Fast Ion Heating in Transient Collisionless Magnetic Reconnection via an Intrinsic Stochastic Mechanism

Abstract

Stochastic heating has been known to be a powerful ion heating mechanism in the solar wind, atmosphere, and flares. In this Letter, we show that stochastic ion heating is inherent to transient collisionless magnetic reconnection. The explanation exploits the connected nature of electron canonical vorticity to show analytically that the in-plane electric and magnetic fields in a typical reconnection geometry satisfy the condition for stochastic heating of ions. Electron fluid simulations, test ion simulations, and comparisons to experiments all support the existence of this mechanism.

Kinetic Verification of the Stochastic Ion Heating Mechanism in Collisionless Magnetic Reconnection

“…It is well known that the Hall term in the generalized Ohm's law is important in collisionless reconnection (Birn et al 2001). Thus, strong in-plane Hall electric fields ( )J B E x x (color) develop; these have also been seen in previous studies (Wygant et al 2005;Li & Horiuchi 2008;Drake et al 2009b;Pritchett 2010;Aunai et al 2011;Yoon & Bellan 2018). Although a finite guide field alters the structure, E x is generally pointing toward x=0, meaning that ions are falling down a potential valley along the inflow.…”

“…in the lab frame. In the vicinity of the reconnection region, the in-plane electric field is mainly balanced by the Hall component (Li & Horiuchi 2008;Yoon & Bellan 2018), so whether Equation (3) satisfies Equation ( 1) is important for determining the existence of stochastic heating.…”

“…Figure 4(a) shows two typical ion outflow trajectories (black solid and dashed line) with an initial spatial separation of 0.04d i for b g =0. Both ions undergo oscillatory motion in the x-direction under the Hall electric field (color; Wygant et al 2005;Drake et al 2009b;Aunai et al 2011;Yoo et al 2013;Yoon & Bellan 2018). Figure 4(b) shows the time-dependent spatial ion separation, which exhibits divergence and thus a positive Lyapunov exponent.…”

“…Thus, stochastic heating is an enticing candidate for the anomalous ion heating mechanism in magnetic reconnection. A previous study (Yoon & Bellan 2018) used a two-fluid, zero-plasma-beta analysis of the generalized Ohm's law to predict that stochastic ion heating occurs in antiparallel reconnection. However, until now a fully self-consistent kinetic verification has been lacking, and the relation of stochastic ion heating to finite guide fields, ion magnetic moments, and ion mass-to-charge ratios has not been demonstrated.…”

Caltech Lab Experiments and the Insights They Provide Into Solar Corona Phenomena

“…These models are not only relevant to solar physics, but also to magnetospheric physics and to astrophysics which can have similar physics but at very different scales from the lab or the Sun. Examples of such theoretical models are the following: A model showing why flux ropes tend to be collimated (Bellan, 2003) A model for how accretion disks and astrophysical jets form a complete electric circuit that transfers angular momentum in a conservative way much like a generator transfers angular momentum via wires to a distant motor (Bellan, 2016a, 2018d) A model providing a time‐dependent analytic solution for an astrophysical jet (Bellan, 2020) A model for how energetic particles are created in the presence of sub‐Dreicer electric fields (Marshall & Bellan, 2019) A model providing an intuitive explanation for fast collisionless magnetic reconnection (Yoon & Bellan, 2017, 2019a) in the electron MHD context, that is, the context where the timescale is so fast that ions can be considered stationary, and the reconnection length scale is short compared to the ion skin depth, so Hall terms and electron inertia are important. A model showing that ions experience fast stochastic heating during fast collisionless reconnection (Yoon & Bellan, 2018, 2019b) A model showing how the reverse current associated with coronal mass ejection drives EUV fronts in the solar corona (Wongwaitayakornkul et al., 2019) …”