New Publication: “Theory Helps Observations: Determination of the Shock Mach Number and Scales From Magnetic Measurements” by Michael Gedalin et al.

The Mach number is one of the key parameters of collisionless shocks. Understanding shock physics requires knowledge of the spatial scales in the shock transition layer. The standard methods of determining the Mach number and the spatial scales require simultaneous measurements of the magnetic field and the particle density, velocity, and temperature. While magnetic field measurements are usually of high quality and resolution, particle measurements are often either unavailable or not properly adjusted to the plasma conditions. We show that theoretical arguments can be used to overcome the limitations of observations and determine the Mach number and spatial scales of the low-Mach number shock when only magnetic field data are available.

The magnetic field in MSO coordinates: Bx (green), By (red), Bz (blue), and |B| (black) for the shock crossing 2011/083/12:25:00.

Full article:
Gedalin, M. (SHARP), Golbraikh, E. (SHARP), Russell, C. T. (SHARP), Dimmock, A. P. (SHARP) (2022). Theory Helps Observations: Determination of the Shock Mach Number and Scales From Magnetic Measurements. Frontiers in Physics, 10, doi: 10.3389/fphy.2022.852720

License: CC BY 4.0

First version of shock database is now publicly available

We use a machine learning approach to automatically identify shock crossings from the Magnetospheric Multiscale (MMS) spacecraft. We compile a database of 2797 crossings including various spacecraft related and shock related parameters for each event. Furthermore, for each event we provide an overview plot containing key parameters of the shock crossing

Version 1.0 of the database including all overview plots is now available on Zenodo:

A Technical report detailing the content of the database can be found here: (submitted to JGR: space physics)

Example of an overview plot from the shock database.

New Publication: “Probabilities of Ion Scattering at the Shock Front” by Michael Gedalin et al.

Collisionless shocks efficiently convert the energy of the directed ion flow into their thermal energy. Ion distributions change drastically at the magnetized shock crossing. Even in the absence of collisions, ion dynamics within the shock front is non-integrable and gyrophase dependent. The downstream distributions just behind the shock are not gyrotropic but become so quickly due to the kinematic gyrophase mixing even in laminar shocks. During the gyrotropization all information about gyrophases is lost. Here we develop a mapping of upstream and downstream gyrotropic distributions in terms of scattering probabilities at the shock front. An analytical expression for the probability is derived for directly transmitted ions in the narrow shock approximation. The dependence of the probability on the magnetic compression and the cross-shock potential is demonstrated.

Full article:
Gedalin, M. (SHARP), Pogorelov, N. V. and Roytershteyn, V. (2022). Probabilities of Ion Scattering at the Shock Front. Journal of Plasma Physics, 88(1), doi: 10.1017/S0022377822000034

License: CC BY 4.0

New Publication: “Collisionless Shocks in the Heliosphere: Foot Width Revisited” by Michael Balikhin and Michael Gedalin

For single-point measurements of quasi-perpendicular shocks, analytical measurements of the foot width are often used to evaluate the velocity of the shock relative to the satellite. This velocity is of crucial importance for in situ observations because it enables the identification of the spatial scale of other regions of the shock front such as a magnetic ramp for which the comprehensive understanding of their formation is not yet achieved. Knowledge of the spatial scale is one of the key parameters for the validation of theoretical models that are developed to explain the formation of these regions. Previously available estimates of the foot width for a quasi-perpendicular shock are based on several simplifications such as zero upstream ion temperature and specular ion reflection by the cross-shock electrostatic potential. The occurrence of specular reflection implies high values of the cross-shock electrostatic potential that significantly exceed the values obtained from in situ measurements. In this paper the effects of nonzero ion temperature and nonspecular ion reflection on the foot width are investigated. It is shown that in the case of nonspecular reflection the foot width can be as small as half of the size of the standard widely used estimate. Results presented here enable more reliable identification of the shock velocity from single-point observations.

The shock magnetic profile (black), the positions and vy of the reflected ions in the reflection point (blue), and the positions and vy of the reflected ions in the turning point (red). The two red lines mark the beginning of the ramp up to the overshoot maximum.

Full article:
Balikhin, M. and Gedalin, M. (SHARP) (2022). Collisionless Shocks in the Heliosphere: Foot Width Revisited. The Astrophysical Journal, 925, doi: 10.3847/1538-4357/ac3bb3

License: CC BY 4.0

New Publication: “Quasi-Parallel Shock Reformation Seen by Magnetospheric Multiscale and Ion-Kinetic Simulations” by Andreas Johlander et al.

A shock wave forms when a supersonic flow encounters an obstacle. Shock waves can even form in the ionized plasma that inhabits most of the seemingly empty space in our solar system, galaxy, and the rest of the universe. One such a shock is found in front of Earth as the fast stream of plasma flowing from the Sun, known as the solar wind, encounters Earth’s magnetic field. Under certain conditions, shock waves can become unsteady and evolve in time. Specifically, it is thought that a new shock can form in front of and replace the old shock in a process known as shock reformation. This process is important for how shock waves heat the plasma and can play a major role in how shocks accelerate particles. In this work, we use data from satellites that fly through Earth’s shock and compare to a computer simulation of the shock wave. We find that a type of magnetic pulsation in front of the shock wave causes it to reform. The method of finding this reformation process presented here can also be used in the future to find shock reformation.

Four-spacecraft observation of the shock transition. (a) and (b) Spacecraft positions relative to the tetrahedron center in the GSE xy and xz planes. The shock orientation and shock normal vector before the shock crossing are shown. (c) B observed by the four spacecraft. (d) Reduced ion distribution as a function of vn observed by MMS1.

Full Article:
Johlander, A. (SHARP), Battarbee, M., Turc, L., Ganse, U., Pfau-Kempf, Y., Grandin, M., et al. (2022). Quasi-parallel shock reformation seen by Magnetospheric Multiscale and ion-kinetic simulations. Geophysical Research Letters, 49, doi: 10.1029/2021GL096335

License: CC BY 4.0

Imaging X-ray Polarimetry Explorer launched successfully

The SHARP collaboration welcomes the successful launch of the NASA SMEX mission Imaging X-ray Polarimetry Explorer (IXPE) on December 9, 2021. The IXPE mission will measure among others the X-ray polarisation from supernova remnants, which helps to probe magnetic-field turbulence in the vicinity of collisionless shocks of these objects. SHARP members Dr. Jacco Vink and Dmitry Prokhorov are scientific members of the IXPE science team and are involved in the analysis of the first science observation done with IXPE, which started on January 10.

Find out more about IXPE here.