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Register a new userPossible phase transition in plasma mirror modes
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Mirror modes in collisionless high-temperature plasmas represent macroscopic high-temperature quasi-superconductors. We explicitly calculate the bouncing electron contribution to the ion-mode growth rate, diamagnetic surface current responsible for the Meissner effect, and the weak attracting electric field. The mean electric field turns out to be negligible. Pairing is a second-order effect of minor importance. The physically important effect is the resonant interaction between bouncing electrons and the thermal ion-sound background. It is responsible for the mirror mode to evolve as a phase transition from normal to quasi-superconducting state.
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We perform direct analysis of mirror mode instabilities from the general dielectric tensor for several model distributions, in the longwavelength limit. The growth rate at the instability threshold depends on the derivative of the distribution for zero parallel energy. The maximum growth rate is always $sim k_parallel v_{Tparallel}$ and the instability is of nonresonant kind. The instability growth rate and its dependence on the propagation angle depend on the shape of the ion and electron distribution functions.
Using kinetic particle-in-cell (PIC) simulations, we simulate reconnection conditions appropriate for the magnetosheath and solar wind, i.e., plasma beta (ratio of gas pressure to magnetic pressure) greater than 1 and low magnetic shear (strong guide field). Changing the simulation domain size, we find that the ion response varies greatly. For reconnecting regions with scales comparable to the ion Larmor radius, the ions do not respond to the reconnection dynamics leading to electron-only reconnection with very large quasi-steady reconnection rates. The transition to more traditional ion-coupled reconnection is gradual as the reconnection domain size increases, with the ions becoming frozen-in in the exhaust when the magnetic island width in the normal direction reaches many ion inertial lengths. During this transition, the quasi-steady reconnection rate decreases until the ions are fully coupled, ultimately reaching an asymptotic value. The scaling of the ion outflow velocity with exhaust width during this electron-only to ion-coupled transition is found to be consistent with a theoretical model of a newly reconnected field line. In order to have a fully frozen-in ion exhaust with ion flows comparable to the reconnection Alfven speed, an exhaust width of at least several ion inertial lengths is needed. In turbulent systems with reconnection occurring between magnetic bubbles associated with fluctuations, using geometric arguments we estimate that fully ion-coupled reconnection requires magnetic bubble length scales of at least several tens of ion inertial lengths.
The mirror mode evolving in collisionless magnetised high-temperature thermally anisotropic plasmas is shown to develop an interesting macro-state. Starting as a classical zero frequency ion fluid instability it saturates quasi-linearly at very low magnetic level, while forming elongated magnetic bubbles which trap the electron component to perform an adiabatic bounce motion along the magnetic field. {Further evolution of the mirror mode towards a stationary state is determined by the bouncing trapped electrons which interact with the thermal level of ion sound waves, generate attractive wake potentials which give rise to formation of electron pairs in the lowest-energy singlet state of two combined electrons. Pairing takes preferentially place near the bounce-mirror points where the pairs become spatially locked with all their energy in the gyration. The resulting large anisotropy of pairs enters the mirror growth rate in the quasi-linearly stable mirror mode. It breaks the quasilinear stability and causes further growth. Pressure balance is either restored by dissipation of the pairs and their anisotropy or inflow of plasma from the environment. In the first case new pairs will continuously form until equilibrium is reached. In the final state the fraction of pairs can be estimated. This process is open to experimental verification. To our knowledge it is the only process where in high temperature plasma pairing may occur and has an important observable macroscopic effect: breaking the quasilinear limit and generation of localised diamagnetism.}
We present estimates of the turbulent energy cascade rate, derived from a Hall-MHD third-order law. We compute the contribution from the Hall term and the MHD term to the energy flux. We use MMS data accumulated in the magnetosheath and the solar wind, and compare the results with previously established simulation results. We find that in observation, the MHD contribution is dominant at inertial scales, as in the simulations, but the Hall term becomes significant in observations at larger scales than in the simulations. Possible reasons are offered for this unanticipated result.
FMS modes are studied in the model of the magnetotail as a cylinder with plasma sheet. The presence of the plasma sheet leads to a significant modification of the modes existing in the magnetotail in the form of a cylinder with no plasma sheet. Azimuthal scales of the FMS modes differ significantly between the lobes and the plasma sheet. The azimuthal scale in the plasma sheet is much smaller than that in the magnetotail lobes. FMS waves with certain parameters are strongly reflected from the boundary between the lobes and the plasma sheet and are very weak in the plasma sheet.
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