Electron energization at quasi-perpendicular oblique shocks in Supernova remnants
Final Report Abstract
Astrophysical shocks occur ubiquitously in our universe, with detections of non-thermal X-ray emission from sources such as supernova remnants (SNRs) implying the presence of accelerated electrons. Diffusive shock acceleration is widely accepted to be the primary acceleration mechanism, yet a pre-requisite for it is that the particles can easily pass through the shock, which is not the case for electrons at thermal energies. It follows that they require some pre-acceleration, and our physical understanding of it is essential to fully comprehend many astrophysical phenomena. To understand any electron-scale phenomena responsible for pre-acceleration, we require a method capable of resolving very small spatial scales. Particle-in-cell (PIC) simulations fulfil this criterion and have previously helped us to establish the contribution to electron pre-acceleration at perpendicular shocks from mechanisms such as shock surfing acceleration, magnetic reconnection, and stochastic Fermi acceleration. At oblique shocks, where the angle between large-scale magnetic field and the direction of motion of the shock front is θBn ≈ 20◦ –70◦ , reflected particles, in particular electrons, can escape upstream along the magnetic field, creating an extended region known as the foreshock. Any particles upstream of an oblique shock must encounter the foreshock before reaching the proper shock, and therefore its properties are important, as any turbulence in this region has the opportunity to influence upstream plasma before it can encounter and be accelerated at the shock. Escaped electrons can drive additional instabilities in the far upstream region, which in turn disturb and heat the plasma. Some electrons may return to the shock and experience additional stages of acceleration. An investigation of this behaviour is of high importance in shock research. Our simulations show that the turbulence in the foreshock evolves into compact structures of strong electromagnetic fields immediately ahead of the shock. Even for a magnetic-field obliquity of θBn ≈ 60◦ , for which only electrons can produce a foreshock, the turbulence driven in the foreshock has substantial consequences. Pre-energization of electron inside the shock is stronger than previously estimated, on account of the strong electromagnetic field driven there. Also, the energy density of electrons reflected off the shock into the foreshock region does not reach a steady state, but continuously increases throughout the simulations, suggesting that the overall efficiencies of turbulence driving and electron energization are larger than expected.
Publications
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Consequences of electron reflection back upstream in oblique shocks. Proceedings of 37th International Cosmic Ray Conference — PoS(ICRC2021), 448. Sissa Medialab.
Morris, Paul; Bohdan, Artem & Pohl, Martin
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Electron-beam instabilities in the foreshock of high Mach number oblique shocks. Proceedings of 37th International Cosmic Ray Conference — PoS(ICRC2021), 479. Sissa Medialab.
Weidl, Martin; Bohdan, Artem; Morris, Paul & Pohl, Martin
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Magnetic field amplification at planetary and astrophysical high Mach-number shocks. Proceedings of 37th International Cosmic Ray Conference — PoS(ICRC2021), 443. Sissa Medialab.
Bohdan, Artem; Pohl, Martin; Niemiec, Jacek; Morris, Paul; Matsumoto, Yosuke; Amano, Takanobu; Hoshino, Masahiro & Sulaiman, Ali
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Magnetic Field Amplification by the Weibel Instability at Planetary and Astrophysical Shocks with High Mach Number. Physical Review Letters, 126(9).
Bohdan, Artem; Pohl, Martin; Niemiec, Jacek; Morris, Paul J.; Matsumoto, Yosuke; Amano, Takanobu; Hoshino, Masahiro & Sulaiman, Ali
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Preacceleration in the Electron Foreshock. I. Electron Acoustic Waves. The Astrophysical Journal, 931(2), 129.
Morris, Paul J.; Bohdan, Artem; Weidl, Martin S. & Pohl, Martin
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The electron foreshock at high-Mach-number non-relativistic oblique shocks. Physics of Plasmas, 29(5).
Bohdan, Artem; Weidl, Martin S.; Morris, Paul J. & Pohl, Martin
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Pre-acceleration in the Electron Foreshock. II. Oblique Whistler Waves. The Astrophysical Journal, 944(1), 13.
Morris, Paul J.; Bohdan, Artem; Weidl, Martin S.; Tsirou, Michelle; Fulat, Karol & Pohl, Martin
