COSMIC-RAY CURRENT-DRIVEN TURBULENCE AND MEAN-FIELD DYNAMO EFFECT

Journal Article · Sun Jul 01 00:00:00 EDT 2012 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/753/1/6· OSTI ID:22036971

Rogachevskii, Igor; Kleeorin, Nathan ^[1]; Brandenburg, Axel ^[2]; Eichler, David ^[3]

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1. Department of Mechanical Engineering, Ben-Gurion University of the Negev, POB 653, Beer-Sheva 84105 (Israel)
2. NORDITA, Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, SE-10691 Stockholm (Sweden)
3. Department of Physics, Ben-Gurion University of the Negev, POB 653, Beer-Sheva 84105 (Israel)

We show that an {alpha} effect is driven by the cosmic-ray (CR) Bell instability exciting left-right asymmetric turbulence. Alfven waves of a preferred polarization have maximally helical motion, because the transverse motion of each mode is parallel to its curl. We show how large-scale Alfven modes, when rendered unstable by CR streaming, can create new net flux over any finite region, in the direction of the original large-scale field. We perform direct numerical simulations (DNSs) of a magnetohydrodynamic fluid with a forced CR current and use the test-field method to determine the {alpha} effect and the turbulent magnetic diffusivity. As follows from DNS, the dynamics of the instability has the following stages: (1) in the early stage, the small-scale Bell instability that results in the production of small-scale turbulence is excited; (2) in the intermediate stage, there is formation of larger-scale magnetic structures; (3) finally, quasi-stationary large-scale turbulence is formed at a growth rate that is comparable to that expected from the dynamo instability, but its amplitude over much longer timescales remains unclear. The results of DNS are in good agreement with the theoretical estimates. It is suggested that this dynamo is what gives weakly magnetized relativistic shocks such as those from gamma-ray bursts (GRBs) a macroscopic correlation length. It may also be important for large-scale magnetic field amplification associated with CR production and diffusive shock acceleration in supernova remnants (SNRs) and blast waves from GRBs. Magnetic field amplification by Bell turbulence in SNRs is found to be significant, but it is limited owing to the finite time available to the super-Alfvenicly expanding remnant. The effectiveness of the mechanisms is shown to be dependent on the shock velocity. Limits on magnetic field growth in longer-lived systems, such as the Galaxy and unconfined intergalactic CRs, are also discussed.

• OSTI ID: 22036971
• Journal Information: Astrophysical Journal, Vol. 753, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
ACCELERATION
ALFVEN WAVES
AMPLIFICATION
ASTROPHYSICS
ASYMMETRY
COMPUTERIZED SIMULATION
CORRELATIONS
COSMIC GAMMA BURSTS
GAMMA ASTRONOMY
INSTABILITY
INTERGALACTIC SPACE
MAGNETIC FIELDS
MEAN-FIELD THEORY
MILKY WAY
POLARIZATION
RELATIVISTIC RANGE
SUPERNOVA REMNANTS
TURBULENCE