Diffusive Shock Acceleration and Reconnection Acceleration Processes

Abstract

Shock waves, as shown by simulations and observations, can generate high levels of downstream vortical turbulence, including magnetic islands. We consider a combination of diffusive shock acceleration (DSA) and downstream magnetic-island-reconnection-related processes as an energization mechanism for charged particles. Observations of electron and ion distributions downstream of interplanetary shocks and the heliospheric termination shock (HTS) are frequently inconsistent with the predictions of classical DSA. We utilize a recently developed transport theory for charged particles propagating diffusively in a turbulent region filled with contracting and reconnecting plasmoids and small-scale current sheets. Particle energization associated with the antireconnection electric field, a consequence of magnetic island merging, and magnetic island contraction, are considered. For the former only, we find that (i) the spectrum is a hard power law in particle speed, and (ii) the downstream solution is constant. For downstream plasmoid contraction only, (i) the accelerated spectrum is a hard power law in particle speed; (ii) the particle intensity for a given energy peaks downstream of the shock, and the distance to the peak location increases with increasing particle energy, and (iii) the particle intensity amplification for a particular particle energy, f x c c f c c , 0 , , 0 0 () () is not 1, as predicted by DSA, but increases with increasing particle energy. The general solution combines both the reconnection-induced electric field and plasmoid contraction. The observed energetic particle intensity profile observed by Voyager 2 downstream of the HTS appears to support a particle acceleration mechanism that combines both DSA and magnetic-island-reconnectionrelated processes.

Particle acceleration and reconnection in the solar wind

“…We adopt a planar shock geometry, and solve a spatially 1D form of the transport equation (1) for prescribed constant upstream and downstream flows U 1 and U 2 . The normalized solution is (see details in [17])…”

“…The location of the particle 070011-3 intensity peak appears to increase with increasing energy. In Figure 2, we plot a set of normalized particle intensity solutions for the general case in the same format as the observations (see [17] for relevant parameters). The theoretical curves show that for the corresponding set of normalized energies, we obtain an amplification factor that increases with increasing energy, and for which the amplification factor is roughly consistent across the observed energies.…”

Anomalous Galactic Cosmic Rays in the Framework of AMS-02

“…In this mechanism, depending on Larmor radius, particles bounce back and forth across the shock front due to their interactions with magnetic irregularities process and gain energy. However, DSA is not able to explain all energetic particle observations in astrophysical sources and an alternative acceleration mechanism, namely magnetic reconnection (see Zank et al 2015), has been applied for some Galactic and extra-galactic sources to explain the CR acceleration (e.g., Khiali et al 2015a,b).…”

Combining Diffusive Shock Acceleration With Acceleration by Contracting and Reconnecting Small-Scale Flux Ropes at Heliospheric Shocks

“…This work is partly motivated by observational evidence that (i) accelerated particle spectra at heliospheric shocks are sometimes harder than expected from standard DSA theory (Desai et al 2004;Decker et al 2008;Ho et al 2008), (ii) energetic particle intensities often peak behind heliospheric shocks (Lario et al 2005;Stone et al 2005Stone et al , 2008Ho et al 2008;Fisk & Gloeckler 2014;Zank et al 2015aZank et al , 2015b) instead of at the shock front as standard DSA theory predicts, and (iii) regions behind shocks, especially close to the heliospheric current sheet and other primary current sheets, appear to be filled with contracting and reconnecting (merging) small-scale flux ropes that are linked to particle acceleration (Giacalone & Jokipii 2007;Chian & Munoz 2011;Song et al 2012;Karimabadi et al 2014;Khabarova et al 2015aKhabarova et al , 2015bKhabarova et al , 2016. Zank et al (2015aZank et al ( , 2015b) concentrated on combining DSA with the downstream acceleration by the mean reconnection electric field of many merging flux ropes plus acceleration by the mean compression of many contracting flux ropes. The results show that whereas both flux-rope mechanisms were found to harden the power-law spectrum of DSA, it was the mean compression of many flux ropes that resulted in the particle intensity peaking downstream.…”

“…In this paper, the kinetic transport formalism of le Roux et al (2015aRoux et al ( , 2015b) is utilized to further investigate the promising paradigm of Zank et al (2015aZank et al ( , 2015b that suprathermal charged particle acceleration at collisionless heliospheric shocks can be viewed as a combination of DSA and acceleration by contracting and reconnecting small-scale flux ropes downstream of these shocks. Zank et al (2015aZank et al ( , 2015b makes a case for this paradigm by emphasizing flux-rope acceleration mechanisms involving field-aligned guiding center motion in the mean reconnection electric field of many merging flux ropes plus first-order Fermi acceleration by the mean compression of many contracting flux ropes.…”