49 research outputs found
Hawking radiation in an electro-magnetic wave-guide?
It is demonstrated that the propagation of electro-magnetic waves in an
appropriately designed wave-guide is (for large wave-lengths) analogous to that
within a curved space-time -- such as around a black hole. As electro-magnetic
radiation (e.g., micro-weaves) can be controlled, amplified, and detected (with
present-day technology) much easier than sound, for example, we propose a
set-up for the experimental verification of the Hawking effect. Apart from
experimentally testing this striking prediction, this would facilitate the
investigation of the trans-Planckian problem. PACS: 04.70.Dy, 04.80.-y,
42.50.-p, 84.40.Az.Comment: 4 pages RevTeX, 1 figur
A nanoflare model of quiet Sun EUV emission
Nanoflares have been proposed as the main source of heating of the solar
corona. However, detecting them directly has so far proved elusive, and
extrapolating to them from the properties of larger brightenings gives
unreliable estimates of the power-law exponent characterising their
distribution. Here we take the approach of statistically modelling light curves
representative of the quiet Sun as seen in EUV radiation. The basic assumption
is that all quiet-Sun EUV emission is due to micro- and nanoflares, whose
radiative energies display a power-law distribution. Radiance values in the
quiet Sun follow a lognormal distribution. This is irrespective of whether the
distribution is made over a spatial scan or over a time series. We show that
these distributions can be reproduced by our simple model.Comment: 13 pages, 18 figures, accepted for publication by A&
Magnetohydrodynamics dynamical relaxation of coronal magnetic fields. II. 2D magnetic X-points
We provide a valid magnetohydrostatic equilibrium from the collapse of a 2D
X-point in the presence of a finite plasma pressure, in which the current
density is not simply concentrated in an infinitesimally thin, one-dimensional
current sheet, as found in force-free solutions. In particular, we wish to
determine if a finite pressure current sheet will still involve a singular
current, and if so, what is the nature of the singularity. We use a full MHD
code, with the resistivity set to zero, so that reconnection is not allowed, to
run a series of experiments in which an X-point is perturbed and then is
allowed to relax towards an equilibrium, via real, viscous damping forces.
Changes to the magnitude of the perturbation and the initial plasma pressure
are investigated systematically. The final state found in our experiments is a
"quasi-static" equilibrium where the viscous relaxation has completely ended,
but the peak current density at the null increases very slowly following an
asymptotic regime towards an infinite time singularity. Using a high grid
resolution allows us to resolve the current structures in this state both in
width and length. In comparison with the well known pressureless studies, the
system does not evolve towards a thin current sheet, but concentrates the
current at the null and the separatrices. The growth rate of the singularity is
found to be tD, with 0 < D < 1. This rate depends directly on the initial
plasma pressure, and decreases as the pressure is increased. At the end of our
study, we present an analytical description of the system in a quasi-static
non-singular equilibrium at a given time, in which a finite thick current layer
has formed at the null
Comment on `Electromagnetic force on a moving dipole'
Using the Lagrangian formalism, the electromagnetic force on a moving dipole
derived by Kholmetskii, Missevitch and Yarman (2011, Eur. J. Phys. 32, 873) is
found to be missing some important terms.Comment: The version as accepted by Eur. J. Phys.; 4 page
A Nanoflare Distribution Generated by Repeated Relaxations Triggered by Kink Instability
Context: It is thought likely that vast numbers of nanoflares are responsible
for the corona having a temperature of millions of degrees. Current
observational technologies lack the resolving power to confirm the nanoflare
hypothesis. An alternative approach is to construct a magnetohydrodynamic
coronal loop model that has the ability to predict nanoflare energy
distributions.
Aims: This paper presents the initial results generated by such a model. It
predicts heating events with a range of sizes, depending on where the
instability threshold for linear kink modes is encountered. The aims are to
calculate the distribution of event energies and to investigate whether kink
instability can be predicted from a single parameter.
Methods: The loop is represented as a straight line-tied cylinder. The
twisting caused by random photospheric motions is captured by two parameters,
representing the ratio of current density to field strength for specific
regions of the loop. Dissipation of the loop's magnetic energy begins during
the nonlinear stage of the instability, which develops as a consequence of
current sheet reconnection. After flaring, the loop evolves to the state of
lowest energy where, in accordance with relaxation theory, the ratio of current
to field is constant throughout the loop and helicity is conserved.
Results: The results suggest that instability cannot be predicted by any
simple twist-derived property reaching a critical value. The model is applied
such that the loop undergoes repeated episodes of instability followed by
energy-releasing relaxation. Hence, an energy distribution of the nanoflares
produced is collated.
Conclusions: The final energy distribution features two nanoflare populations
that follow different power laws. The power law index for the higher energy
population is more than sufficient for coronal heating.Comment: 13 pages, 18 figure
Solar Particle Acceleration at Reconnecting 3D Null Points
Context: The strong electric fields associated with magnetic reconnection in
solar flares are a plausible mechanism to accelerate populations of high
energy, non-thermal particles. One such reconnection scenario occurs at a 3D
magnetic null point, where global plasma flows give rise to strong currents in
the spine axis or fan plane. Aims: To understand the mechanism of charged
particle energy gain in both the external drift region and the diffusion region
associated with 3D magnetic reconnection. In doing so we evaluate the
efficiency of resistive spine and fan models for particle acceleration, and
find possible observables for each. Method: We use a full orbit test particle
approach to study proton trajectories within electromagnetic fields that are
exact solutions to the steady and incompressible magnetohydrodynamic equations.
We study single particle trajectories and find energy spectra from many
particle simulations. The scaling properties of the accelerated particles with
respect to field and plasma parameters is investigated. Results: For fan
reconnection, strong non-uniform electric drift streamlines can accelerate the
bulk of the test particles. The highest energy gain is for particles that enter
the current sheet, where an increasing "guide field" stabilises particles
against ejection. The energy is only limited by the total electric potential
energy difference across the fan current sheet. The spine model has both slow
external electric drift speed and weak energy gain for particles reaching the
current sheet. Conclusions: The electromagnetic fields of fan reconnection can
accelerate protons to the high energies observed in solar flares, gaining up to
0.1 GeV for anomalous values of resistivity. However, the spine model, which
gave a harder energy spectrum in the ideal case, is not an efficient
accelerator after pressure constraints in the resistive model are included.Comment: 15 pages, 14 figures. Submitted to Astronomy and Astrophysic
Particle interactions with single or multiple 3D solar reconnecting current sheets
The acceleration of charged particles (electrons and protons) in flaring
solar active regions is analyzed by numerical experiments. The acceleration is
modelled as a stochastic process taking place by the interaction of the
particles with local magnetic reconnection sites via multiple steps. Two types
of local reconnecting topologies are studied: the Harris-type and the X-point.
A formula for the maximum kinetic energy gain in a Harris-type current sheet,
found in a previous work of ours, fits well the numerical data for a single
step of the process. A generalization is then given approximating the kinetic
energy gain through an X-point. In the case of the multiple step process, in
both topologies the particles' kinetic energy distribution is found to acquire
a practically invariant form after a small number of steps. This tendency is
interpreted theoretically. Other characteristics of the acceleration process
are given, such as the mean acceleration time and the pitch angle distributions
of the particles.Comment: 18 pages, 9 figures, Solar Physics, in pres
Star-planet magnetic interaction and activity in late-type stars with close-in planets
Late-type stars interact with their close-in planets through their coronal
magnetic fields. We introduce a theory for the interaction between the stellar
and planetary fields focussing on the processes that release magnetic energy in
the stellar coronae. We consider the energy dissipated by the reconnection
between the stellar and planetary magnetic fields as well as that made
available by the modulation of the magnetic helicity of the coronal field
produced by the orbital motion of the planet. We estimate the powers released
by both processes in the case of axisymmetric and non-axisymmetric, linear and
non-linear force-free coronal fields finding that they scale as v_r (B_s)^(4/3)
(B_p)^(2/3) (R_p)^2, where v_r is the relative velocity between the stellar and
planetary fields, B_s the mean stellar surface field, B_p the planetary field
at the poles, and R_p the radius of the planet. A chromospheric hot spot or a
flaring activity phased to the orbital motion of the planet are found only when
the stellar field is axisymmetric. In the case of a non-axisymmetric field, the
time modulation of the energy release is multiperiodic and can be easily
confused with the intrinsic stellar variability. We apply our theory to the
systems with some reported evidence of star-planet magnetic interaction finding
a dissipated power at least one order of magnitude smaller than that emitted by
the chromospheric hot spots. The phase lags between the planets and the hot
spots are reproduced by our models in all the cases except for upsilon And. In
conclusion, the chromospheric hot spots rotating in phase with the planets
cannot be explained by the energy dissipation produced by the interaction
between stellar and planetary fields as considered by our models and require a
different mechanism.Comment: 16 pages, 3 figures, accepted by Astronomy and Astrophysic
Magnetic Pinching of Hyperbolic Flux Tubes: I. Basic Estimations
The concept of hyperbolic flux tubes (HFTs) is a generalization of the
concept of separator field lines for coronal magnetic fields with a trivial
magnetic topology. An effective mechanism of a current layer formation in HFTs
is proposed. This mechanism is called magnetic pinching and it is caused by
large-scale shearing motions applied to the photospheric feet of HFTs in a way
as if trying to twist the HFT. It is shown that in the middle of an HFT such
motions produce a hyperbolic flow that causes an exponentially fast growth of
the current density in a thin force-free current layer. The magnetic energy
associated with the current layer that is built up over a few hours is
sufficient for a large flare. Other implications of HFT pinching for solar
flares are discussed as well.Comment: 31 pages, 12 figures, accepted to Astrophysical Journal, added typos
in Eq. (A9) and new comments to Sections 2 and 7, references update
Particle acceleration in three-dimensional tearing configurations
In three-dimensional electromagnetic configurations that result from unstable
resistive tearing modes particles can efficiently be accelerated to
relativistic energies. To prove this resistive magnetohydrodynamic simulations
are used as input configurations for successive test particle simulations. The
simulations show the capability of three-dimensional non-linearly evolved
tearing modes to accelerate particles perpendicular to the plane of the
reconnecting magnetic field components. The simulations differ considerably
from analytical approaches by involving a realistic three-dimensional electric
field with a non-homogenous component parallel to the current direction. The
resulting particle spectra exhibit strong pitch-angle anisotropies. Typically,
about 5-8 % of an initially Maxwellian distribution is accelerated to the
maximum energy levels given by the macroscopic generalized electric potential
structure. Results are shown for both, non-relativistic particle acceleration
that is of interest, e.g., in the context of auroral arcs and solar flares, and
relativistic particle energization that is relevant, e.g., in the context of
active galactic nuclei.Comment: Physics of Plasmas, in prin