Magnetic Null Points in Kinetic Simulations of Space Plasmas

Abstract

We present a systematic attempt to study magnetic null points and the associated magnetic energy conversion in kinetic particle-in-cell simulations of various plasma configurations. We address three-dimensional simulations performed with the semi-implicit kinetic electromagnetic code iPic3D in different setups: variations of a Harris current sheet, dipolar and quadrupolar magnetospheres interacting with the solar wind,and a relaxing turbulent configuration with multiple null points. Spiral nulls are more likely created in space plasmas: in all our simulations except lunar magnetic anomaly (LMA) and quadrupolar mini-magnetosphere the number of spiral nulls prevails over the number of radial nulls by a factor of 3-9. We show that often magnetic nulls do not indicate the regions of intensive energy dissipation. Energy dissipation events caused by topological bifurcations at radial nulls are rather rare and short-lived. The so-called X-lines formed by the radial nulls in the Harris current sheet and LMA simulations are rather stable and do not exhibit any energy dissipation. Energy dissipation is more powerful in the vicinity of spiral nulls enclosed by magnetic flux ropes with strong currents at their axes (their crosssections resemble 2D magnetic islands). These null lines reminiscent of Z-pinches efficiently dissipate magnetic energy due to secondary instabilities such as the two-stream or kinking instability, accompanied by changes in magnetic topology. Current enhancements accompanied by spiral nulls may signal magnetic energy conversion sites in the observational data.

Reiner Gamma albedo features reproduced by modeling solar wind standoff

“…The 3-D simulated reflected proton flux distribution is highly non-uniform, both spatially and rather an ensemble to provide insight into the magnetic topology at steady state. Indicated as well are the regions where the charge-separation electric field is greatest 18 (bright green shading: E x > 40 mV m −1 ) and where magnetic null points might be found 62 (red shading: B j j < 10 −4 nT). The bright green volume above the tadpole's head has a maximum thickness of 8.2 km.…”

Nonlinear waves and instabilities leading to secondary reconnection in reconnection outflows

“…We use the technique based on the topological degree method (Greene 1992) as described in Olshevsky et al. (2016) to locate and classify magnetic nulls. Indeed, in the simulation reported here, no magnetic nulls are present in the central current sheet as summarized in figure 5.…”

“…This sort of magnetic reconnection, however, does not require the field to become exactly zero (hence, no magnetic nulls are formed) on the reconnection site. We use the technique based on the topological degree method (Greene 1992) as described in Olshevsky et al (2016) to locate and classify magnetic nulls. Indeed, in the simulation reported here, no magnetic nulls are present in the central current sheet as summarized in figure 5.…”

Properties of Turbulence in the Reconnection Exhaust: Numerical Simulations Compared with Observations

“…It is sometimes referred to as the dissipative term, even though the energy transfer from magnetic field to parti- cles is not always an irreversible process, and so it does not strictly imply dissipation. Despite this fact, for the purpose of our paper we will keep this definition, also used elsewhere (Zenitani et al 2011;Olshevsky et al 2015Olshevsky et al , 2016, and from now on we will use D l as a proxy for dissipation or more properly energy release from the electromagnetic field (in the laboratory frame). A 2D plot of D l integrated in the z direction is shown in Figure 2.…”