33 research outputs found
On extra atmospheric radioastronomical investigations
Extraterrestrial radio waves in radio astronom
Broadband microwave burst produced by electron beams
Theoretical and experimental study of fast electron beams attracts a lot of
attention in the astrophysics and laboratory. In the case of solar flares the
problem of reliable beam detection and diagnostics is of exceptional
importance. This paper explores the fact that the electron beams moving oblique
to the magnetic field or along the field with some angular scatter around the
beam propagation direction can generate microwave continuum bursts via
gyrosynchrotron mechanism. The characteristics of the microwave bursts produced
by beams differ from those in case of isotropic or loss-cone distributions,
which suggests a new tool for quantitative diagnostics of the beams in the
solar corona. To demonstrate the potentiality of this tool, we analyze here a
radio burst occurred during an impulsive flare 1B/M6.7 on 10 March 2001 (AR
9368, N27W42). Based on detailed analysis of the spectral, temporal, and
spatial relationships, we obtained firm evidence that the microwave continuum
burst is produced by electron beams. For the first time we developed and
applied a new forward fitting algorithm based on exact gyrosynchrotron formulae
and employing both the total power and polarization measurements to solve the
inverse problem of the beam diagnostics. We found that the burst is generated
by a oblique beam in a region of reasonably strong magnetic field ( G) and the burst is observed at a quasi-transverse viewing angle. We
found that the life time of the emitting electrons in the radio source is
relatively short, s, consistent with a single reflection
of the electrons from a magnetic mirror at the foot point with the stronger
magnetic field. We discuss the implications of these findings for the electron
acceleration in flares and for beam diagnostics.Comment: Astrophysical Journal, accepted: 26 pages, 8 figure
Transport of Cosmic Rays in Chaotic Magnetic Fields
The transport of charged particles in disorganised magnetic fields is an
important issue which concerns the propagation of cosmic rays of all energies
in a variety of astrophysical environments, such as the interplanetary,
interstellar and even extra-galactic media, as well as the efficiency of Fermi
acceleration processes. We have performed detailed numerical experiments using
Monte-Carlo simulations of particle propagation in stochastic magnetic fields
in order to measure the parallel and transverse spatial diffusion coefficients
and the pitch angle scattering time as a function of rigidity and strength of
the turbulent magnetic component. We confirm the extrapolation to high
turbulence levels of the scaling predicted by the quasi-linear approximation
for the scattering frequency and parallel diffusion coefficient at low
rigidity. We show that the widely used Bohm diffusion coefficient does not
provide a satisfactory approximation to diffusion even in the extreme case
where the mean field vanishes. We find that diffusion also takes place for
particles with Larmor radii larger than the coherence length of the turbulence.
We argue that transverse diffusion is much more effective than predicted by the
quasi-linear approximation, and appears compatible with chaotic magnetic
diffusion of the field lines. We provide numerical estimates of the Kolmogorov
length and magnetic line diffusion coefficient as a function of the level of
turbulence. Finally we comment on applications of our results to astrophysical
turbulence and the acceleration of high energy cosmic rays in supernovae
remnants, in super-bubbles, and in jets and hot spots of powerful
radio-galaxies.Comment: To be published in Physical Review D, 20 pages 9 figure
Diffusion and drift of cosmic rays in highly turbulent magnetic fields
We determine numerically the parallel, perpendicular, and antisymmetric
diffusion coefficients for charged particles propagating in highly turbulent
magnetic fields, by means of extensive Monte Carlo simulations. We propose
simple expressions, given in terms of a small set of fitting parameters, to
account for the diffusion coefficients as functions of magnetic rigidity and
turbulence level, and corresponding to different kinds of turbulence spectra.
The results obtained satisfy scaling relations, which make them useful for
describing the cosmic ray origin and transport in a variety of different
astrophysical environments.Comment: 17 pages, 7 figure
Interstellar Turbulence II: Implications and Effects
Interstellar turbulence has implications for the dispersal and mixing of the
elements, cloud chemistry, cosmic ray scattering, and radio wave propagation
through the ionized medium. This review discusses the observations and theory
of these effects. Metallicity fluctuations are summarized, and the theory of
turbulent transport of passive tracers is reviewed. Modeling methods, turbulent
concentration of dust grains, and the turbulent washout of radial abundance
gradients are discussed. Interstellar chemistry is affected by turbulent
transport of various species between environments with different physical
properties and by turbulent heating in shocks, vortical dissipation regions,
and local regions of enhanced ambipolar diffusion. Cosmic rays are scattered
and accelerated in turbulent magnetic waves and shocks, and they generate
turbulence on the scale of their gyroradii. Radio wave scintillation is an
important diagnostic for small scale turbulence in the ionized medium, giving
information about the power spectrum and amplitude of fluctuations. The theory
of diffraction and refraction is reviewed, as are the main observations and
scintillation regions.Comment: 46 pages, 2 figures, submitted to Annual Reviews of Astronomy and
Astrophysic
On the Existence of Ionospheric Feedback Instability in the Earth's Magnetosphere‐Ionosphere System
The ionospheric feedback instability (IFI) has been considered one of the main generation mechanisms for large-amplitude ultralow frequency waves and small-scale field-aligned currents in the auroral and subauroral regions for more than 40 years. Sydorenko and Rankin (2017, https://doi.org/10.1002/2017GL073415) have recently challenged the very existence of the IFI for any realistic geophysical conditions in the Earth\u27s ionosphere-magnetosphere system. Because this conclusion contradicts numerous theoretical, numerical, and experimental works successfully used IFI to explain and predict results from observations for more than four decades, it deserves special attention. We show that this conclusion is mainly based on the specific ionospheric density profile and boundary conditions used in two runs of simulations presented in Sydorenko and Rankin (2017), and the generalization of this result is not justified. The effect of the collisions between ionospheric ions and neutrals on the development of the instability has been well studied since 1981, and these studies demonstrate that it does not prevent the development of the instability. Furthermore, excellent agreement of the theoretical and numerical results with observations verify without doubt the IFI existence and significance in the Earth\u27s magnetosphere-ionosphere system
Effects of the Hall Conductivity in Ionospheric Heating Experiments
We investigate the role of Hall conductivity in ionospheric heating experiments. Ionosphericheating by powerful X-mode waves changes the Hall and Pedersen conductances in theEandDregions,which lead to the generation of ultra-low frequency (ULF)/extremely-low frequency/very low frequencywaves, when the electric field exists in the ionosphere. The importance of the Hall currents in themagnetosphere-ionosphere interactions, carried by ULF waves and field-aligned currents, has beenconsistently overlooked in studies devoted tothe active experiments. Simulations of the three-dimensionaltwo-fluid magnetohydrodynamic (MHD) model, presented in this paper, demonstrate that the Hallconductivity changes (1) the growth rate and the amplitude of ULF waves generated by the heating and (2)the orientation and the direction of propagation of the generated waves. These findings provide insight inthe experiments where the waves were generated with a geometric modulation technique and suggest anew and more efficient approach for conducting such experiments in the future