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1. Scattering of Superthermal Ions at Shocks: Dependence on Energy

   https://doi.org/10.3847/1538-4357/ad04dd  

   Gedalin, Michael; Ganushkina, Natalia; Pogorelov, Nikolai V; Roytershteyn, Vadim (November 2023, The Astrophysical Journal)

   Diffusive shock acceleration requires the production of backstreaming superthermal ions (injection) as a first step. Such ions can be generated in the process of scattering of ions in the superthermal tail off the shock front. Knowledge of the scattering of high-energy ions is essential for matching conditions of upstream and downstream distributions at the shock transition. Here we analyze the generation of backstreaming ions as a function of their initial energy in a model stationary shock and in a similar rippled shock. Rippling substantially enhances ion reflection and the generation of backstreaming ions for slightly and moderately superthermal energies, and thus is capable of ensuring ion injection into a further diffusive shock acceleration process. For high-energy ions, there is almost no difference in the fraction of backstreaming ions produced and the ion distributions between the planar stationary shock and the rippled shock. 

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2. Scattering of Ions at a Rippled Shock

   https://doi.org/10.3847/1538-4357/acd63c  

   Gedalin, Michael; Pogorelov, Nikolai V.; Roytershteyn, Vadim (July 2023, The Astrophysical Journal)

   Abstract In a collisionless shock the energy of the directed flow is converted to heating and acceleration of charged particles, and to magnetic compression. In low-Mach number shocks the downstream ion distribution is made of directly transmitted ions. In higher-Mach number shocks ion reflection is important. With the increase of the Mach number, rippling develops, which is expected to affect ion dynamics. Using ion tracing in a model shock front, downstream distributions of ions are analyzed and compared for a planar stationary shock with an overshoot and a similar shock with ripples propagating along the shock front. It is shown that rippling results in the distributions, which are substantially broader and more diffuse in the phase space. Gyrotropization is sped up. Rippling is able to generate backstreaming ions, which are absent in the planar stationary case. 

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3. Role of the overshoot in the shock self-organization

   https://doi.org/10.1017/S0022377823000090  

   Gedalin, Michael; Dimmock, Andrew P.; Russell, Christopher T.; Pogorelov, Nikolai V.; Roytershteyn, Vadim (April 2023, Journal of Plasma Physics)

   A collisionless shock is a self-organized structure where fields and particle distributions are mutually adjusted to ensure a stable mass, momentum and energy transfer from the upstream to the downstream region. This adjustment may involve rippling, reformation or whatever else is needed to maintain the shock. The fields inside the shock front are produced due to the motion of charged particles, which is in turn governed by the fields. The overshoot arises due to the deceleration of the ion flow by the increasing magnetic field, so that the drop of the dynamic pressure should be compensated by the increase of the magnetic pressure. The role of the overshoot is to regulate ion reflection, thus properly adjusting the downstream ion temperature and kinetic pressure and also speeding up the collisionless relaxation and reducing the anisotropy of the eventually gyrotropized distributions. 

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4. Change of Rankine–Hugoniot Relations during Postshock Relaxation of Anisotropic Distributions

   https://doi.org/10.3847/1538-4357/ac958d  

   Gedalin, Michael; Golan, Michal; Pogorelov, Nikolai V.; Roytershteyn, Vadim (November 2022, The Astrophysical Journal)

   Abstract 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. 

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5. Localized Heating of the Martian Topside Ionosphere Through the Combined Effects of Magnetic Pumping by Large‐Scale Magnetosonic Waves and Pitch Angle Diffusion by Whistler Waves

   https://doi.org/10.1029/2019GL086408  

   Fowler, C. M.; Agapitov, O. V.; Xu, S.; Mitchell, D. L.; Andersson, L.; Artemyev, A.; Espley, J.; Ergun, R. E.; Mazelle, C. (March 2020, Geophysical Research Letters)

   Abstract We present Mars Atmosphere and Volatile EvolutioN (MAVEN) observations of periodic (25 s) large‐scale (hundreds of km) magnetosonic waves propagating into the Martian dayside upper ionosphere. These waves adiabatically modulate the superthermal electron distribution function, and the induced electron temperature anisotropies drive the generation of observed electromagnetic whistler waves. The localized (in altitude) minimum in the ratio_pe/_ce provides conditions favorable for the local enhancement of efficient wave‐particle interactions, so that the induced whistlers act back on the superthermal electron population to isotropize the plasma through pitch angle scattering. These wave‐particle interactions break the adiabaticity of the large‐scale magnetosonic wave compressions, leading to local heating of the superthermal electrons during compressive wave “troughs.” Further evidence of this heating is observed as the subsequent phase shift between the observed perpendicular‐to‐parallel superthermal electron temperatures and compressive wave fronts. This heating mechanism may be important at other unmagnetized bodies. 

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