 Publication Date:
 NSFPAR ID:
 10344783
 Journal Name:
 The Astrophysical Journal
 Volume:
 922
 Issue:
 1
 Page Range or eLocationID:
 1
 ISSN:
 0004637X
 Sponsoring Org:
 National Science Foundation
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We have performed twodimensional hybrid simulations of nonrelativistic collisionless shocks in the presence of preexisting energetic particles (‘seeds’); such a study applies, for instance, to the reacceleration of galactic cosmic rays (CRs) in supernova remnant (SNR) shocks and solar wind energetic particles in heliospheric shocks. Energetic particles can be effectively reflected and accelerated regardless of shock inclination via a process that we call diffusive shock reacceleration. We find that reaccelerated seeds can drive the streaming instability in the shock upstream and produce effective magnetic field amplification. This can eventually trigger the injection of thermal protons even at oblique shocks that ordinarily cannot inject thermal particles. We characterize the current in reflected seeds, finding that it tends to a universal value $J\simeq en_{\text{CR}}v_{\text{sh}}$ , where $en_{\text{CR}}$ is the seed charge density and $v_{\text{sh}}$ is the shock velocity. When applying our results to SNRs, we find that the reacceleration of galactic CRs can excite the Bell instability to nonlinear levels in less than ${\sim}10~\text{yr}$ , thereby providing a minimum level of magnetic field amplification for any SNR shock. Finally, we discuss the relevance of diffusive shock reacceleration also for other environments, such as heliospheric shocks, galactic superbubbles and clusters of galaxies.

The possibility that charged particles are accelerated statistically in a “sea” of smallscale interacting magnetic flux ropes in the supersonic solar wind is gaining credence. In this Letter, we extend the Zank et al. statistical transport theory for a nearly isotopic particle distribution by including an escape term corresponding to particle loss from a finite acceleration region. Steadystate 1D solutions for both the accelerated particle velocity distribution function and differential intensity are derived. We show Ulysses observations of an energetic particle flux enhancement event downstream of a shock near 5 au that is inconsistent with the predictions of classical diffusive shock acceleration (DSA) but may be explained by local acceleration associated with magnetic islands. An automated GradShafranov reconstruction approach is employed to identify smallscale magnetic flux ropes behind the shock. For the first time, the observed energetic particle “timeintensity” profile and spectra are quantitatively compared with theoretical predictions. The results show that stochastic acceleration by interacting magnetic islands accounts successfully for the observed (i) peaking of particle intensities behind the shock instead of at the shock front as standard DSA predicts; (ii) increase in the particle flux amplification factor with increasing particle energy; (ii) increase in distance between the particlemore »

The possibility that charged particles are accelerated statistically in a “sea” of smallscale interacting magnetic flux ropes in the supersonic solar wind is gaining credence. In this Letter, we extend the Zank et al. statistical transport theory for a nearly isotopic particle distribution by including an escape term corresponding to particle loss from a finite acceleration region. Steadystate 1D solutions for both the accelerated particle velocity distribution function and differential intensity are derived. We show Ulysses observations of an energetic particle flux enhancement event downstream of a shock near 5 au that is inconsistent with the predictions of classical diffusive shock acceleration (DSA) but may be explained by local acceleration associated with magnetic islands. An automated GradShafranov reconstruction approach is employed to identify smallscale magnetic flux ropes behind the shock. For the first time, the observed energetic particle “timeintensity” profile and spectra are quantitatively compared with theoretical predictions. The results show that stochastic acceleration by interacting magnetic islands accounts successfully for the observed (i) peaking of particle intensities behind the shock instead of at the shock front as standard DSA predicts; (ii) increase in the particle flux amplification factor with increasing particle energy; (ii) increase in distance between the particlemore »

ABSTRACT We present the first simulations evolving resolved spectra of cosmic rays (CRs) from MeV–TeV energies (including electrons, positrons, (anti)protons, and heavier nuclei), in live kineticmagnetohydrodynamics galaxy simulations with star formation and feedback. We utilize new numerical methods including terms often neglected in historical models, comparing Milky Way analogues with phenomenological scattering coefficients ν to Solarneighbourhood [Local interstellar medium (LISM)] observations (spectra, B/C, e+/e−, $\mathrm{\bar{p}}/\mathrm{p}$, 10Be/9Be, ionization, and γrays). We show it is possible to reproduce observations with simple singlepowerlaw injection and scattering coefficients (scaling with rigidity R), similar to previous (nondynamical) calculations. We also find: (1) The circumgalactic medium in realistic galaxies necessarily imposes an $\sim 10\,$ kpc CR scattering halo, influencing the required ν(R). (2) Increasing the normalization of ν(R) renormalizes CR secondary spectra but also changes primary spectral slopes, owing to source distribution and loss effects. (3) Diffusive/turbulent reacceleration is unimportant and generally subdominant to gyroresonant/streaming losses, which are subdominant to adiabatic/convective terms dominated by $\sim 0.11\,$ kpc turbulent/fountain motions. (4) CR spectra vary considerably across galaxies; certain features can arise from local structure rather than transport physics. (5) Systematic variation in CR ionization rates between LISM and molecular clouds (or Galactic position) arises naturally without invoking alternativemore »

Abstract The structure of shocks and turbulence are strongly modified during the acceleration of cosmic rays (CRs) at a shock wave. The pressure and the collisionless viscous stress decelerate the incoming thermal gas and thus modify the shock structure. A CR streaming instability ahead of the shock generates the turbulence on which CRs scatter. The turbulent magnetic field in turn determines the CR diffusion coefficient and further affects the CR energy spectrum and pressure distribution. The dissipation of turbulence contributes to heating the thermal gas. Within a multicomponent fluid framework, CRs and thermal gas are treated as fluids and are closely coupled to the turbulence. The system equations comprise the gas dynamic equations, the CR pressure evolution equation, and the turbulence transport equations, and we adopt typical parameters for the hot ionized interstellar medium. It is shown that the shock has no discontinuity but possesses a narrow but smooth transition. The selfgenerated turbulent magnetic field is much stronger than both the largescale magnetic field and the preexisting turbulent magnetic field. The resulting CR diffusion coefficient is substantially suppressed and is more than three orders smaller near the shock than it is far upstream. The results are qualitatively consistent with certainmore »