New Publication “Implications of weak rippling of the shock ramp on the pattern of the electromagnetic field and ion distributions” by Michael Gedalin et al.

Collisionless shocks undergo structural changes with the increase of Mach number. Observations and numerical simulations indicate development of time-dependent rippling. It is not known at present what causes the rippling. However, effects of such rippling on the field pattern and ion motion and distributions can be studied without precise knowledge of the causes and detailed shape. It is shown that deviations of the normal component of the magnetic field from the constant value indicate certain spatial dependence of the rippling. Deviations of the motional electric field from the constant value indicate time dependence. It is argued that whistler waves should propagate towards upstream and downstream regions from the rippled ramp. It is shown that the downstream pattern of the fields and ion distributions should follow the rippling pattern, while collisionless relaxation should be faster than in the stationary planar case.

Full article:
Gedalin, M. (SHARP) and Ganushkina, N. (SHARP) (2022). Implications of weak rippling of the shock ramp on the pattern of the electromagnetic field and ion distributions. Journal of Plasma Physics, 88(3), doi: 10.1017/S0022377822000356


New Publication: “Analysis of multiscale structures at the quasi-perpendicular Venus bow shock – Results from Solar Orbiter’s first Venus flyby” by Andrew Dimmock et al.

This study aims to investigate the outbound Venus bow shock crossing measured by Solar Orbiter during the first flyby. We study the complex features of the bow shock traversal in which multiple large amplitude magnetic field and density structures were observed as well as higher frequency waves. Our aim is to understand the physical mechanisms responsible for these high amplitude structures, characterize the higher frequency waves, determine the source of the waves, and put these results into context with terrestrial bow shock observations.

The Venus bow shock at a moderately high Mach number (∼5) in the quasi-perpendicular regime exhibits complex features similar to the Earth’s bow shock at comparable Mach numbers. The study highlights the need to be able to distinguish between large amplitude waves and spatial structures such as shock rippling. The simultaneous high frequency observations also demonstrate the complex nature of energy dissipation at the shock and the important question of understanding cross-scale coupling in these complex regions. These observations will be important to interpreting future planetary missions and additional gravity assist maneuvers.

Shock substructure in density and magnetic field. Panels a and b: electron density determined from the spacecraft potential and the magnetic field modulus. A wavelet coherency spectrum is shown in panel c, which demonstrates that panels a and b share common variations around 0.5–1 Hz during the shock front, which are in phase.

Full Article:
Dimmock, A. P. (SHARP), Khotyaintsev, Yu. V. (SHARP), Lalti, A. (SHARP), Yordanova, E., Edberg, N. J. T., Steinvall, K., Graham, D. B. (SHARP), et al. (2022). Analysis of multiscale structures at the quasi-perpendicular Venus bow shock – Results from Solar Orbiter’s first Venus flyby. Astronomy and Astrophysics, 660, doi: 10.1051/0004-6361/202140954

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


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: https://doi.org/10.5281/zenodo.6343989

A Technical report detailing the content of the database can be found here: https://doi.org/10.48550/arXiv.2203.04680 (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

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