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.
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
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.
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
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.
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
Nonthermal, pickup ions (PUIs) represent an energetic component of the solar wind (SW). While a number of theoretical models have been proposed to describe the PUI flow, of major importance are in situ measurements providing us with the vital source of model validation. The Solar Wind Ion Composition Spectrometer (SWICS) instrument on board the Ulysses spacecraft was specifically designed for this purpose. Zhang et al. proposed a new, accurate method for the derivation of ion velocity distribution function in the SW frame on the basis of count rates collected by SWICS. We calculate the moments of these distribution functions for protons (H+) and He+ ions along the Ulysses trajectory for a period of 2 months including the Halloween 2003 solar storm. This gives us the time distributions of PUI density and temperature. We compare these with the results obtained earlier for the same interval of time, in which the ion spectra are converted to the SW frame using the narrow-beam approximation. Substantial differences are identified, which are of importance for the interpretation of PUI distributions in the 3D, time-dependent heliosphere. We also choose one of the shocks crossed by Ulysses during this time interval and analyze the distribution functions and PUI bulk properties in front of and behind it. The results are compared with the test-particle calculations and diffusive shock acceleration theory.
Full Article: Smith, W. P., Renfroe, K., Pogorelov, N. V., Zhang, M., Gedalin, M. (SHARP) and Kim, T. K. (2022). Bulk Properties of Pickup Ions Derived from the Ulysses Solar Wind Ion Composition Spectrometer Data. The Astrophysical Journal, 933, doi: 10.3847/1538-4357/ac73f2
Observations with imaging atmospheric Cherenkov telescopes (IACTs) have enhanced our knowledge of nearby supernova (SN) remnants with ages younger than 500 yr by establishing Cassiopeia A and the remnant of Tycho’s SN as very-high-energy (VHE) γ-ray sources. The remnant of Kepler’s SN, which is the product of the most recent naked-eye SN in our Galaxy, is comparable in age to the other two, but is significantly more distant. If the γ-ray luminosities of the remnants of Tycho’s and Kepler’s SNe are similar, then the latter is expected to be one of the faintest γ-ray sources within reach of the current generation IACT arrays. Here we report evidence at a statistical level of 4.6σ for a VHE signal from the remnant of Kepler’s SN based on deep observations by the High Energy Stereoscopic System (H.E.S.S.) with an exposure of 152 h. The measured integral flux above an energy of 226 GeV is ∼0.3% of the flux of the Crab Nebula. The spectral energy distribution (SED) reveals a γ-ray emitting component connecting the VHE emission observed with H.E.S.S. to the emission observed at GeV energies with Fermi-LAT. The overall SED is similar to that of the remnant of Tycho’s SN, possibly indicating the same nonthermal emission processes acting in both these young remnants of thermonuclear SNe.
Full Article: H. E. S. S. Collaboration, Aharonian, F., Ait Benkhali, F., Angüner, O., et al. (2022). Evidence for γ-ray emission from the remnant of Kepler’s supernova based on deep H.E.S.S. observations. Astronomy and Astrophysics, 662, doi: 10.1051/0004-6361/202243096
Since the day of its explosion, supernova (SN) 1987A has been closely monitored to study its evolution and to detect its central compact relic. In fact, the formation of a neutron star is strongly supported by the detection of neutrinos from the SN. However, besides the detection in the Atacama Large Millimeter/submillimeter Array (ALMA) data of a feature that is compatible with the emission arising from a protopulsar wind nebula (PWN), the only hint of the existence of such an elusive compact object is provided by the detection of hard emission in NuSTAR data up to ∼20 keV. We report on the simultaneous analysis of multiepoch observations of SN 1987A performed with Chandra, XMM-Newton, and NuSTAR. We also compare the observations with a state-of-the-art three-dimensional magnetohydrodynamic simulation of SN 1987A. A heavily absorbed power law, consistent with the emission from a PWN embedded in the heart of SN 1987A, is needed to properly describe the high-energy part of the observed spectra. The spectral parameters of the best-fit power law are in agreement with the previous estimate, and exclude diffusive shock acceleration as a possible mechanism responsible for the observed nonthermal emission. The information extracted from our analysis is used to infer the physical characteristics of the pulsar and the broadband emission from its nebula, in agreement with the ALMA data. Analysis of the synthetic spectra also shows that, in the near future, the main contribution to the Fe K emission line will originate in the outermost shocked ejecta of SN 1987A.
Full Article: Greco, E. (SHARP), Miceli, M., Orlando, S., Olmi, B., Bocchino, F., Nagataki, S., Sun, L., Vink, J. (SHARP), et al. (2022). Additional Evidence for a Pulsar Wind Nebula in the Heart of SN 1987A from Multiepoch X-Ray Data and MHD Modeling. The Astrophysical Journal, 931, doi: 10.3847/1538-4357/ac679d
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
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.
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