861 research outputs found
Ambipolar Diffusion-Mediated Thermal Fronts in the Neutral ISM
In a thermally bistable medium, cold, dense gas is separated from warm,
rareified gas by thin phase transition layers, or fronts, in which heating,
radiative cooling, thermal conduction, and convection of material are balanced.
We calculate the steady-state structure of such fronts in the presence of
magnetic fields, including the processes of ion-neutral drift and ion-neutral
frictional heating. We find that ambipolar diffusion efficiently transports the
magnetic field across the fronts, leading to a flat magnetic field strength
profile. The thermal profiles of such fronts are not significantly different
from those of unmagnetized fronts. The near uniformity of the magnetic field
strength across a front is consistent with the flat field strength-gas density
relation that is observed in diffuse interstellar gas.Comment: 17 pages, 12 figures, 1 table, accepted for publication in Ap
Application of the MHD energy principle to magnetostatic atmospheres
The MHD energy principle is applied to the stability of a magnetized atmosphere which is bounded below by much denser fluid, as is the solar corona. The two fluids are treated as ideal; the approximation is consistent with the energy principle, and the dynamical conditions that must hold at a fluid-fluid interface are used to show that if vertical displacements of the lower boundary are premitted, then the lower atmosphere must be perturbed as well. However, displacements which do not perturb the coronal boundary can be properly treated as isolated perturbations of the corona alone
Turbulence in the Star-forming Interstellar Medium: Steps toward Constraining Theories with Observations
Increasingly sophisticated observational tools and techniques are now being
developed for probing the nature of interstellar turbulence. At the same time,
theoretical advances in understanding the nature of turbulence and its effects
on the structure of the ISM and on star formation are occurring at a rapid
pace, aided in part by numerical simulations. These increased capabilities on
both fronts open new opportunities for strengthening the links between
observation and theory,and for meaningful comparisons between the two.Comment: 9 pages, 2 figures, Summary of Interstellar Turbulence Sessions at
the Workshop on Magnetic Fields and Star Formation: Theory versus
Observation
Environments for Magnetic Field Amplification by Cosmic Rays
We consider a recently discovered class of instabilities, driven by cosmic
ray streaming, in a variety of environments. We show that although these
instabilities have been discussed primarily in the context of supernova driven
interstellar shocks, they can also operate in the intergalactic medium and in
galaxies with weak magnetic fields, where, as a strong source of helical
magnetic fluctuations, they could contribute to the overall evolution of the
magnetic field. Within the Milky Way, these instabilities are strongest in warm
ionized gas, and appear to be weak in hot, low density gas unless the injection
efficiency of cosmic rays is very high.Comment: 9 pages, 8 figures; Accepted to Ap
The Fermi Bubbles: Gamma-ray, Microwave, and Polarization Signatures of Leptonic AGN Jets
The origin of the Fermi bubbles and the microwave haze is yet to be
determined. To disentangle different models requires detailed comparisons
between theoretical predictions and multi-wavelength observations. Our previous
simulations have demonstrated that the primary features of the Fermi bubbles
could be successfully reproduced by recent jet activity from the central active
galactic nucleus (AGN). In this work, we generate gamma-ray and microwave maps
and spectra based on the simulated properties of cosmic rays (CRs) and magnetic
fields in order to examine whether the observed bubble and haze emission could
be explained by leptons contained in the AGN jets. We also investigate the
model predictions of the polarization properties of the Fermi bubbles. We find
that: (1) The same population of leptons can simultaneously explain the bubble
and haze emission given that the magnetic fields within the bubbles are very
close to the exponentially distributed ambient field, which can be explained by
mixing in of the ambient field followed by turbulent field amplification; (2)
The centrally peaked microwave profile suggests CR replenishment, which is
consistent with the presence of a more recent second jet event; (3) The bubble
interior exhibits a high degree of polarization because of ordered radial
magnetic field lines stretched by elongated vortices behind the shocks;
highly-polarized signals could also be observed inside the draping layer; (4)
Enhancement of rotation measures could exist within the shock-compressed layer
because of increased gas density and more amplified and ordered magnetic
fields. We discuss the possibility that the deficient haze emission at b<-35
degrees is due to the suppression of magnetic fields, which is consistent with
the existence of lower-energy CRs causing the polarized emission at 2.3 GHz.
Possible AGN jet composition in the leptonic scenario is also discussed.Comment: 15 pages, 9 figures, matched with MNRAS published versio
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