25 research outputs found
Revisiting the Equipartition Assumption in Star-forming Galaxies
Energy equipartition between cosmic rays and magnetic fields is often assumed
to infer magnetic field properties from the synchrotron observations of
star-forming galaxies. However, there is no compelling physical reason to
expect the same. We aim to explore the validity of the energy equipartition
assumption. After describing popular arguments in favour of the assumption, we
first discuss observational results which support it at large scales and how
certain observations show significant deviations from equipartition at scales
smaller than , probably related to the propagation
length of the cosmic rays. Then we test the energy equipartition assumption
using test-particle and MHD simulations. From the results of the simulations,
we find that the energy equipartition assumption is not valid at scales smaller
than the driving scale of the ISM turbulence ( in
spiral galaxies), which can be regarded as the lower limit for the scale beyond
which equipartition is valid. We suggest that one must be aware of the
dynamical scales in the system before assuming energy equipartition to extract
magnetic field information from synchrotron observations. Finally, we present
ideas for future observations and simulations to investigate in more detail
under which conditions the equipartition assumption is valid or not.Comment: Invited review article for the special issue "New Perspectives on
Galactic Magnetism" of the journal "Galaxies", accepted for publicatio
Seed magnetic fields in turbulent small-scale dynamos
Magnetic fields in galaxies and galaxy clusters are amplified from a very
weak seed value to the observed strengths by the turbulent dynamo.
The seed magnetic field can be of primordial or astrophysical origin. The
strength and structure of the seed field, on the galaxy or galaxy cluster
scale, can be very different, depending on the seed-field generation mechanism.
The seed field first encounters the small-scale dynamo, thus we investigate the
effects of the strength and structure of the seed field on the small-scale
dynamo action. Using numerical simulations of driven turbulence and considering
three different seed-field configurations: 1) uniform field, 2) random field
with a power-law spectrum, and 3) random field with a parabolic spectrum, we
show that the strength and statistical properties of the dynamo-generated
magnetic fields are independent of the details of the seed field. We
demonstrate that, even when the small-scale dynamo is not active, small-scale
magnetic fields can be generated and amplified linearly due to the tangling of
the large-scale field. In the presence of the small-scale dynamo action, we
find that any memory of the seed field for the non-linear small-scale dynamo
generated magnetic fields is lost and thus, it is not possible to trace back
seed-field information from the evolved magnetic fields in a turbulent medium.Comment: 12 pages, 9 figures, 1 table, accepted for publication in MNRA
Cosmic ray propagation in turbulent galactic magnetic fields
PhD ThesisCosmic rays and magnetic fields are important non-thermal components of the interstellar
medium in galaxies. This thesis explores the intermittent structure of the magnetic field
generated by a fluctuation dynamo and the interaction of cosmic rays with small-scale random
magnetic fields.
First, the nonlinear state of fluctuation dynamo is described in terms of the statistical
and structural properties of magnetic and velocity fields. Using three-dimensional fluctuation
dynamo simulations, we study their properties in the kinematic and saturated stages. The
alignment of the magnetic field, electric current density, and velocity field are analyzed to
suggest a possible saturation mechanism for the fluctuation dynamo. Furthermore, we also
study the change in the diffusion of magnetic fields by calculating local magnetic Reynolds
number in the kinematic and saturated stages. We show that both the amplification and diffusion of magnetic fields are affected by nonlinearity. The dynamo-generated magnetic field is
intermittent, i.e., concentrated in filaments, ribbons, and sheets. Minkowski functionals are
used to characterize the shape of the magnetic structures and study its dependence on magnetic Reynolds number. We find that all three length scales of magnetic structures decreases
on saturation. We also propose that observing magnetic fields in elliptical galaxies, via a grid
of the Faraday rotation measures from background polarized sources, would serve as a probe
of the fluctuation dynamo action in a galactic environment.
Next, the effect of magnetic field intermittency on cosmic ray propagation is studied.
Using test-particle simulations, it is shown that the diffusivity of low energy cosmic rays is
enhanced when the magnetic field is intermittent. It is demonstrated that the cosmic ray diffusion in any random magnetic field (Gaussian or intermittent) can be better described as a
correlated random walk rather the usual Brownian motion. Then, the energy equipartition
between magnetic fields and cosmic rays usually assumed to infer magnetic field strength
from synchrotron intensity observations is discussed. Using test-particle and magnetohydrodynamic simulations, it is shown that the cosmic ray and magnetic field energy densities are
not correlated on scales less than the driving scale of the turbulence. Even when the cosmic
ray and magnetic field energy densities are uncorrelated, small-scale structures are seen in
the spatial distribution of cosmic rays as they are trapped between random magnetic mirrors.
These results exclude the possibility of local energy equipartition between cosmic rays and
magnetic fields
Multiphase Neutral Interstellar Medium: Analyzing Simulation with H I 21cm Observational Data Analysis Techniques
Several different methods are regularly used to infer the properties of the
neutral interstellar medium (ISM) using atomic hydrogen (H I) 21cm absorption
and emission spectra. In this work, we study various techniques used for
inferring ISM gas phase properties, namely the correlation between brightness
temperature and optical depth , at each channel velocity
, and decomposition into Gaussian components, by creating mock spectra
from a 3D magnetohydrodynamic simulation of a two-phase, turbulent ISM. We
propose a physically motivated model to explain the
distribution and relate the model parameters to properties like warm gas spin
temperature and cold cloud length scales. Two methods based on Gaussian
decomposition -- using only absorption spectra and both absorption and emission
spectra -- are used to infer the column density distribution as a function of
temperature. In observations, such analysis reveals the puzzle of large amounts
(significantly higher than in simulations) of gas with temperature in the
thermally unstable range of to and
a lack of the expected bimodal (two-phase) temperature distribution. We show
that, in simulation, both methods are able to recover the true gas distribution
till temperatures (and the two-phase distribution in
general) reasonably well. We find our results to be robust to a range of
effects such as noise, varying emission beam size, and simulation resolution.
This shows that the observational inferences are unlikely to be artifacts, thus
highlighting a tension between observations and simulations. We discuss
possible reasons for this tension and ways to resolve it.Comment: 21 pages (including appendixes), 15 figures, 3 tables, Submitted to
MNRAS, Comments are welcom
Growth or decay: universality of the turbulent dynamo saturation
The small-scale turbulent dynamo (SSD) is likely to be responsible for the
magnetisation of the interstellar medium (ISM) that we observe in the Universe
today. The SSD efficiently converts kinetic energy into magnetic
energy , and is often used to explain how an initially weak
magnetic field with is amplified, and then
maintained at a level . Usually, this process
is studied by initialising a weak seed magnetic field and letting the
turbulence grow it to saturation. However, in this study, using
three-dimensional, non-ideal magnetohydrodynamical turbulence simulations, we
show that the same saturated state can also be achieved if initially or . This is realised
through a two-stage exponential decay (1. a slow backreaction that converts
into , and 2. Ohmic dissipation concentrated in
anisotropic current sheets) into the saturated state, for which we provide an
analytical model. This means that even if there are temporary local
enhancements of in the ISM, such that , e.g., through amplifications such as compressions, over a long enough
time the field will decay into the saturated state set by the SSD, which is
determined by the turbulence and magnetic dissipation. However, we also provide
analytical models for the decay times and utilise wait-time statistics from
compressive supernova events to show that if the magnetic field is enhanced
above the saturated state, it will not have enough time to decay the field
before the next supernova event. Hence, unless there exists a mechanism for
destroying magnetic fields that is not in our non-ideal MHD models, the
amplitudes of interstellar magnetic fields may also be a... (abridged).Comment: 18 pages. 14 figures. Submitted to MNRAS. Comments welcom
Saturation mechanism of the fluctuation dynamo at Pr-M >= 1
The presence of magnetic fields in many astrophysical objects is due to dynamo action, whereby a part of the kinetic energy is converted into magnetic energy. A turbulent dynamo that produces magnetic field structures on the same scale as the turbulent flow is known as the fluctuation dynamo. We use numerical simulations to explore the nonlinear, statistically
steady state of the fluctuation dynamo in driven turbulence. We demonstrate that as the magnetic field growth saturates, its amplification and diffusion are both affected by the back-reaction of the Lorentz force upon the flow. The amplification of the magnetic field is reduced due to stronger alignment between the velocity field, magnetic field, and electric current density. Furthermore, we confirm that the amplification decreases due to a weaker stretching of the magnetic field lines. The enhancement in diffusion relative to the field line stretching is quantified by a decrease in the computed local value of the magnetic Reynolds number. Using the Minkowski functionals, we quantify the shape of the magnetic structures produced by the dynamo as magnetic filaments and ribbons in both kinematic and saturated dynamos and derive the scalings of the typical length, width, and thickness
of the magnetic structures with the magnetic Reynolds number. We show that all three of these magnetic length scales increase as the dynamo saturates. The magnetic intermittency, strong in the kinematic dynamo (where the magnetic field strength grows exponentially), persists in the statistically steady state, but intense magnetic filaments and ribbons are more volume-filling.We acknowledge financial support of the STFC (Grant No. ST/N000900/1,
Project 2) and the Leverhulme Trust (Grant No. RPG-2014-427)