Collisionless shock meeting

As part of the SHARP project, the Swedish Institute for Space Physics (IRF) organised a Collisionless shock meeting on January 26-27th in Uppsala, Sweden. The meeting consisted of sessions on interplanetary shocks, astrophysical shocks, foreshock/sheath plasma regions and planetary bow shocks. The program is available here.

New Publication: “Electron Heating Scales in Collisionless Shocks Measured by MMS” by Andreas Johlander at al.

Electron heating at collisionless shocks in space is a combination of adiabatic heating due to large-scale electric and magnetic fields and non-adiabatic scattering by high-frequency fluctuations. The scales at which heating happens hints to what physical processes are taking place. In this letter, we study electron heating scales with data from the Magnetospheric Multiscale (MMS) spacecraft at Earth’s quasi-perpendicular bow shock. We utilize the tight tetrahedron formation and high-resolution plasma measurements of MMS to directly measure the electron temperature gradient. From this, we reconstruct the electron temperature profile inside the shock ramp and find that the electron temperature increase takes place on ion or sub-ion scales. Further, we use Liouville mapping to investigate the electron distributions through the ramp to estimate the deHoffmann-Teller potential and electric field. We find that electron heating is highly non-adiabatic at the high-Mach number shocks studied here.

Electron temperature profiles for the three shock crossing events. The x-axes show the profile along urn:x-wiley:00948276:media:grl65419:grl65419-math-0015 which means that upstream is at higher values regardless of which direction the spacecraft crossed the shock. Units are km on the bottom and di,u on the top. The shortest distance where half the temperature increase takes place is marked in gray.

Full Article:
Johlander, A. (SHARP), Khotyaintsev, Y. V. (SHARP), Dimmock, A. P. (SHARP), Graham, D. B. (SHARP), & Lalti, A. (SHARP) (2023). Electron heating scales in collisionless shocks measured by MMS. Geophysical Research Letters, 50, doi: 10.1029/2022GL100400

License: CC BY 4.0

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: “An update on Fermi-LAT transients in the Galactic plane, including strong activity of Cygnus X-3 in mid-2020” by Dmitry Prokhorov et al.

We present a search for Galactic transient γ-ray sources using 13 yr of the Fermi Large Area Telescope data. The search is based on a recently developed variable-size sliding-time-window (VSSTW) analysis and aimed at studying variable γ-ray emission from binary systems, including novae, γ-ray binaries, and microquasars. Compared to the previous search for transient sources at random positions in the sky with 11.5 yr of data, we included γ-rays with energies down to 500 MeV, increased a number of test positions, and extended the data set by adding data collected between 2020 February and 2021 July. These refinements allowed us to detect additional three novae, V1324 Sco, V5855 Sgr, V357 Mus, and one γ-ray binary, PSR B1259-63, with the VSSTW method. Our search revealed a γ-ray flare from the microquasar, Cygnus X-3, occurred in 2020. When applied to equal quarters of the data, the analysis provided us with detections of repeating signals from PSR B1259-63, LS I +61°303, PSR J2021+4026, and Cygnus X-3. While the Cygnus X-3 was bright in γ-rays in mid-2020, it was in a soft X-ray state and we found that its γ-ray emission was modulated with the orbital period.

The significance map of γ-ray transient emission in σ showing the microquasar Cygnus X-3, the nova V407 Cyg, and the pulsar PSR J2021+4026.

Full Article:
Prokhorov, D. A. (SHARP), Moraghan, A. (2022). An update on Fermi-LAT transients in the Galactic plane, including strong activity of Cygnus X-3 in mid-2020. Monthly Notices of the Royal Astronomical Society, 519, doi: 10.1093/mnras/stac3453

License: CC BY 4.0

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