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

Abstract

Computational and observational evidence is accruing that heliospheric shocks, as emitters of vorticity, can produce downstream magnetic flux ropes and filaments. This led Zank et al. to investigate a new paradigm whereby energetic particle acceleration near shocks is a combination of diffusive shock acceleration (DSA) with downstream acceleration by many small-scale contracting and reconnecting (merging) flux ropes. Using a model where flux-rope acceleration involves a first-order Fermi mechanism due to the mean compression of numerous contracting flux ropes, Zank et al. provide theoretical support for observations that power-law spectra of energetic particles downstream of heliospheric shocks can be harder than predicted by DSA theory and that energetic particle intensities should peak behind shocks instead of at shocks as predicted by DSA theory. In this paper, a more extended formalism of kinetic transport theory developed by le Roux et al. is used to further explore this paradigm. We describe how second-order Fermi acceleration, related to the variance in the electromagnetic fields produced by downstream small-scale flux-rope dynamics, modifies the standard DSA model. The results show that (i) this approach can qualitatively reproduce observations of particle intensities peaking behind the shock, thus providing further support for the new paradigm, and (ii) stochastic acceleration by compressible flux ropes tends to be more efficient than incompressible flux ropes behind shocks in modifying the DSA spectrum of energetic particles.

Formation of Power-law Electron Energy Spectra in Three-dimensional Low-β Magnetic Reconnection

“…An estimate of the power-law index by evaluating the acceleration rate α and the escape rate 1/τ esc for high-energy electrons (ε > 40ε th ) in the major acceleration region, where |v E · κ| is larger than a threshold, as indicated in Figure 4 This shows that the electron power-law energy spectrum is a dynamical balance between acceleration and escape, as in the classical Fermi-type acceleration processes. Several comprehensive models have been developed for studying particle acceleration in non-relativistic reconnection (Drake et al 2006(Drake et al , 2013(Drake et al , 2018le Roux et al 2015le Roux et al , 2016le Roux et al , 2018Li et al 2018b;Montag et al 2017;Zank et al 2014Zank et al , 2015Zhao et al 2018Zhao et al , 2019Adhikari et al 2019), and they all predict the formation of power-law energy distributions in certain regimes. The new 3D simulations allow us to study power-laws in non-relativistic studies and provide opportunities for testing those models.…”

The Energetic Particle Detector

“…Besides the previously mentioned sources of accelerated particles, recent observations of energetic particle enhancements in turbulent wakes of interplanetary shocks, near reconnecting current sheets, and within magnetic cavities filled with magnetic islands strongly support theoretical expectations of particle energisation to kilo-or even megaelectronvolt energies, in the presence of coherent structures in the solar wind (Zank et al 2014(Zank et al , 2015le Roux et al 2015le Roux et al , 2016. Many examples of such atypical energetic particle enhancements (AEPEs) in the heliosphere at radial distances ≥ 1 au have been reported (Khabarova et al 2018, and references therein).…”

The Re-Acceleration of Galactic Electrons at the Heliospheric Termination Shock

“…This mechanism, simply explained, entails that reaccelerated electrons can be transported from different positions along the TS to the spacecraft during good magnetic connections, which may change significantly with time and position. Although the modeling presented here illustrates the case for the involvement of DSA, alternative re-acceleration mechanisms must necessarily be considered in future work, such as stochastic acceleration and adiabatic heating in the heliosheath (e.g., Strauss et al 2010), magnetic reconnection (Zank et al 2014;le Roux et al 2015), or a combination of different processes with DSA (Zank et al 2015;le Roux et al 2016).…”