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.

SHARP at ‘Astronomy Day and Night’ Festival in Uppsala, Sweden

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.

SHARP at SERPENTINE user workshop in Kiel

The SERPENTINE (Solar energetic particle analysis platform for the inner heliosphere) project organised a user workshop on September 5.-9. at Christian-Albrechts University in Kiel, Germany. At the workshop, Andrew Dimmock gave an overview of the SHARP project and the first version of the shock database. Max van de Kamp presented the online interface for searching and downloading data from the shock database.

Try the online database interface yourself:
Learn more about SERPENTINE here:

New Publication: “Different heating of Maxwellian and kappa distributions at shocks” by Michael Gedalin and Natalia Ganushkina

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

License: CC BY 4.0

New Publication: “The supernova remnant SN 1006 as a Galactic particle accelerator” by Roberta Giuffrida et al.

The origin of cosmic rays is a pivotal open issue of high-energy astrophysics. Supernova remnants are strong candidates to be the Galactic factory of cosmic rays, their blast waves being powerful particle accelerators. However, supernova remnants can power the observed flux of cosmic rays only if they transfer a significant fraction of their kinetic energy to the accelerated particles, but conclusive evidence for such efficient acceleration is still lacking. In this scenario, the shock energy channeled to cosmic rays should induce a higher post-shock density than that predicted by standard shock conditions. Here we show this effect, and probe its dependence on the orientation of the ambient magnetic field, by analyzing deep X-ray observations of the Galactic remnant of SN 1006. By comparing our results with state-of-the-art models, we conclude that SN 1006 is an efficient source of cosmic rays and obtain an observational support for the quasi-parallel acceleration mechanism.

SN 1006 in different energy bands.

Full Article:
Giuffrida, R., Miceli, M., Caprioli, D., et al. (2022). The supernova remnant SN 1006 as a Galactic particle accelerator. Nature Communications, 13, doi: 10.1038/s41467-022-32781-4

License: CC BY 4.0

New Publication: “A Database of MMS Bow Shock Crossings Compiled Using Machine Learning” by Ahmad Lalti et al.

Identifying collisionless shock crossings in data sent from spacecraft has so far been done manually or using basic algorithms. It is a tedious job that shock physicists have to go through if they want to conduct case studies or perform statistical studies. We use a machine learning approach to automatically identify shock crossings from the Magnetospheric Multiscale (MMS) spacecraft. We compiled a database of 2,797 shock crossings, spanning a period from October 2015 to December 2020, including various spacecraft-related and shock-related parameters for each event. Furthermore, we show that the shock crossings in the database are spread out in space, from the subsolar point to the far flanks. On top of that, we show that they cover a wide range of parameter space. We also present a possible scientific application of the database by looking for correlations between ion acceleration efficiency at shocks with different shock parameters, such as the angle between the upstream magnetic field and the shock normal θBn and the Alfvénic Mach number MA. We find no clear correlation between the acceleration efficiency and MA; however, we find that quasi-parallel shocks are more efficient at accelerating ions than quasi-perpendicular shocks.

Location of the crossings of all of the shocks in the database projected on the ecliptic plane and normalized to Earth radius. Color bar in panel (a) represents time, (b) dynamic pressure, (c) MA, and (d) θBn with the y coordinate of the crossing flipped if the magnetic field is in the ortho-Parker spiral configuration

Full Article:
Lalti, A. (SHARP), Khotyaintsev, Yu. V. (SHARP), Dimmock, A. P. (SHARP), Johlander, A. (SHARP), Graham, D. B. (SHARP), Olshevsky, V. (2022). A database of MMS bow shock crossings compiled using machine learning. Journal of Geophysical Research: Space Physics, 127, doi: 10.1029/2022JA030454

License: CC BY 4.0