Turbulence, Magnetic Reconnection in Turbulent Fluids and Energetic Particle Acceleration

Abstract

Turbulence is ubiquitous in astrophysics. It radically changes many astrophysical phenomena, in particular, the propagation and acceleration of cosmic rays. We present the modern understanding of compressible magnetohydrodynamic (MHD) turbulence, in particular its decomposition into Alfvén, slow and fast modes, discuss the density structure of turbulent subsonic and supersonic media, as well as other relevant regimes of astrophysical turbulence. All this information is essential for understanding the energetic par- ticle acceleration that we discuss further in the review. For instance, we show how fast and slow modes accelerate energetic particles through the second order Fermi acceleration, while density fluctuations generate magnetic fields in pre-shock regions enabling the first order Fermi acceleration of high energy cosmic rays. Very importantly, however, the first order Fermi cosmic ray acceleration is also possible in sites of magnetic reconnection. In the presence of turbulence this reconnection gets fast and we present numerical evidence supporting the predictions of the Lazarian & Vishniac (1999) model of fast reconnection. The efficiency of this process suggests that magnetic reconnection can release substantial amounts of energy in short periods of time. As the particle tracing numerical simulations show that the particles can be efficiently accelerated during the reconnection, we argue that the process of magnetic reconnection may be much more important for particle acceleration than it is currently accepted. In particular, we discuss the acceleration arising from reconnection as a possible origin of the anomalous cosmic rays measured by Voyagers as well as the origin cosmic ray excess in the direction of Heliotail.

Additional acceleration of solar-wind particles in current sheets of the heliosphere

“…In these 2-D simulations the y dimension is chosen to be invariant. Since we solve a system of partial differential equations in a limited simulation region, we consider the periodic boundary conditions in the z direction, similar to other studies of such a problem (Makov and Payne, 1995;Verboncoeur and Gladd, 1995;Zeiler et al, 2002), so that a particle that leaves the system through the right or left boundary (see Fig. 7 for the model cartoon) appears on the opposite boundary.…”

“…These current sheets in the solar-wind environment can be associated with streamers and filaments and can be produced by turbulence, created during the solar wind's ejection from the corona, while smaller-scale current sheets observed at several AU may originate from in situ turbulence in the solar wind (Greco et al, 2010;Lazarian et al, 2012). However, some alternative suggestions can be also considered, such as these current sheets being caused by certain processes in the interplanetary space, e.g.…”

Mission Oriented Support and Theory (MOST) for MMS—the Goddard Space Flight Center/University of California Los Angeles Interdisciplinary Science Program

“…One such threedimensional kinetic simulation is described in Daughton et al (2011) and the argument that the simulation reflects the generation of intermittent cascading turbulence is discussed in Leonardis et al (2013). Second, magnetofluid turbulence itself can give rise to reconnection (Matthaeus and Montgomery 1980;Matthaeus and Lamkin 1985;Servidio et al 2011;Lazarian et al 2012;Donato et al 2013). The third way in which turbulence and reconnection are related occurs when the background within which reconnection is initiated is permeated by turbulence.…”

“…The third way in which turbulence and reconnection are related occurs when the background within which reconnection is initiated is permeated by turbulence. In that situation, estimates of reconnection rates and classical pictures involving slow shock waves are likely to be modified dramatically by the background turbulence (Matthaeus and Lamkin 1985;Lazarian and Vishniac 1999;Kowal et al 2009;Eyink et al 2011;Lazarian et al 2012). The turbulence can nudge fluid elements into thin current sheets and initiate reconnection.…”

Topics in Microphysics of Relativistic Plasmas

“…Studies of the First order Fermi acceleration has been performed so far for non-relativistic reconnection (see de Gouveia dal Pino & Lazarian 2005, Lazarian 2005, Drake et al 2006, Lazarian & Opher 2009, Drake et al 2010, Lazarian & Desiati 2010, Kowal et al 2012. We briefly summarize those below.…”