On the Polarisation of Radio Relics

Radio relics are extended radio emission features which trace shock waves in the periphery of galaxy clusters originating from cluster mergers. Some radio relics show a highly polarised emission, which make relics an excellent probe for the magnetisation of the intra-cluster medium. The origin of the relic polarisation is still debated. It could be a result of tangentially stretching the magnetic field at the shock surface. This scenario would naturally explain the alignment of the polarisation (E-vectors) with the shock normal. We have implemented a toy model for the relic polarisation according to this scenario. We find that the magnetic field strength itself crucially affects the fractional polarisation. Moreover, we find that the shock strength has surprisingly little effect on the overall polarisation fraction. Finally, we find that the fractional polarisation may decrease downstream depending on the magnetic field strength. Our results demonstrates that the shock compression scenario provides a very plausible explanation for the radio relic polarisation which specific features permitting to test the origin of radio relic polarisation.

Intensification of non-uniformity in convergent near-conical hypersonic flow

The inevitable formation of a Mach disk at the central axis of a convergent conical shock wave may suffer from fundamental changes when the flow deviates from the axisymmetric condition. In this paper, the behaviours of near-conical shocks, which are generated by a circular ring wedge of $10^{\circ }$ at typical angles of attack (AoAs), are investigated at a free stream Mach number of 6 in a shock tunnel. To reveal the characteristics and mechanism of the flow, numerical analyses are carried out under the same conditions. The results indicate that when the flow deviates from axial symmetry, the circumferential non-uniformity is remarkably intensified as the shock converges downstream. The converging centre shifts against the inclination of the incoming flow and moves to the leeward side. For a sufficiently small AoA, the formation of a Mach disk remains similar to that in the axisymmetric case, although the Mach disk shrinks in size and is slightly flattened. As the circumferential non-uniformity of the shock increases at an AoA of approximately $3^{\circ }$ , a pair of kinks separate the shock surface into two discontinuous segments with the stronger shock segment on the windward side and the weaker shock segment on the leeward side. When the AoA increases further, the shrinkage of the Mach disk continuously occurs, and the Mach disk is eventually replaced by a regular reflection. The discontinuity of a convergent shock with flattening on the separated shock segments and the insufficient strength increase during the subsequent convergence are responsible for the appearance of regular reflection.

Foreshock ULF fluctuations near the Moon: THEMIS observations

<p>The foreshock is a region filled with a turbulent plasma located upstream the Earth’s bow shock where interplanetary magnetic field (IMF) lines are connected to the bow shock surface. In this region, ultra-low frequency (ULF) waves are generated due to the interaction of the solar wind plasma with particles reflected from the bow shock back into the solar wind. It is assumed that excited waves grow and they are convected through the solar wind/foreshock, thus the inner spacecraft (close to the bow shock) would observe larger wave amplitudes than the outer (far from the bow shock) spacecraft. The paper presents a statistical analysis of excited ULF fluctuations observed simultaneously by two closely separated THEMIS spacecraft orbiting the Moon under a nearly radial IMF. We found that ULF fluctuations (in the plasma rest frame) can be characterized as a mixture of transverse and compressional modes with different properties at both locations. We discuss the growth and/or damping of ULF waves during their propagation.</p>

Understanding the Origin of Variable Compositions of Gradual Solar Energetic Particle Events by Combining Observations and Numerical Simulations

<p>One of the mysteries of solar energetic particle (SEP) events is the compositional variability in those events that are clearly shock-related and may be called gradual events.<span>  </span>In particular, the reason for the enhancement of Fe with respect to O or C at high energies has been debated over the past two decades, and yet it is still unsettled.<span>  </span>One hypothesis relates the compositional variability with whether the CME-driven shock is quasi-parallel or quad-perpendicular near the Sun, bu<span>t this may not be easily tested using</span> remote-sensing data alone. In recent years, however, CME-driven shock waves have been modelled by fitting shock-like features in EUV and white-light images with relatively simple shapes, and in combination with magnetic field models, ir is possible to compute shock parameters at the shock surface. In this presentation, we simulate a few CMEs whose associated SEP events show widely different Fe/O, using the Alfven wave Solar Model (AWSoM) that is part of the Space Weather Modeling Framework (SWMF). We constrain the input parameters of the simulations so that the observed pre-eruption corona, erup<span>tion and CME are well-reproduced. The shock surface, across which the shock parameters are highly non-uniform, is carefully traced, and the time-dependent connectivity of the shock surface with the observer at multiple spacecraft is compared with the SEP properties including composition. We discuss how much about the compositional variability of SEP events can be learned with this technique.</span></p>

Diagnosing cosmic ray modified shocks with H α polarimetry

ABSTRACT A novel diagnostic of cosmic ray modified shocks by polarimetry of H α emissions is suggested. In a cosmic ray modified shock, the pressure of cosmic rays is sufficiently high compared to the upstream ram pressure to force the background plasma to decelerate (measured in the shock rest frame). Simultaneously, a fraction of the hydrogen atoms co-existing in the upstream plasma collide with the decelerated protons and undergo charge-exchange reactions. As a result, hydrogen atoms with the same bulk velocity of the decelerated protons are generated. We show that when the shock is observed from edge-on, the H α radiated by these upstream hydrogen atoms is linearly polarized with a sizable degree of a few per cent as a result of resonant scattering of Ly β. The polarization direction depends strongly on the velocity modification; the direction is parallel to the shock surface for the case of no modification, while the direction is parallel to the shock velocity for the case of a modified shock.

A Novel Laser Shock Surface Patterning Process Toward Tribological Applications

We report a novel laser-based surface processing process, laser shock surface patterning (LSSP) integrating both surface strengthening and patterning effects might lead to broader impacts in tribology research and applications. This process utilizes the laser-induced shockwave loadings to introduce the surface strengthening and patterning effects simultaneously, leading to the fabrication of arrays of micro-indentations or protrusions for the enhanced wear resistance and manipulated friction values. Two process designs, direct-LSSP and indirect-LSSP were proposed and carried out on AZ31B Mg alloys and AISI 1045 steels, respectively. The 3D surface profiles of the samples after LSSP were characterized. The hardness of surface patterns prepared by laser processing was measured. The friction values as affected by laser processing parameters were measured by sliding tests. The relationships among laser processing parameters, micro-feature characteristics, and COF were discussed.