SHARP Working meeting

SHARP Working meeting on the future synthesis of heliospheric and astrophysical shocks with participation of colleagues from SERPENTINE project was held on December 1st, 2022.

New Publication: “Change of Rankine–Hugoniot Relations during Postshock Relaxation of Anisotropic Distributions” by Michael Gedalin et al.

Collisionless shocks channel the energy of the directed plasma flow into the heating of the plasma species and magnetic field enhancement. The kinetic processes at the shock transition cause the ion distributions just behind the shock to be nongyrotropic. Gyrotropization and subsequent isotropization occur at different spatial scales. Accordingly, for a given upstream plasma and magnetic field state, there would be different downstream states corresponding to the anisotropic and isotropic regions. Thus, at least two sets of Rankine–Hugoniot relations are needed, in general, to describe the connection of the downstream measurable parameters to the upstream ones. We establish the relation between the two sets.

Top: the magnetic field magnitude, normalized to the upstream magnetic field magnitude. Middle: the three eigenvalues of the ion temperature tensor, normalized to the upstream ion temperature. Bottom: the three eigenvalues of the electron temperature tensor, normalized to the upstream electron temperature. The smallest eigenvalue is in blue, while the largest one is in black.

Full Article:
Gedalin, M. (SHARP), Golan, M., Pogorelov, N. V. and Roytershteyn, V. (2022). Change of Rankine–Hugoniot Relations during Postshock Relaxation of Anisotropic Distributions. The Astrophysical Journal, 940, doi: 10.3847/1538-4357/ac958d

License: CC BY 4.0

New Publication: “Combining Rankine–Hugoniot relations, ion dynamics in the shock front, and the cross-shock potential” by Michael Gedalin

RankineHugoniot relations (RH) connect the upstream and downstream plasma states. They allow us to determine the magnetic compression, the density compression, and the plasma heating as functions of the Mach number, shock angle, and upstream temperature. RH are based on the conservation laws in the hydrodynamical form. In collisionless shocks, the ion distributions behind the shock transition are determined by ion dynamics in the macroscopic fields of the shock front. The ion parameters upon crossing the shock are directly related to the magnetic compression and the cross-shock potential. For given upstream parameters, RH provide the magnetic compression. If there is no substantial overshoot, an analytical estimate provides the cross-shock potential as a function of the magnetic compression and the Mach number. Numerical tracing of ions across a shock profile with the derived parameters provides the ion pressure, which is in good agreement with the combination of the two theoretical approaches.

The normalized model magnetic field (black curve), the magnetic field derived from the pressure balance (blue curve), and the reduced distribution function (log scale), for M = 4.3, ?=60°, and ??/??=3, with overshoot and undershoot added.

Full Article:
Gedalin, M. (SHARP) (2022), Combining Rankine-Hugoniot relations, ion dynamics in the shock front, and the cross-shock potential. Physics of Plasmas, 29, doi: 10.1063/5.0120578

License: CC BY 4.0

New Publication: “Mirror Mode Storms Observed by Solar Orbiter” by Andrew Dimmock et al.

Mirror modes (MMs) are ubiquitous in space plasma and grow from pressure anisotropy. Together with other instabilities, they play a fundamental role in constraining the free energy contained in the plasma. This study focuses on MMs observed in the solar wind by Solar Orbiter (SolO) for heliocentric distances between 0.5 and 1 AU. Typically, MMs have timescales from several to tens of seconds and are considered quasi-MHD structures. In the solar wind, they also generally appear as isolated structures. However, in certain conditions, prolonged and bursty trains of higher frequency MMs are measured, which have been labeled previously as MM storms. At present, only a handful of existing studies have focused on MM storms, meaning that many open questions remain. In this study, SolO has been used to investigate several key aspects of MM storms: their dependence on heliocentric distance, association with local plasma properties, temporal/spatial scale, amplitude, and connections with larger-scale solar wind transients. The main results are that MM storms often approach local ion scales and can no longer be treated as quasi-magnetohydrodynamic, thus breaking the commonly used long-wavelength assumption. They are typically observed close to current sheets and downstream of interplanetary shocks. The events were observed during slow solar wind speeds and there was a tendency for higher occurrence closer to the Sun. The occurrence is low, so they do not play a fundamental role in regulating ambient solar wind but may play a larger role inside transients.

Mirror modes (MMs) observed on 19 July 2021. Plotted in panels (a and b) are |B| and Brtn, a wavelet spectrogram of B is shown in panel (c), and the ellipticity of the magnetic field is shown in panel (d). Panels (e–k) depict Ni, |Vi|, Ti, differential energy flux, βi, and RMM, respectively. Regions that are highlighted in yellow correspond to localized reductions in ellipticity and the manifestation of MM structures since they should have zero ellipticity.

Full Article:
Dimmock, A. P. (SHARP), Yordanova, E., Graham, D. B. (SHARP), Khotyaintsev, Y. V. (SHARP), Blanco-Cano, X., Kajdič, P., et al. (2022). Mirror mode storms observed by Solar Orbiter. Journal of Geophysical Research: Space Physics, 127, doi: 10.1029/2022JA030754

License: CC BY 4.0

NASA’S IXPE Helps Unlock the Secrets of Cassiopeia A

NASA’s Imaging X-ray Polarimetry Explorer (IXPE) was launched on Dec. 9, 2021. Now the first results, analysing the X-ray polarization of the young supernova remnant Cassiopeia A, have been published. SHARP members Dr. Jacco Vink and Dmitry Prokhorov are scientific members of the IXPE science team and were involved in the analysis of the first science observation done with IXPE

Read NASA’s press release here:

Scientific publication:
Vink, J. (SHARP), Prokhorov, D. (SHARP), Ferrazzoli, R. et al. (2022). X-ray polarization detection of Cassiopeia A with IXPE. The Astrophyscial Journal, 938, doi: 10.3847/1538-4357/ac8b7b

New Publication: “X-Ray Polarization Detection of Cassiopeia A with IXPE” by Jacco Vink et al.

We report on a ∼5σ detection of polarized 3–6 keV X-ray emission from the supernova remnant Cassiopeia A (Cas A) with the Imaging X-ray Polarimetry Explorer (IXPE). The overall polarization degree of 1.8% ± 0.3% is detected by summing over a large region, assuming circular symmetry for the polarization vectors. The measurements imply an average polarization degree for the synchrotron component of ∼2.5%, and close to 5% for the X-ray synchrotron-dominated forward shock region. These numbers are based on an assessment of the thermal and nonthermal radiation contributions, for which we used a detailed spatial-spectral model based on Chandra X-ray data. A pixel-by-pixel search for polarization provides a few tentative detections from discrete regions at the ∼ 3σ confidence level. Given the number of pixels, the significance is insufficient to claim a detection for individual pixels, but implies considerable turbulence on scales smaller than the angular resolution. Cas A’s X-ray continuum emission is dominated by synchrotron radiation from regions within ≲1017 cm of the forward and reverse shocks. We find that (i) the measured polarization angle corresponds to a radially oriented magnetic field, similar to what has been inferred from radio observations; (ii) the X-ray polarization degree is lower than in the radio band (∼5%). Since shock compression should impose a tangential magnetic-field structure, the IXPE results imply that magnetic fields are reoriented within ∼1017 cm of the shock. If the magnetic-field alignment is due to locally enhanced acceleration near quasi-parallel shocks, the preferred X-ray polarization angle suggests a size of 3 × 1016 cm for cells with radial magnetic fields.

Left: IXPE three color Stokes I image with square-root brightness scaling. Center: hardness ratio map based on Chandra X-ray data. Right: the MDP99 levels for the 3–6 keV band for the IXPE observations of Cas A.

Full Article:
Vink, J. (SHARP), Prokhorov, D. (SHARP), Ferrazzoli, R. et al. (2022). X-ray polarization detection of Cassiopeia A with IXPE. The Astrophyscial Journal, 938, doi: 10.3847/1538-4357/ac8b7b

License: CC BY 4.0

Shocks Workshop, October 20th and 21st

The SHARP project organised a Shocks Workshop on October 20th and 21st at the Finnish Meteorological Institute in Helsinki, Finland. The format of the workshop was hybrid. Half of the participants attended in person, while the other half joined online. There were also participants from the related projects SERPENTINE and EUHFORIA.

The porgramme involved various talks about shock physics, an introduction to the SHARP project and presentations about recent results from the project. Besides the presentations, two discussion sessions were organised. One session focused on discussing the comparison of astrophysical and heliospheric shocks and the other session consisted of discussions on the collaboration between SERPENTINE and SHARP.