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The origin of cosmic magnetic fields is an unsolved problem and magnetogenesis could have occurred in the early Universe. We study the evolution of such primordial magnetic fields across the cosmological recombination epoch via 3D magnetohydrodynamic numerical simulations. We compute the effective or net heating rate of baryons due to decaying magnetic fields and its dependence on the magnetic field strength and spectral index. In the drag-dominated regime (⁠|$z$| ≳ 1500), prior to recombination, we find no real heating is produced. Our simulations allow us to smoothly trace a new transition regime (600 ≲ |$z$| ≲ 1500), where magnetic energy decays, at first, into the kinetic energy of baryons. A turbulent velocity field is built up until it saturates, as the net heating rate rises from a low value at recombination to its peak towards the end of the transition regime. This is followed by a turbulent decay regime (⁠|$z$| ≲ 600) where magnetic energy dissipates via turbulent decay of both magnetic and velocity fields while net heating remains appreciable and declines slowly. Both the peak of the net heating rate and the onset of turbulent decay are delayed significantly beyond recombination, by up to 0.5 Myr (until |$z$| ≃ 600–700), for scale-invariant magnetic fields. We concentrate on low magnetic field strength to avoid confusion with magnetic field-generated density fluctuations. Analytic approximations are provided and we present numerical results for a range of field strengths (≃10−3− 2 × 10−2nG) and spectral indices, illustrating the redshift-dependence of dissipation and net heating rates. These can be used to study cosmic microwave background constraints on primordial magnetic fields. [ABSTRACT FROM AUTHOR]

Current observations of supernova remnant (SNR) RX J1713.7-3946 favor the leptonic scenario for the TeV emission, where the radio to X-ray emission is produced via the synchrotron process and the γ -ray emission is produced via the inverse Comptonization of soft background photons, and the electron distribution can be inferred from the observed γ -ray spectrum with a spectral inversion method. It is shown that the observed correlation between the X-ray and γ -ray brightness of SNR RX J1713.7-3946 can be readily explained with the assumption that the energy density of energetic electrons is proportional to that of the magnetic field in such a scenario. A two-dimensional magnetohydrodynamic simulation is then carried out to model the overall emission spectrum. It is found that the total energy of electrons above ∼1 GeV is equal to that of the magnetic field. This is the first piece of observational evidence for energy equipartition between energetic electrons and magnetic field in the downstream of strong collisionless astrophysical shocks of SNRs. [ABSTRACT FROM AUTHOR]

We perform three-dimensional, vertically-stratified, local shearing-box ideal MHD simulations of the magnetorotational instability (MRI) that include a net vertical magnetic flux, which is characterized by midplane plasma β0 (ratio of gas to magnetic pressure). We have considered β0 = 102, 103, and 104, and in the first two cases the most unstable linear MRI modes are well resolved in the simulations. We find that the behavior of the MRI turbulence strongly depends on β0: the radial transport of angular momentum increases with net vertical flux, achieving α ~ 0.08 for β = 104 and α ≳ 1.0 for β0 = 100, where α is the height-integrated and mass-weighted Shakura-Sunyaev parameter. A critical value lies at β0 ~ 103: for β0 ≳ 103, the disk consists of a gas pressure dominated midplane and a magnetically dominated corona. The turbulent strength increases with net flux, and angular momentum transport is dominated by turbulent fluctuations. The magnetic dynamo that leads to cyclic flips of large-scale fields still exists, but becomes more sporadic as net flux increases. For β0 ≲ 103, the entire disk becomes magnetically dominated. The turbulent strength saturates, and the magnetic dynamo is fully quenched. Stronger large-scale fields are generated with increasing net flux, which dominates angular momentum transport. A strong outflow is launched from the disk by the magnetocentrifugal mechanism, and the mass flux increases linearly with net vertical flux and shows sign of saturation at β0 ≲ 102. However, the outflow is unlikely to be directly connected to a global wind: for β0 ≳ 103, the large-scale field has no permanent bending direction due to dynamo activities, while for β0 ≲ 103, the outflows from the top and bottom sides of the disk bend towards opposite directions, inconsistent with a physical disk wind geometry. Global simulations are needed to address the fate of the outflow. [ABSTRACT FROM AUTHOR]

We use magnetic field and ion moment data from the MFI and SWE instruments on board the Wind spacecraft to study the nature of solar wind turbulence at ion-kinetic scales. We analyze the spectral properties of magnetic field fluctuations between 0.1 and 5.4 Hz during 2012 using an automated routine, computing high-resolution 92 s power and magnetic helicity spectra. To ensure the spectral features are physical, we make the first in-flight measurement of the MFI “noise-floor” using tail-lobe crossings of the Earth’s magnetosphere during early 2004. We utilize Taylor’s hypothesis to Doppler-shift into the spacecraft frequency frame, finding that the spectral break observed at these frequencies is best associated with the proton cyclotron resonance scale, 1/kc, rather than the proton inertial length, di, or proton gyroscale, ρi. This agreement is strongest when we consider periods where , and is consistent with a spectral break at di for and at ρi for . We also find that the coherent magnetic helicity signature observed at these frequencies is bounded at low frequencies by 1/kc, and its absolute value reaches a maximum at ρi. These results hold in both slow and fast wind streams, but with a better correlation in the more Alfvénic fast wind where the helicity signature is strongest. We conclude that these findings are consistent with proton cyclotron resonance as an important mechanism for dissipation of turbulent energy in the solar wind, occurring at least half the time in our selected interval. However, we do not rule out additional mechanisms. [ABSTRACT FROM AUTHOR]

We present the first statistical study of the anisotropy of the magnetic field turbulence in the solar wind between 1 and 200 Hz, i.e., from proton to sub-electron scales. We consider 93 ten-minute intervals of the Cluster/STAFF measurements. We find that the fluctuations are not gyrotropic at a given frequency f, a property already observed at larger scales ( means parallel/perpendicular to the average magnetic ). This non-gyrotropy gives indications of the angular distribution of the wave vectors : at 10 Hz, we find that , mainly in the fast wind; at 10 Hz, fluctuations with a non-negligible k∥ are also present. We then consider the anisotropy ratio , which is a measure of the magnetic compressibility of the fluctuations. This ratio, always smaller than 1, increases with f. It reaches a value showing that the fluctuations are more or less isotropic at electron scales, for . From 1 to 15–20 Hz, there is a strong correlation between the observed compressibility and the one expected for the kinetic Alfvén waves (KAWs), which only depends on the total plasma β. For , the observed compressibility is larger than expected for KAWs, and it is stronger in the slow wind: this could be an indication of the presence of a slow-ion acoustic mode of fluctuations, which is more compressive and is favored by the larger values of the electron to proton temperature ratio generally observed in the slow wind. [ABSTRACT FROM AUTHOR]

On the basis of the modern understanding of MHD turbulence, we propose a new way of using synchrotron radiation: using synchrotron intensity gradients (SIGs) for tracing astrophysical magnetic fields. We successfully test the new technique using synthetic data obtained with 3D MHD simulations and provide the demonstration of the practical utility of the technique by comparing the directions of magnetic fields that are obtained with PLANCK synchrotron intensity data to the directions obtained with PLANCK synchrotron polarization data. We demonstrate that the SIGs can reliably trace magnetic fields in the presence of noise and can provide detailed maps of magnetic field directions. We also show that the SIGs are relatively robust for tracing magnetic fields while the low spatial frequencies of the synchrotron image are removed. This makes the SIGs applicable to the tracing of magnetic fields using interferometric data with single-dish measurement absent. We discuss the synergy of using the SIGs together with synchrotron polarization in order to find the actual direction of the magnetic fields and quantify the effects of Faraday rotation as well as with other ways of studying astrophysical magnetic fields. We test our method in the presence of noise and the resolution effects. We stress the complementary nature of the studies using the SIG technique and those employing the recently introduced velocity gradient techniques that trace magnetic fields using spectroscopic data. [ABSTRACT FROM AUTHOR]

The investigation of the diffusive transport of charged particles in a turbulent magnetic field remains a subject of considerable interest. Research has most frequently concentrated on determining the diffusion coefficient in the presence of a mean magnetic field. Here we consider the diffusion of charged particles in fully three-dimensional isotropic turbulent magnetic fields with no mean field, which may be pertinent to many astrophysical situations. We identify different ranges of particle energy depending upon the ratio of Larmor radius to the characteristic outer length scale of turbulence. Two different theoretical models are proposed to calculate the diffusion coefficient, each applicable to a distinct range of particle energies. The theoretical results are compared to those from computer simulations, showing good agreement. [ABSTRACT FROM AUTHOR]