Electron and ion velocity distribution functions across one-dimensional tangential discontinuities: particle-in-cell simulations

<div><span>Tangential discontinuities are finite-width current sheets separating two magnetized plasmas with different macroscopic properties. Such structures have been measured in-situ in the solar wind plasma by various space missions. Also, under certain conditions, the terrestrial magnetopause can be approximated with a tangential discontinuity. Studying the microstructure of tangential discontinuities is fundamentally important to understand the transfer of mass, momentum and energy in space plasmas. The propagation of solar wind discontinuities and their interaction with the terrestrial magnetosphere play a significant role for space weather science. In this paper we use 1d3v electromagnetic particle-in-cell simulations to study the kinetic structure and stability of one-dimensional tangential discontinuities. The simulation setup corresponds to a plasma slab configuration which allows the simultaneous investigation of two discontinuities at the interface between the slab population and the background plasma. The initial discontinuities are infinitesimal and evolve rapidly towards finite-width transition layers. We focus on tangential discontinuities with and without perpendicular velocity shear. Three-dimensional velocity distribution functions are computed in different locations across the discontinuities, at different time instances, for both electrons and ions. We emphasize the space and time evolution of the velocity distribution functions inside the transition layers and discuss their deviation from the initial Maxwellian distributions. The simulated distributions show similar features with the theoretical solutions provided by Vlasov equilibrium models. </span></div>

Plasma and magnetic-field structures near the Martian induced magnetosphere boundary

The interaction between the solar wind and the Martian induced magnetosphere can lead to the formation of various regions with different plasma and magnetic-field characteristics. In this paper, these structures are investigated based on the plasma and magnetic-field measurements from Mars Atmosphere and Volatile EvolutioN (MAVEN). We find that the structures upstream of Mars are similar to those around Earth: both have a bow shock, magnetosheath, magnetopause, and a magnetosphere or induced magnetosphere. The inner part of Martian magnetosheath is called a plasma depletion region (PDR), similar to the plasma depletion layer upstream of the Earth’s magnetopause, in which the magnetosheath magnetic fields are piled up and the magnetosheath plasmas (including ions and electrons) are partially depleted. Several cases of PDRs are examined in detail. The hotter plasmas in PDRs are squeezed out along the enhanced magnetic field, resulting in the decrease of the plasma beta, the plasma density, and the ion temperature. The boundary between the magnetosheath and the induced magnetosphere is called the magnetopause, which can be identified as a magnetohydrodynamic discontinuity, either tangential discontinuity (TD) or rotational discontinuity, where the magnetic field changes its orientation. Tangential discontinuities with an insignificant normal component (BN ≈ 0) of the magnetic field are the focus of this study. This discontinuity separates the magnetosheath H+ ions from the heavy ions (e.g. O+, O2+) in the induced magnetosphere. Inside a TD, ions from both sides are mixed. There are 3332 boundary crossings by MAVEN in 2015, 1075 cases of which are identified as the TD (including the potential TD). Tangential discontinuities at Mars are at higher locations in the southern hemisphere and have an average thickness of ~200 km, mostly ranging from 50 to 400 km. The sample of TD is a decreasing function of θ (θ is the magnetic field rotation angle on the two sides of the TD). The PDRs in front of TDs are thicker in the northern hemisphere. From the sub-solar point to the Mars tail, PDR thickness increases and the proton number density and temperature decrease.

Deprojecting galaxy-cluster cold fronts: evidence for bulk, magnetized spiral flows

ABSTRACT Tangential discontinuities known as cold fronts (CFs) are abundant in groups and clusters of galaxies (GCs). The relaxed, spiral-type CFs were initially thought to be isobaric, but a significant, $10{{\ \rm per\ cent}}$–$20{{\ \rm per\ cent}}$ jump in the thermal pressure Pt was reported when deprojected CFs were stacked, interpreted as missing Pt below the CFs (i.e. at smaller radii r) due to a locally enhanced non-thermal pressure Pnt. We report a significant (∼4.3σ) deprojected jump in Pt across a single sharp CF in the Centaurus cluster. Additional seven CFs are deprojected in the GCs A2029, A2142, A2204, and Centaurus, all found to be consistent (stacked: ∼1.9σ) with similar pressure jumps. Combining our sample with high quality deprojected CFs from the literature indicates pressure jumps at significance levels ranging between 2.7σ and 5.0σ, depending on assumptions. Our nominal results are consistent with Pnt ≃ (0.1–0.3)Pt just below the CF. We test different deprojection and analysis methods to confirm that our results are robust, and show that without careful deprojection, an opposite pressure trend may incorrectly be inferred. Analysing all available deprojected data, we also find: (i) small variations around the mean density and temperature CF contrast q within each GC, monotonically increasing with the GC mass M200 as $q\propto M_{200}^{0.23\pm 0.04}$; (ii) hydrostatic mass discontinuities indicating fast bulk tangential flows below all deprojected CFs, with a mean Mach number ∼0.76; and (iii) the newly deprojected CFs are consistent (stacked: ∼2.9σ) with a $1.25^{+0.09}_{-0.08}$ metallicity drop across the CF. These findings suggest that GCs quite generally harbour extended spiral flows.

Visualization of gas dynamics discontinuities in supersonic flows using digital image processing methods

Рассматриваются подходы к выделению особенностей газодинамических полей, полученных при помощи методов сквозного счета. Для определения положения и типа разрыва из численного решения привлекаются идеи и методы цифровой обработки изображений, в частности методы выделения контуров объектов, основанные на разрывах яркости изображения. Для классификации газодинамических разрывов (нормальные ударные волны, косые ударные волны, тангенциальные разрывы, контактные разрывы) используются условия динамической совместности. Разработанный подход применим к результатам расчетов, полученных любым методом сквозного счета, облегчая и ускоряя обработку результатов численного моделирования, а также повышая объективность интерпретации полученных результатов. Приводятся примеры визуализации газодинамических разрывов, возникающих при дифракции и рефракции ударных волн. A number of approaches to the detection of features of gas dynamics fields obtained with shockcapturing methods are considered. In order to determine the location and type of gas dynamic discontinuities from the numerical solution, the ideas and methods of digital image processing are applied, in particular, the methods of detection of image contours based on the brightness of the image. Conditions of dynamic compatibility are used to classify gas the dynamic discontinuities, such as the normal shock waves, the oblique shock waves, the tangential discontinuities, the contact discontinuities, and the compression waves. The developed approach is not dependent on a specific type of the problem to be solved and is applicable to the calculations obtained with any shockcapturing method, facilitating and speeding up the processing of simulation results and increasing the objectivity of the interpretation of the results. Some examples of visualization of gas dynamic discontinuities arising in the diffraction and refraction of shock waves are given.

An overview of flux braiding experiments

In a number of papers dating back to the 1970s, Parker has hypothesized that, in a perfectly ideal environment, complex photospheric motions acting on a continuous magnetic field will result in the formation of tangential discontinuities corresponding to singular currents. I review direct numerical simulations of the problem and find that the evidence points to a tendency for thin but finite-thickness current layers to form, with thickness exponentially decreasing in time. Given a finite resistivity, these layers will eventually become important and cause the dynamical process of energy release. Accordingly, a body of work focuses on evolution under continual boundary driving. The coronal volume evolves into a highly dynamic but statistically steady state where quantities have a temporally and spatially intermittent nature and where the Poynting flux and dissipation are decoupled on short time scales. Although magnetic braiding is found to be a promising coronal heating mechanism, much work remains to determine its true viability. Some suggestions for future study are offered.