News

New Publication: “Shock Heating of Incident Thermal and Superthermal Populations of Different Ion Species” by Michael Gedalin et al.

Using ion tracing in a model shock front we study heating of thermal (Maxwellian) and superthermal (Vasyliunas–Siscoe) populations of protons, singly charged helium, and alpha particles. It is found that heating of thermal and superthermal populations is different, mainly because of substantially higher ion reflection in the superthermal populations. Accordingly, the temperature increase of initially superthermal populations is substantially higher than that of the thermal ions. Heating per mass decreases with the increase of the mass-to-charge ratio because of the reduced effect of the cross-shock potential and, accordingly, weaker ion reflection. The findings are supported by two-dimensional hybrid simulations.

The upstream (left panel) and downstream (right panel) distribution of the initially VS distributed α particles (AV). The distributions are shown on a log scale.

Full Article:
Gedalin, M. (SHARP), Roytershteyn, V., Pogorelov, N. V. (2023). Shock Heating of Incident Thermal and Superthermal Populations of Different Ion Species. The Astrophysical Journal, 945, doi: 10.3847/1538-4357/acb13a

License: CC BY 4.0

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

New Publication: “Transmission of foreshock waves through Earth’s bow shock” by Lucile Turc et al.

The Earth’s magnetosphere and its bow shock, which is formed by the interaction of the supersonic solar wind with the terrestrial magnetic field, constitute a rich natural laboratory enabling in situ investigations of universal plasma processes. Under suitable interplanetary magnetic field conditions, a foreshock with intense wave activity forms upstream of the bow shock. So-called 30 s waves, named after their typical period at Earth, are the dominant wave mode in the foreshock and play an important role in modulating the shape of the shock front and affect particle reflection at the shock. These waves are also observed inside the magnetosphere and down to the Earth’s surface, but how they are transmitted through the bow shock remains unknown. By combining state-of-the-art global numerical simulations and spacecraft observations, we demonstrate that the interaction of foreshock waves with the shock generates earthward-propagating, fast-mode waves, which reach the magnetosphere. These findings give crucial insight into the interaction of waves with collisionless shocks in general and their impact on the downstream medium.

a, Colour map of the magnetic field strength fluctuations in the simulation plane at time t = 500 s from the beginning of the simulation. We subtract <B>50s, which is a 50 s average of the field magnitude, from B to reveal the fluctuations of the magnetic field magnitude. The black curve shows the approximate magnetopause location. The black arrows show the IMF direction, and the purple arrows depict the shock normal direction nshock at two positions along the bow shock. b, PSD of the total magnetic field fluctuations at the three locations marked by coloured circles in a. c, PSD of the magnetic field fluctuations parallel and perpendicular to the mean magnetic field at the virtual spacecraft location in the magnetosheath. The perpendicular directions are defined such that B⊥1 lies in the simulation (x–y) plane while B⊥2 completes the right-handed set.

Full Article:
Turc, L., Roberts, O.W., Verscharen, D., Dimmock, A. P. (SHARP) et al. (2022). Transmission of foreshock waves through Earth’s bow shock. Nature Physics, 19, doi: 10.1038/s41567-022-01837-z

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

Rankine–Hugoniot 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