A collisionless shock is a self-organized structure where fields and particle distributions are mutually adjusted to ensure a stable mass, momentum and energy transfer from the upstream to the downstream region. This adjustment may involve rippling, reformation or whatever else is needed to maintain the shock. The fields inside the shock front are produced due to the motion of charged particles, which is in turn governed by the fields. The overshoot arises due to the deceleration of the ion flow by the increasing magnetic field, so that the drop of the dynamic pressure should be compensated by the increase of the magnetic pressure. The role of the overshoot is to regulate ion reflection, thus properly adjusting the downstream ion temperature and kinetic pressure and also speeding up the collisionless relaxation and reducing the anisotropy of the eventually gyrotropized distributions.
The magnetic field magnitude, normalized to the upstream magnetic field magnitude (black curve) and the reduced ion distribution function.
Full Article: Gedalin, M. (SHARP), Dimmock, A. (SHARP), Russell, C. (SHARP), Pogorelov, N., & Roytershteyn, V. (2023). Role of the overshoot in the shock self-organization. Journal of Plasma Physics, 89(2), doi: 10.1017/S0022377823000090
Supernova remnants are commonly considered to produce most of the Galactic cosmic rays via diffusive shock acceleration. However, many questions regarding the physical conditions at shock fronts, such as the magnetic-field morphology close to the particle acceleration sites, remain open. Here we report the detection of a localized polarization signal from some synchrotron X-ray emitting regions of Tycho’s supernova remnant made by the Imaging X-ray Polarimetry Explorer. The derived degree of polarization of the X-ray synchrotron emission is 9% ± 2% averaged over the whole remnant, and 12% ± 2% at the rim, higher than the value of polarization of 7%–8% observed in the radio band. In the west region, the degree of polarization is 23% ± 4%. The degree of X-ray polarization in Tycho is higher than for Cassiopeia A, suggesting a more ordered magnetic field or a larger maximum turbulence scale. The measured tangential direction of polarization corresponds to the radial magnetic field, and is consistent with that observed in the radio band. These results are compatible with the expectation of turbulence produced by an anisotropic cascade of a radial magnetic field near the shock, where we derive a magnetic-field amplification factor of 3.4 ± 0.3. The fact that this value is significantly smaller than those expected from acceleration models is indicative of highly anisotropic magnetic-field turbulence, or that the emitting electrons either favor regions of lower turbulence, or accumulate close to where the orientation of the magnetic field is preferentially radially oriented due to hydrodynamical instabilities.
Polarization map in the 3–6 keV energy band with a 60” pixel size. Only the pixels with significance higher than 1σ are shown. The blue bars represent the direction of the polarization (that is, the direction of the electric vector polarization angle) and their length is proportional to the degree of polarization. The thicker cyan bars mark the pixels with significance higher than 2σ. The orientation of the magnetic field is perpendicular to the direction of the polarization. Superimposed in green are the 4–6 keV Chandra contours.
Full Article: Ferrazzoli, R., Slane, P., Prokhorov, D. (SHARP), Zhou, P., Vink, J. (SHARP), et al. (2023). X-Ray Polarimetry Reveals the Magnetic-field Topology on Sub-parsec Scales in Tycho’s Supernova Remnant. The Astrophysical Journal, 945, doi: 10.3847/1538-4357/acb496
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.
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
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
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
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
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.
Andrew Dimmock gave a presentation with the title “Solar Wind: What is it and how does it affect Earth and other planets?” at the Biotopia museum in Uppsala as part of the ‘Astronomy Day and Night’ festival, which is a space festival with events throughout Sweden. The festival is built up by organizers around Sweden who draw attention to astronomy and space travel in different ways. This year ‘Astronomy Day and Night’ had the theme of all the solar systems of the Universe.
Ion heating in collisionless shocks is non-adiabatic and efficient. The amount of heating and the downstream distributions depend on the shock parameters and on the incident ion distribution. The number of reflected ions and their distribution depend on the detailed shape of the tail of the distribution. In supercritical shocks the reflected ion contribution is significant. Kappa distributed ions are heated more strongly and have a larger fraction of reflected ions than Maxwellian distributed ions with the same upstream temperature and the same shock parameters. For kappa distributions the phase space dips are shallower.
The upstream (left-hand side) and downstream (right-hand side) gyrotropic distributions for initially κ-distributed ions, on a log scale.
Full Article: Gedalin, M. (SHARP) and Ganushkina, N. (SHARP) (2022). Different heating of Maxwellian and kappa distributions at shocks. Journal of Plasma Physics,88(5), doi: 10.1017/S0022377822000824
