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1. In Situ Measurement of the Energy Fraction in Suprathermal and Energetic Particles at ACE, Wind, and PSP Interplanetary Shocks

   https://doi.org/10.3847/1538-4357/ac54af  

   David, Liam ; Fraschetti, Federico ; Giacalone, Joe ; Wimmer-Schweingruber, Robert F. ; Berger, Lars ; Lario, David ( March 2022 , The Astrophysical Journal)

   The acceleration of charged particles by interplanetary shocks (IPs) can drain a nonnegligible fraction of the plasma pressure. In this study, we have selected 17 IPs observed in situ at 1 au by the Advanced Composition Explorer and the Wind spacecraft, and 1 shock at 0.8 au observed by Parker Solar Probe. We have calculated the time-dependent partial pressure of suprathermal and energetic particles (smaller and greater than 50 keV for protons and 30 keV for electrons, respectively) in both the upstream and downstream regions. The particle fluxes were averaged for 1 hr before and 1 hr after the shock time to remove short timescale effects. Using the MHD Rankine–Hugoniot jump conditions, we find that the fraction of the total upstream energy flux transferred to suprathermal and energetic downstream particles is typically ≲16%, in agreement with previous observations and simulations. Notably, by accounting for errors on all measured shock parameters, we have found that for any given fast magnetosonic Mach number,M_f< 7, the angle between the shock normal and average upstream magnetic field,θ_Bn, is not correlated with the energetic particle pressure; in particular, the partial pressure of energized particles does not decrease forθ_Bn≳ 45°. The downstream electron-to-proton energy ratio in the range ≳ 140 eV for electrons and ≳ 70 keV for protons exceeds the expected ∼1% and nears equipartition (>0.1) for the Wind events.

    

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2. Electron Heating in 2D Particle-in-cell Simulations of Quasi-perpendicular Low-beta Shocks

   https://doi.org/10.3847/1538-4357/ad1f69  

   Tran, Aaron ; Sironi, Lorenzo ( April 2024 , The Astrophysical Journal)

   We measure the thermal electron energization in 1D and 2D particle-in-cell simulations of quasi-perpendicular, low-beta (β_p= 0.25) collisionless ion–electron shocks with mass ratiom_i/m_e= 200, fast Mach number${\mathcal{M}}_{\mathrm{ms}}=1$–4, and upstream magnetic field angleθ_Bn= 55°–85° from the shock normal$\hat{\mathit{n}}$. It is known that shock electron heating is described by an ambipolar,B-parallel electric potential jump, Δϕ_∥, that scales roughly linearly with the electron temperature jump. Our simulations have$\mathrm{\Delta }{\varphi }_{\parallel }/(0.5m_{\mathrm{i}}{u_{\mathrm{sh}}}^2)\sim 0.1$–0.2 in units of the pre-shock ions’ bulk kinetic energy, in agreement with prior measurements and simulations. Different ways to measureϕ_∥, including the use of de Hoffmann–Teller frame fields, agree to tens-of-percent accuracy. Neglecting off-diagonal electron pressure tensor terms can lead to a systematic underestimate ofϕ_∥in our low-β_pshocks. We further focus on twoθ_Bn= 65° shocks: a${\mathcal{M}}_{\mathrm{s}}=4$(${\mathcal{M}}_{\mathrm{A}}=1.8$) case with a long, 30d_iprecursor of whistler waves along$\hat{\mathit{n}}$, and a${\mathcal{M}}_{\mathrm{s}}=7$(${\mathcal{M}}_{\mathrm{A}}=3.2$) case with a shorter, 5d_iprecursor of whistlers oblique to both$\hat{\mathit{n}}$andB;d_iis the ion skin depth. Within the precursors,ϕ_∥has a secular rise toward the shock along multiple whistler wavelengths and also has localized spikes within magnetic troughs. In a 1D simulation of the${\mathcal{M}}_{\mathrm{s}}=4$,θ_Bn= 65° case,ϕ_∥shows a weak dependence on the electron plasma-to-cyclotron frequency ratioω_pe/Ω_ce, andϕ_∥decreases by a factor of 2 asm_i/m_eis raised to the true proton–electron value of 1836.

    

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3. Secondary Proton Beam Instability in a Turbulent Solar Wind at 50 R ⊙

   https://doi.org/10.3847/1538-4357/acf656  

   Markovskii, S. A. ; Vasquez, Bernard J. ( September 2023 , The Astrophysical Journal)

   We investigate a secondary proton beam instability coexisting with the ambient solar wind turbulence at 50R_☉. Three-dimensional hybrid numerical simulations (particle ions and a quasi-neutralizing electron fluid) are carried out with the plasma parameters in the observed range. In the turbulent background, the particle distribution function, in particular the slope of the “bump-on-tail” responsible for the instability, is time-dependent and inhomogeneous. The presence of the turbulence substantially reduces the growth rate and saturation level of the instability. We derive magnetic power spectra from the observational data and perform a statistical analysis to evaluate the average turbulence intensity at 50R_☉. This information is used to link the observed frequency spectrum to the wavenumber spectrum in the simulations. We verify that Taylor’s frozen-in hypothesis is valid for this purpose to a sufficient extent. To reproduce the typical magnetic power spectrum of the instability observed concurrently with the background turbulence, an artificial spacecraft probe is run through the simulation box. The thermal-ion instabilities are often seen as power elevations in the kinetic range of scales above an extrapolation of the turbulence spectrum from larger scales. We show that the elevated power in the simulations is much higher than the background level. Therefore, the turbulence at the average intensity does not obscure the secondary proton beam instability, as opposed to the solar wind at 1 au, in which the ambient turbulence typically obscures thermal-ion instabilities.

    

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4. Electron Injection via Modified Diffusive Shock Acceleration in High-Mach-number Collisionless Shocks

   https://doi.org/10.3847/2041-8213/ad0cf9  

   Grassi, A. ; Rinderknecht, H. G. ; Swadling, G. F. ; Higginson, D. P. ; Park, H.-S. ; Spitkovsky, A. ; Fiuza, F. ( November 2023 , The Astrophysical Journal Letters)

   The ability of collisionless shocks to efficiently accelerate nonthermal electrons via diffusive shock acceleration (DSA) is thought to require an injection mechanism capable of preaccelerating electrons to high enough energy where they can start crossing the shock front potential. We propose, and show via fully kinetic plasma simulations, that in high-Mach-number shocks electrons can be effectively injected by scattering in kinetic-scale magnetic turbulence produced near the shock transition by the ion Weibel, or current filamentation, instability. We describe this process as a modified DSA mechanism where initially thermal electrons experience the flow velocity gradient in the shock transition and are accelerated via a first-order Fermi process as they scatter back and forth. The electron energization rate, diffusion coefficient, and acceleration time obtained in the model are consistent with particle-in-cell simulations and with the results of recent laboratory experiments where nonthermal electron acceleration was observed. This injection model represents a natural extension of DSA and could account for electron injection in high-Mach-number astrophysical shocks, such as those associated with young supernova remnants and accretion shocks in galaxy clusters.

    

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5. The Maximum Energy of Shock-accelerated Cosmic Rays

   https://doi.org/10.3847/1538-4357/ad00b1  

   Diesing, Rebecca ( November 2023 , The Astrophysical Journal)

   Identifying the accelerators of Galactic cosmic ray (CR) protons with energies up to a few PeV (10¹⁵eV) remains a theoretical and observational challenge. Supernova remnants (SNRs) represent strong candidates because they provide sufficient energetics to reproduce the CR flux observed at Earth. However, it remains unclear whether they can accelerate particles to PeV energies, particularly after the very early stages of their evolution. This uncertainty has prompted searches for other source classes and necessitates comprehensive theoretical modeling of the maximum proton energy,$E_{\max }$, accelerated by an arbitrary shock. While analytic estimates of$E_{\max }$have been put forward in the literature, they do not fully account for the complex interplay between particle acceleration, magnetic field amplification, and shock evolution. This paper uses a multizone, semianalytic model of particle acceleration based on kinetic simulations to place constraints on$E_{\max }$for a wide range of astrophysical shocks. In particular, we develop relationships between$E_{\max }$, shock velocity, size, and ambient medium. We find that SNRs can only accelerate PeV particles under a select set of circumstances, namely, if the shock velocity exceeds ∼10⁴km s⁻¹and escaping particles drive magnetic field amplification. However, older and slower SNRs may still produce observational signatures of PeV particles due to populations accelerated when the shock was younger. Our results serve as a reference for modelers seeking to quickly produce a self-consistent estimate of the maximum energy accelerated by an arbitrary astrophysical shock.¹¹

   Presented as a thesis to the Department of Astronomy and Astrophysics, The University of Chicago, in partial fulfillment of the requirements for a Ph.D. degree.

    

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