17 research outputs found
Turbulent Reacceleration of Streaming Cosmic Rays
Subsonic, compressive turbulence transfers energy to cosmic rays (CRs), a
process known as non-resonant reacceleration. It is often invoked to explain
observed ratios of primary to secondary CRs at energies,
assuming wholly diffusive CR transport. However, such estimates ignore the
impact of CR self-confinement and streaming. We study these issues in stirring
box magnetohydrodynamic (MHD) simulations using Athena++, with field-aligned
diffusive and streaming CR transport. For diffusion only, we find CR
reacceleration rates in good agreement with analytic predictions. When
streaming is included, reacceleration rates depend on plasma . Due to
streaming-modified phase shifts between CR and gas variables, they are slower
than canonical reacceleration rates in low- environments like the
interstellar medium (ISM) but remain unchanged in high- environments
like the intracluster medium (ICM). We also quantify the streaming energy loss
rate in our simulations. For sub-Alfv\'{e}nic turbulence, it is
resolution-dependent (hence unconverged in large scale simulations) and heavily
suppressed -- by an order of magnitude -- compared to the isotropic loss rate
, due to
misalignment between the mean field and isotropic CR gradients.
Counterintuitively, and unlike acceleration efficiencies, CR losses are almost
independent of magnetic field strength over and are,
therefore, not the primary factor behind lower acceleration rates when
streaming is included. While this paper is primarily concerned with how
turbulence affects CRs, in a follow-up paper (Bustard and Oh, in prep), we
consider how CRs affect turbulence by diverting energy from the MHD cascade,
altering the pathway to gas heating and steepening the turbulent power
spectrum.Comment: 20 pages, 7 figures, comments welcome
Cosmic Ray Drag and Damping of Compressive Turbulence
While it is well-known that cosmic rays (CRs) can gain energy from turbulence
via second order Fermi acceleration, how this energy transfer affects the
turbulent cascade remains largely unexplored. Here, we show that damping and
steepening of the compressive turbulent power spectrum are expected once the
damping time becomes comparable to the turbulent cascade time. Magnetohydrodynamic
(MHD) simulations of stirred compressive turbulence in a gas-CR fluid with
diffusive CR transport show clear imprints of CR-induced damping, saturating at
, where is the
turbulent energy input rate. In that case, almost all the energy in large scale
motions is absorbed by CRs and does not cascade down to grid scale. Through a
Hodge-Helmholtz decomposition, we confirm that purely compressive forcing can
generate significant solenoidal motions, and we find preferential CR damping of
the compressive component in simulations with diffusion and streaming,
rendering small-scale turbulence largely solenoidal, with implications for
thermal instability and proposed resonant scattering of GeV CRs by
fast modes. When CR transport is streaming dominated, CRs also damp large scale
motions, with kinetic energy reduced by up to to an order of magnitude in
realistic scenarios, but turbulence (with a reduced
amplitude) still cascades down to small scales with the same power spectrum.
Such large scale damping implies that turbulent velocities obtained from the
observed velocity dispersion may significantly underestimate turbulent forcing
rates, i.e. .Comment: Accepted to ApJ. Additions include Section 4.5 which shows decomposed
solenoidal and compressive spectra, Figure 7 showing CR and magnetic energy
spectra, new Figure 3 panel showing modified Burgers spectra, and small
corrections to Figure
Cosmic-Ray Drag and Damping of Compressive Turbulence
While it is well known that cosmic rays (CRs) can gain energy from turbulence via second-order Fermi acceleration, how this energy transfer affects the turbulent cascade remains largely unexplored. Here, we show that damping and steepening of the compressive turbulent power spectrum are expected once the damping time becomes comparable to the turbulent cascade time. Magnetohydrodynamic simulations of stirred compressive turbulence in a gas-CR fluid with diffusive CR transport show clear imprints of CR-induced damping, saturating at , where is the turbulent energy input rate. In that case, almost all of the energy in large-scale motions is absorbed by CRs and does not cascade down to grid scale. Through a Hodge–Helmholtz decomposition, we confirm that purely compressive forcing can generate significant solenoidal motions, and we find preferential CR damping of the compressive component in simulations with diffusion and streaming, rendering small-scale turbulence largely solenoidal, with implications for thermal instability and proposed resonant scattering of E ≳ 300 GeV CRs by fast modes. When CR transport is streaming dominated, CRs also damp large-scale motions, with kinetic energy reduced by up to 1 order of magnitude in realistic E _CR ∼ E _g scenarios, but turbulence (with a reduced amplitude) still cascades down to small scales with the same power spectrum. Such large-scale damping implies that turbulent velocities obtained from the observed velocity dispersion may significantly underestimate turbulent forcing rates, i.e.,