162 research outputs found
Scatter broadening of pulsars and implications on the interstellar medium turbulence
Observations reveal a uniform Kolmogorov turbulence throughout the diffuse
ionized interstellar medium (ISM) and supersonic turbulence preferentially
located in the Galactic plane. Correspondingly, we consider the Galactic
distribution of electron density fluctuations consisting of not only a
Kolmogorov density spectrum but also a short-wave-dominated density spectrum
with the density structure formed at small scales due to shocks. The resulting
dependence of the scatter broadening time on the dispersion measure (DM)
naturally interprets the existing observational data for both low and high-DM
pulsars. According to the criteria that we derive for a quantitative
determination of scattering regimes over wide ranges of DMs and frequencies
, we find that the pulsars with low DMs are primarily scattered by the
Kolmogorov turbulence, while those at low Galactic latitudes with high DMs
undergo more enhanced scattering dominated by the supersonic turbulence, where
the corresponding density spectrum has a spectral index . Besides,
by considering a volume filling factor of the density structures with the
dependence on as in the supersonic turbulence, our
model can also explain the observed shallower scaling of the scattering
time than the Kolmogorov scaling for the pulsars with relatively large DMs. The
comparison between our analytical results and the scattering measurements of
pulsars in turn makes a useful probe of the properties of the large-scale ISM
turbulence, e.g., an injection scale of pc, and also characteristics
of small-scale density structures.Comment: 11 pages, 3 figures, accepted for publication in Ap
Linewidth Differences of Neutrals and Ions Induced by MHD Turbulence
We address the problem of the difference of line widths of neutrals and ions
observed from molecular clouds and explore whether this difference can arise
from the effects of magnetohydrodynamic (MHD) turbulence acting on partially
ionized gas. Among the three fundamental modes of MHD turbulence, we find fast
modes do not contribute to linewidth differences, whereas slow modes can have
an effect on different line widths for certain parameters. We focus on
Alfv\'{e}nic component because they contain most of the turbulent energy, and
consider the damping of this component taking into account both neutral-ion
collisions and neutral viscosity. We consider different regimes of turbulence
corresponding to different media magnetizations and turbulent drivings. In the
case of super-Alfv\'{e}nic turbulence, when the damping scale of Alfv\'{e}nic
turbulence is below , where is the injection scale of anisotropic
GS95-type turbulence, the linewidth difference does not depend on the magnetic
field strength. While for other turbulent regimes, the dependence is present.
For instance, the difference between the squares of the neutral and ion
velocity dispersions in strong sub-Alfv\'{e}nic turbulence allows evaluation of
magnetic field. We discuss earlier findings on the neutral-ion linewidth
differences in the literature and compare the expressions for magnetic field we
obtain with those published earlier.Comment: 26 pages, 8 figure
Gamma-Ray Bursts Induced by Turbulent Reconnection
We revisit the Internal-Collision-induced MAgnetic Reconnection and Turbulence model of gamma-ray bursts (GRBs) in view of the advances made in understanding of both relativistic magnetic turbulence and relativistic turbulent magnetic reconnection. We identify the kink instability as the most natural way of changing the magnetic configuration to release the magnetic free energy through magnetic reconnection, as well as driving turbulence that enables fast turbulent reconnection. We show that this double role of the kink instability is important for explaining the prompt emission of GRBs. Our study confirms the critical role that turbulence plays in boosting reconnection efficiency in GRBs and suggests that the GRB phenomena can be modeled in the magnetohydrodynamics approximation. That is, the modeling is not constrained by reproducing the detailed microphysical properties of relativistic magnetized plasmas
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