A new scalar measure of the gyrotropy of a pressure tensor is defined. Previously suggested measures are shown to be incomplete by means of examples for which they give unphysical results. To ...demonstrate its usefulness as an indicator of magnetic topology, the new measure is calculated for electron data taken from numerical simulations of magnetic reconnection, shown to peak at separatrices and X points, and compared to the other measures. The new diagnostic has potential uses in analyzing spacecraft observations, and so a method for calculating it from measurements performed in an arbitrary coordinate system is derived.
Key Points
The new quantity measures pressure tensor gyrotropy
Previous measures do not correctly describe gyrotropy
The new measure identifies regions of interesting magnetic topology
The first self-consistent simulations of electron acceleration during magnetic reconnection in a macroscale system are presented. Consistent with solar flare observations, the spectra of energetic ...electrons take the form of power laws that extend more than two decades in energy. The drive mechanism for these nonthermal electrons is Fermi reflection in growing and merging magnetic flux ropes. A strong guide field suppresses the production of nonthermal electrons by weakening the Fermi drive mechanism. For a weak guide field the total energy content of nonthermal electrons dominates that of the hot thermal electrons even though their number density remains small. Our results are benchmarked with the hard x-ray, radio, and extreme ultraviolet observations of the X8.2-class solar flare on September 10, 2017.
The structure of magnetic flux ropes injected into the solar wind during reconnection in the coronal atmosphere is explored with particle-in-cell simulations and compared with in situ measurements of ...magnetic “switchbacks” from the Parker Solar Probe. We suggest that multi-x-line reconnection between open and closed flux in the corona injects flux ropes into the solar wind and that these flux ropes convect outward over long distances before eroding due to reconnection. Simulations that explore the magnetic structure of flux ropes in the solar wind reproduce the following key features of the switchback observations: a rapid rotation of the radial magnetic field into the transverse direction, which is a consequence of reconnection with a strong guide field; and the potential to reverse the radial field component. The potential implication of the injection of large numbers of flux ropes in the coronal atmosphere for understanding the generation of the solar wind is discussed.
Abstract
We conduct two-dimensional particle-in-cell simulations to investigate the scattering of electron heat flux by self-generated oblique electromagnetic waves. The heat flux is modeled as a ...bi-kappa distribution with a
T
∥
>
T
⊥
temperature anisotropy maintained by continuous injection at the boundaries. The anisotropic distribution excites oblique whistler waves and filamentary-like Weibel instabilities. Electron velocity distributions taken after the system has reached a steady state show that these instabilities inhibit the heat flux and drive the total distributions toward isotropy. Electron trajectories in velocity space show a circular-like diffusion along constant energy surfaces in the wave frame. The key parameter controlling the scattering rate is the average speed, or drift speed
v
d
, of the heat flux compared with the electron Alfvén speed
v
Ae
, with higher drift speeds producing stronger fluctuations and a more significant reduction of the heat flux. Reducing the density of the electrons carrying the heat flux by 50% does not significantly affect the scattering rate. A scaling law for the electron scattering rate versus
v
d
/
v
Ae
is deduced from the simulations. The implications of these results for understanding energetic electron transport during energy release in solar flares are discussed.
The waves generated by high-energy proton and alpha particles streaming from solar flares into regions of colder plasma are explored using particle-in-cell simulations. Initial distribution functions ...for the protons and alphas consist of two populations: an energetic, streaming population represented by an anisotropic (T∥ > T⊥), one-sided kappa function and a cold, Maxwellian background population. The anisotropies and nonzero heat fluxes of these distributions destabilize oblique waves with a range of frequencies below the proton cyclotron frequency. These waves scatter particles out of the tails of the initial distributions along constant-energy surfaces in the wave frame. Overlap of the nonlinear resonance widths allows particles to scatter into near-isotropic distributions by the end of the simulations. The dynamics of 3He are explored using test particles. Their temperatures can increase by a factor of nearly 20. Propagation of such waves into regions above and below the flare site can lead to heating and transport of 3He into the flare acceleration region. The amount of heated 3He that will be driven into the flare site is proportional to the wave energy. Using values from our simulations, we show that the abundance of 3He driven into the acceleration region should approach that of 4He in the corona. Therefore, waves driven by energetic ions produced in flares are a strong candidate to drive the enhancements of 3He observed in impulsive flares.
Non-Maxwellian electron velocity distribution functions composed of a warm bulk population and a cold beam are directly measured during electron-only reconnection with a strong out-of-plane (guide) ...magnetic field in a laboratory plasma. Electron heating is localized to the separatrix, and the electron temperature increases continuously along the separatrix. The measured gain in enthalpy flux is 70% of the incoming Poynting flux. The electron beams are oppositely directed on either side of the X point, and their velocities are comparable to, and scale with, the electron Alfvén speed. Particle-in-cell simulations are consistent with the measurements. The experimental results are consistent with, and go beyond, recent observations in the magnetosheath.
During magnetic reconnection, the field lines must break and reconnect to release the energy that drives solar and stellar flares and other explosive events in space and in the laboratory. Exactly ...how this happens has been unclear, because dissipation is needed to break magnetic field lines and classical collisions are typically weak. Ion-electron drag arising from turbulence, dubbed 'anomalous resistivity', and thermal momentum transport are two mechanisms that have been widely invoked. Measurements of enhanced turbulence near reconnection sites in space and in the laboratory support the anomalous resistivity idea but there has been no demonstration from measurements that this turbulence produces the necessary enhanced drag. Here we report computer simulations that show that neither of the two previously favoured mechanisms controls how magnetic field lines reconnect in the plasmas of greatest interest, those in which the magnetic field dominates the energy budget. Rather, we find that when the current layers that form during magnetic reconnection become too intense, they disintegrate and spread into a complex web of filaments that causes the rate of reconnection to increase abruptly. This filamentary web can be explored in the laboratory or in space with satellites that can measure the resulting electromagnetic turbulence.
Kinetic aspects of energy conversion and dissipation near a dipolarization front (DF) in the magnetotail are considered using fully kinetic 3‐D particle‐in‐cell simulations. The energy conversion is ...described in terms of the pressure dilatation, as well as the double contraction of deviatoric pressure tensor and traceless strain rate tensor, also known as the Pi‐D parameter in turbulence studies. It is shown that in contrast to the fluid dissipation measure, the Joule heating rate, which cannot distinguish between ion and electron dissipation and reveals deep negative dips at the DF, the Pi‐D parameters, as kinetic analogs of the Joule heating rate, are largely positive and drastically different for ions and electrons. Further analysis of these parameters suggests that ions are heated at and ahead of the DF due to their reflection from the front, while electrons are heated at and behind the DF due to the long‐wavelength lower‐hybrid drift instability.
Plain Language Summary
We explore new measures of plasma dissipation in rapidly contracting tubes of magnetic flux and plasma on the nightside of the terrestrial magnetosphere. These contracting tubes make the stretched tail‐like magnetic field more dipolar and have sharp profiles of plasma density and magnetic field at the leading edge. Relaxation of the stretched magnetic field releases the energy, which is spent for plasma acceleration and heating. Since collisions are extremely rare, the energy dissipation processes are different for electrons and ions and hence require special quantitative measures. Here we derive such measures from massively parallel three‐dimensional particle‐in‐cell simulations of the tail plasmas and demonstrate that as expected for measures of dissipation, they are positive on average and different for ions and electrons. The new quantitative measures allow us to reveal specific physical processes responsible for energy dissipation.
Key Points
Newly derived kinetic dissipation parameters are largely positive and different for ions and electrons
Ion dissipation is dominated by ion reflection from fronts
Electron dissipation is dominated by the lower‐hybrid drift instability
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