Modification of magnetohydrodynamic waves by the relativistic Hall effect [CL]

This study shows that a relativistic Hall effect significantly changes the properties of wave propagation by deriving a linear dispersion relation for relativistic Hall magnetohydrodynamics (HMHD). Whereas, in non- relativistic HMHD, the phase and group velocities of fast magnetosonic wave become anisotropic with an increasing Hall effect, the relativistic Hall effect brings upper bounds to the anisotropies. The Alfve\'{e}n wave group velocity with strong Hall effect also becomes less anisotropic than non-relativistic case. Moreover, the group velocity surfaces of Alfv\'{e}n and fast waves coalesce into a single surface in the direction other than near perpendicular to the ambient magnetic field. It is also remarkable that a characteristic scale length of the relativistic HMHD depends on ion temperature, magnetic field strength, and density while the non-relativistic HMHD scale length, i.e., ion skin depth, depends only on density. The modified characteristic scale length increases as the ion temperature increases and decreases as the magnetic field strength increases.

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Y. Kawazura
Fri, 23 Jun 17

Comments: N/A

Early propagation of energetic particles across the mean field in turbulent plasmas [CL]

Propagation of energetic particles across the mean field direction in turbulent magnetic fields is often described as spatial diffusion. Recently, it has been suggested that initially the particles propagate systematically along meandering field lines, and only later reach the time-asymptotic diffusive cross-field propagation. In this paper, we analyse cross-field propagation of 1–100 MeV protons in composite 2D-slab turbulence superposed on a constant background magnetic field, using full-orbit particle simulations, to study the non-diffusive phase of particle propagation with a wide range of turbulence parameters. We show that the early-time non-diffusive propagation of the particles is consistent with particle propagation along turbulently meandering field lines. This results in a wide cross-field extent of the particles already at the initial arrival of particles to a given distance along the mean field direction, unlike when using spatial diffusion particle transport models. The cross-field extent of the particle distribution remains constant for up to tens of hours in turbulence environment consistent with the inner heliosphere during solar energetic particle events. Subsequently, the particles escape from their initial meandering field lines, and the particle propagation across the mean field reaches time-asymptotic diffusion. Our analysis shows that in order to understand solar energetic particle event origins, particle transport modelling must include non-diffusive particle propagation along meandering field lines.

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T. Laitinen, S. Dalla and D. Marriott
Thu, 22 Jun 17

Comments: 11 pages, 9 figures; Accepted for publication in MNRAS

Propagation of Solar Energetic Particles in Three-dimensional Interplanetary Magnetic Fields: Radial Dependence of Peak Intensities [CL]

A functional form I_{max}(R)=kR^{-\alpha}, where R is the radial distance of spacecraft, was usually used to model the radial dependence of peak intensities I_{max}(R) of solar energetic particles (SEPs). In this work, the five-dimensional Fokker-Planck transport equation incorporating perpendicular diffusion is numerically solved to investigate the radial dependence of SEP peak intensities. We consider two different scenarios for the distribution of spacecraft fleet: (1) along the radial direction line, (2) along the Parker magnetic field line. We find that the index \alpha in the above expression varies in a wide range, primarily depending on the properties (e.g., location, coverage) of SEP sources and on the longitudinal/latitudinal separations between the sources and the magnetic footpoints of the observers. Particularly, the situation that whether the magnetic footpoint of the observer is located inside or outside of the SEP source is a crucial factor determining the values of index \alpha. A two-phase phenomenon is found in the radial dependence of peak intensities. The “position” of the breakpoint (transition point/critical point) is determined by the magnetic connection status of the observers. This finding suggests that a very careful examination of magnetic connection between SEP source and each spacecraft should be taken in the observational studies. We obtain a lower limit of R^{-1.7\pm0.1} for empirically modelling the radial dependence of SEP peak intensities. Our findings in this work can be used to explain the majority of the previous multispacecraft survey results, and especially to reconcile the different/conflicting empirical values of index \alpha in the literature.

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H. He, G. Zhou and W. Wan
Tue, 20 Jun 17

Comments: Published in ApJ

Electron plasma wake field acceleration in solar coronal and chromospheric plasmas [SSA]

Three dimensional, particle-in-cell, fully electromagnetic simulations of electron plasma wake field acceleration applicable to solar atmosphere are presented. It is established that injecting driving and trailing electron bunches into solar coronal and chromospheric plasmas, results in electric fields ($-(20-5) \times 10^{6}$ V/m), leading to acceleration of the trailing bunch up to 52 MeV, starting from initial 36 MeV. The results provide one of potentially important mechanisms for the extreme energetic solar flare electrons, invoking plasma wake field acceleration.

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D. Tsiklauri
Mon, 19 Jun 17

Comments: accepted for publication in Phys. Plasmas (July 2017 issue) arXiv admin note: text overlap with arXiv:1606.00367

Modeling Shocks Detected by Voyager 1 in the Local Interstellar Medium [CL]

The magnetometer (MAG) on Voyager 1 (V1) has been sampling the interstellar magnetic field (ISMF) since August 2012. The V1 MAG observations have shown draped ISMF in the very local interstellar medium disturbed occasionally by significant enhancements in magnetic field strength. Using a three-dimensional, data driven, multi-fluid model, we investigated these magnetic field enhancements beyond the heliopause that are supposedly associated with solar transients. To introduce time-dependent effects at the inner boundary at 1 astronomical unit, we used daily averages of the solar wind parameters from the OMNI data set. The model ISMF strength, direction, and proton number density are compared with V1 data beyond the heliopause. The model reproduced the large-scale fluctuations between 2012.652 and 2016.652, including major events around 2012.9 and 2014.6. The model also predicts shocks arriving at V1 around 2017.395 and 2019.502. Another model driven by OMNI data with interplanetary coronal mass ejections (ICMEs) removed at the inner boundary suggests that ICMEs may play a significant role in the propagation of shocks into the interstellar medium.

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T. Kim, N. Pogorelov and L. Burlaga
Thu, 15 Jun 17

Comments: Submitted to Astrophysical Journal Letters

The spherically symmetric gravitational field [CL]

The general solution of the system of General Relativity equations has been found for isotropic Universe with the flat spatial distribution and synchronized time taking into account a perfect dust and the cosmological constant. Schwarzschild, Friedmann and Einstein-de Sitter solutions (as well as all of their fusion with each other) are special cases of the found general solution. A method of generating an infinite number of Tolman’s like solutions has been found. Exact solutions has been found for the spherically symmetric gravitational field of perfect dust clouds in the expanding Universe filled with radiation. A system of ordinary differential equations has been obtained for the spherically symmetric gravitational field of perfect dust clouds in the expanding Universe filled with radiation and nonrelativistic gas. A system of equations has been obtained for the spherically symmetric gravitational field of ultrarelativistic celestial body explosion (supernova, quasar). The problem of a negative density of a perfect dust cloud in General Relativity has been considered.

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S. Gubanov
Thu, 15 Jun 17

Comments: 18 pages, no figures

Kelvin–Helmholtz instability in a cool solar jet in the framework of Hall magnetohydrodynamics: A case study [SSA]

We investigate the conditions under which the magnetohydrodynamic (MHD) modes in a cylindrical magnetic flux tube moving along its axis become unstable against the Kelvin–Helmholtz (KH) instability. We employ the dispersion relations of MHD modes derived from the linearized Hall MHD equations for cool (zero beta) plasma. We assume real wave numbers and complex angular wave frequencies, notably complex wave phase velocities. The dispersion equations are solved numerically at fixed input parameters and varying values of the ratio $l_\mathrm{Hall}/a$, where $l_\mathrm{Hall} = c/\omega_\mathrm{pi}$ ($c$ being the speed of light, and $\omega_\mathrm{pi}$ the ion plasma frequency) and $a$ is the flux tube radius. It is shown that the stability of the MHD modes depends upon four parameters: the density contrast between the flux tube and its environment, the ratio $l_\mathrm{Hall}/a$, and the value of the Alfv\’en Mach number (the ratio of the tube axial velocity to Alfv\’en speed inside the flux tube). It is found that at high density contrasts, for small values of $l_\mathrm{Hall}/a$, the kink ($m = 1$) mode can become unstable against KH instability at some critical Alfv\’en Mach number (or equivalently at critical flow speed), but a threshold $l_\mathrm{Hall}/a$ can suppress the onset of KH instability. At small density contrasts, however, the magnitude of $l_\mathrm{Hall}/a$ does not affect noticeably the condition for instability occurrence—even though it can reduce the critical Alfv\’en Mach number. It is established that the sausage mode ($m = 0$) is not subject to the KH instability.

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I. Zhelyazkov and Z. Dimitrov
Tue, 13 Jun 17

Comments: 15 pages, 7 figures