Large Scale Linear Magnetic Holes with Magnetic Mirror Properties in Hybrid Simulations of Solar Wind Turbulence

TL;DR

This study uses hybrid simulations to demonstrate that magnetic holes in solar wind turbulence originate from ion trapping mechanisms, exhibiting properties consistent with space observations and providing new diagnostic tools for their detection.

Contribution

It introduces a self-consistent simulation approach showing how magnetic holes form from plasma turbulence and ion trapping, with new metrics for identifying MHs in satellite data.

Findings

Magnetic holes develop from initial magnetic perturbations in turbulence.

Ion trapping leads to temperature anisotropy stabilizing MHs.

New diagnostic based on local magnetic and ion temperature measures.

Abstract

Magnetic holes (MHs) are coherent magnetic field dips whose size ranges from fluid to kinetic scale, ubiquitously observed in the heliosphere and in planetary environments. Despite the longstanding effort in interpreting the abundance of observations, the origin and properties of MHs are still debated. In this letter, we investigate the interplay between plasma turbulence and MHs, using a 2D hybrid simulation initialized with solar wind parameters. We show that fully developed turbulence exhibits localized elongated magnetic depressions, whose properties are consistent with linear MHs frequently encountered in space. The observed MHs develop self-consistently from the initial magnetic field perturbations, by trapping hot ions with large pitch angles. Ion trapping produces an enhanced perpendicular temperature anysotropy that makes MHs stable for hundreds of ion gyroperiods, despite the surrounding turbulence. We introduce a new quantity, based on local magnetic field and ion temperature values, to measure the efficiency of ion trapping, with potential applications to the detection of MHs in satellite measurements. We complement this method by analyzing the ion velocity distribution functions inside MHs. Our diagnostics reveal the presence of trapped gyrotropic ion populations, whose velocity distribution is consistent with a loss cone, as expected for the motion of particles inside a magnetic mirror. Our results have potential implications for the theoretical and numerical modelling of MHs.