Category: Uncategorized

We report on proper motion measurements of the forward- and reverse shock regions of the supernova remnant Cassiopeia A (Cas A), including deceleration/acceleration measurements of the forward shock. The measurements combine 19 yr of observations with the Chandra X-ray Observatory, using the 4.2–6 keV continuum band, preferentially targeting X-ray synchrotron radiation. The average expansion rate is 0.218 ± 0.029% yr⁻¹ for the forward shock, corresponding to a velocity of ≈5800 km s⁻¹. The time derivative of the proper motions indicates deceleration in the east, and an acceleration up to 1.1 × 10⁻⁴ yr⁻² in the western part. The reverse shock moves outward in the east, but in the west it moves toward the center with an expansion rate of −0.0225 ± 0.0007 % yr⁻¹, corresponding to −1884 ± 17 km s⁻¹. In the west, the reverse shock velocity in the ejecta frame is ≳3000 km s⁻¹, peaking at ∼8000 km s⁻¹, explaining the presence of X-ray synchrotron emitting filaments there. The backward motion of the reverse shock can be explained by either a scenario in which the forward shock encountered a partial, dense, wind shell, or one in which the shock transgressed initially through a lopsided cavity, created during a brief Wolf–Rayet star phase. Both scenarios are consistent with the local acceleration of the forward shock. Finally we report on the proper motion of the northeastern jet, using both the X-ray continuum band, and the Si xiii K-line emission band. We find expansion rates of, respectively, 0.21% and 0.24% yr⁻¹, corresponding to velocities at the tip of the X-ray jet of 7830–9200 km s⁻¹.

Full Article:
Vink, J. (SHARP), Patnaude, D. J. and Castro, D. (2022). The Forward and Reverse Shock Dynamics of Cassiopeia A. The Astrophysical Journal, 929, doi: 10.3847/1538-4357/ac590f

License: CC BY 4.0

A press release about the shock dynamics of Cassiopeia A has been published on astronomie.nl. The article is based on the recent findings in the paper “The forward and reverse shock dynamics of Cassiopeia A” by Jacco Vink et al. (accepted for publication in The Astrophysical Journal, preprint).

Click here to read the full article on astronomie.nl

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.

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

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)

The 6th SHARP Progress meeting took place on 23rd February, 2022.

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

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.

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

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.

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

The first SHARP Review meeting took place on January 19th and 20th, 2022.

The fifth SHARP Progress meeting took place on 3rd December, 2021.