Equations of State for Collisionless Guide-Field Reconnection

“…Similar pressure anisotropy is observed in fully kinetic reconnection simulations [7]. Recently, Egedal et al introduced a mechanism that accounts for the electron pressure anisotropy [8]. Extending those results, we present here, and confirm using the results of a kinetic simulation, equations of state for p k and p ?…”

“…Note that while the original model in Ref. [8] used a uniform f 1 far from the x line [in the shaded boxes in Fig. 1(a)], due to computational constraints on the simulation size, the PIC code uses an open boundary condition that varies f at the edge to eliminate gradients in the density, fluid flow, and pressure tensor at the boundary.…”

“…Þ derived in Ref. [8], which is generalized below. (Note that our model does not include gyrophase-dependent terms, which are sizable very near the x line [9], but otherwise negligible.)…”

“…The electron distribution in the ambient plasma, f 1 , is assumed gyrotropic (generalizing Ref. [8], which takes a fully isotropic distribution for simplicity) so that fðx; vÞ ¼ f 1 ðE k1 ; E ?1 Þ. Thus, to obtain fðx; vÞ we need only characterize the kinetic energies E k1 and E ?1 that the electron had before it entered the reconnection region.…”

VINETA II: A linear magnetic reconnection experiment

“…This process takes place in the current sheet formed at the boundary between opposed magnetic fields and is thought to play an important role in space phenomena, such as solar flares 4 and at Earth's magnetosphere 5 , as well as in fusion experiments 6,7 . Despite intense research on the topic over the last decades and a vast quantity of information obtained through space observations 8,9 , numerical simulations 10,11 and experiments [12][13][14][15][16][17][18][19] , the underlying processes of magnetic reconnection are still not fully understood. For plasmas with high resistivity and small length scales, simple magneto-hydrondynamic (MHD) models give an adequate description of reconnection 4,20,21 .…”