Magnetic null points in kinetic simulations of space plasmas

TL;DR

This study investigates magnetic null points in kinetic space plasma simulations, revealing that spiral nulls are more common and are associated with significant magnetic energy dissipation, especially within flux ropes, unlike radial nulls.

Contribution

It provides a systematic analysis of magnetic null types and their role in energy conversion in 3D kinetic plasma simulations, highlighting the significance of spiral nulls.

Findings

Spiral nulls are more prevalent than radial nulls in simulations.

Energy dissipation is mainly associated with spiral nulls within flux ropes.

Radial nulls like X-lines are stable and less involved in energy dissipation.

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 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 lunar magnetic anomaly 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.