The Kelvin-Helmholtz Instability From the Perspective of Hybrid Simulations

The flow shear-driven Kelvin-Helmholtz (KH) instability is ubiquitous in planetary magnetospheres. At Earth these surface waves are important along the dawn and dusk flanks of the magnetopause boundary while at Jupiter and Saturn the entire dayside magnetopause boundary can exhibit KH activity due to corotational flows in the magnetosphere. Kelvin-Helmholtz waves can be a major ingredient in the so-called viscous-like interaction with the solar wind. In this paper, we review the KH instability from the perspective of hybrid (kinetic ions, fluid electrons) simulations. Many of the simulations are based on parameters typically found at Saturn’s magnetopause boundary, but the results can be generally applied to any KH-unstable situation. The focus of the discussion is on the ion kinetic scale and implications for mass, momentum, and energy transport at the magnetopause boundary.

Sliding Contacts in Planetary Magnetospheres

In the planetary magnetospheres there are specific places connected with velocity breakdown, reconnection, and dynamo processes. Here we pay attention to sliding layers. Sliding layers are formed in the ionosphere, on separatrix surfaces, at the magnetopauses and boundaries of stellar astrospheres, and at the Alfvén radius in the equatorial magnetosphere of rapidly rotating strongly magnetized giant planets. Although sliding contacts usually occur in thin local layers, their influence on the global structure of the surrounding space is very great. Therefore, they are associated with non-local processes that play a key role on a large scale. There can be an exchange between different forms of energy, a generation of strong field-aligned currents and emissions, and an amplification of magnetic fields. Depending on the conditions in the magnetosphere of the planet/exoplanet and in the flow of magnetized plasma passing it, different numbers of sliding layers with different configurations appear. Some are associated with regions of auroras and possible radio emissions. The search for planetary radio emissions is a current task in the detection of exoplanets.

A modern digital High Frequency Receiver to explore Uranus and Neptune radio emitters

<div class="">Among the known planetary magnetospheres, those of Uranus and Neptune display very similar radio environments so that they have early been referred to as ‘radio twins’. They produce a variety of electromagnetic radio waves ranging from ~0 to a few tens of MHz similar to - although more complex than - those of Saturn or the Earth (Desch et al., 1991, Zarka et al., 1995). These include the well known Uranian/Neptunian Kilometric Radiations (UKR/NKR) below 1MHz or the Uranian/Neptunian Electrostatic Discharges (UED/NED) beyond, which remain only known from Voyager 2 radio observations. Here, we present a modern concept of digital High Frequency Receiver (HFR) within the frame of a general Radio and Plasma Wave (RPW) experiment retained in various mission concepts toward Uranus and Neptune (e.g. Hess et al., 2010 ; Arridge et al., 2011, 2013, 2014 Christophe et al., 2011; Masters et al., 2013; Hofstadter at al., 2019). The presented HFR concept, based on the heritage of Cassini/RPWS/HFR, Bepi-Clompobo/PWI/Sorbet, Solar Orbiter/RPW and JUICE/RPWI/JENRAGE is aimed at providing a light, robust, low-consumption versatile instrument capable of goniopolarimetric and waveform measurements from a few kHz to ~20MHz, devoted to the study of auroral and atmospheric radio and plasma waves or dust impacts.</div>

Magnetohydrodynamic Equilibria

Magnetohydrodynamic equilibria are time-independent solutions of the full magnetohydrodynamic (MHD) equations. An important class are static equilibria without plasma flow. They are described by the magnetohydrostatic equations j×B=∇p+ρ∇Ψ,∇×B=μ0j,∇·B=0. B is the magnetic field, j the electric current density, p the plasma pressure, ρ the mass density, Ψ the gravitational potential, and µ0 the permeability of free space. Under equilibrium conditions, the Lorentz force j×B is compensated by the plasma pressure gradient force and the gravity force. Despite the apparent simplicity of these equations, it is extremely difficult to find exact solutions due to their intrinsic nonlinearity. The problem is greatly simplified for effectively two-dimensional configurations with a translational or axial symmetry. The magnetohydrostatic (MHS) equations can then be transformed into a single nonlinear partial differential equation, the Grad–Shafranov equation. This approach is popular as a first approximation to model, for example, planetary magnetospheres, solar and stellar coronae, and astrophysical and fusion plasmas. For systems without symmetry, one has to solve the full equations in three dimensions, which requires numerically expensive computer programs. Boundary conditions for these systems can often be deduced from measurements. In several astrophysical plasmas (e.g., the solar corona), the magnetic pressure is orders of magnitudes higher than the plasma pressure, which allows a neglect of the plasma pressure in lowest order. If gravity is also negligible, Equation 1 then implies a force-free equilibrium in which the Lorentz force vanishes. Generalizations of MHS equilibria are stationary equilibria including a stationary plasma flow (e.g., stellar winds in astrophysics). It is also possible to compute MHD equilibria in rotating systems (e.g., rotating magnetospheres, rotating stellar coronae) by incorporating the centrifugal force. MHD equilibrium theory is useful for studying physical systems that slowly evolve in time. In this case, while one has an equilibrium at each time step, the configuration changes, often in response to temporal changes of the measured boundary conditions (e.g., the magnetic field of the Sun for modeling the corona) or of external sources (e.g., mass loading in planetary magnetospheres). Finally, MHD equilibria can be used as initial conditions for time-dependent MHD simulations. This article reviews the various analytical solutions and numerical techniques to compute MHD equilibria, as well as applications to the Sun, planetary magnetospheres, space, and laboratory plasmas.