291 research outputs found
Langmuir Wave Electric Fields Induced by Electron Beams in the Heliosphere
Solar electron beams responsible for type III radio emission generate
Langmuir waves as they propagate out from the Sun. The Langmuir waves are
observed via in-situ electric field measurements. These Langmuir waves are not
smoothly distributed but occur in discrete clumps, commonly attributed to the
turbulent nature of the solar wind electron density. Exactly how the density
turbulence modulates the Langmuir wave electric fields is understood only
qualitatively. Using weak turbulence simulations, we investigate how solar wind
density turbulence changes the probability distribution functions, mean value
and variance of the beam-driven electric field distributions. Simulations show
rather complicated forms of the distribution that are dependent upon how the
electric fields are sampled. Generally the higher magnitude of density
fluctuations reduce the mean and increase the variance of the distribution in a
consistent manor to the predictions from resonance broadening by density
fluctuations. We also demonstrate how the properties of the electric field
distribution should vary radially from the Sun to the Earth and provide a
numerical prediction for the in-situ measurements of the upcoming Solar Orbiter
and Solar Probe Plus spacecraft.Comment: 14 pages, 11 figures, published in Astronomy and Astrophysic
Spatial expansion and speeds of type III electron beam sources in the solar corona
A component of space weather, electron beams are routinely accelerated in the
solar atmosphere and propagate through interplanetary space. Electron beams
interact with Langmuir waves resulting in type III radio bursts. Electron beams
expand along the trajectory, and using kinetic simulations, we explore the
expansion as the electrons propagate away from the Sun. Specifically, we
investigate the front, peak and back of the electron beam in space from derived
radio brightness temperatures of fundamental type III emission. The front of
the electron beams travelled at speeds from 0.2c--0.7c, significantly faster
than the back of the beam that travelled between 0.12c--0.35c. The difference
in speed between the front and the back elongates the electron beams in time.
The rate of beam elongation has a 0.98 correlation coefficient with the peak
velocity; in-line with predictions from type III observations. The inferred
speeds of electron beams initially increase close to the acceleration region
and then decrease through the solar corona. Larger starting densities and
harder initial spectral indices result in longer and faster type III sources.
Faster electron beams have higher beam energy densities, produce type IIIs with
higher peak brightness temperatures and shorter FWHM durations. Higher
background plasma temperatures also increase speeds, particularly at the back
of the beam. We show how our predictions of electron beam evolution influences
type III bandwidth and drift-rates. Our radial predictions of electron beam
speed and expansion can be tested by the upcoming in situ electron beam
measurements made by Solar Orbiter and Parker Solar Probe.Comment: 19 pages, 20 figures, submitted to Ap
LOFAR observations of fine spectral structure dynamics in type IIIb radio bursts
Solar radio emission features a large number of fine structures demonstrating
great variability in frequency and time. We present spatially resolved spectral
radio observations of type IIIb bursts in the MHz range made by the Low
Frequency Array (LOFAR). The bursts show well-defined fine frequency
structuring called "stria" bursts. The spatial characteristics of the stria
sources are determined by the propagation effects of radio waves; their
movement and expansion speeds are in the range of 0.1-0.6c. Analysis of the
dynamic spectra reveals that both the spectral bandwidth and the frequency
drift rate of the striae increase with an increase of their central frequency;
the striae bandwidths are in the range of ~20-100 kHz and the striae drift
rates vary from zero to ~0.3 MHz s^-1. The observed spectral characteristics of
the stria bursts are consistent with the model involving modulation of the type
III burst emission mechanism by small-amplitude fluctuations of the plasma
density along the electron beam path. We estimate that the relative amplitude
of the density fluctuations is of dn/n~10^-3, their characteristic length scale
is less than 1000 km, and the characteristic propagation speed is in the range
of 400-800 km/s. These parameters indicate that the observed fine spectral
structures could be produced by propagating magnetohydrodynamic waves
Stopping Frequency of Type III Solar Radio Bursts in Expanding Magnetic Flux Tubes
Understanding the properties of type III radio bursts in the solar corona and
interplanetary space is one of the best ways to remotely deduce the
characteristics of solar accelerated electron beams and the solar wind plasma.
One feature of all type III bursts is the lowest frequency they reach (or
stopping frequency). This feature reflects the distance from the Sun that an
electron beam can drive the observable plasma emission mechanism. The stopping
frequency has never been systematically studied before from a theoretical
perspective. Using numerical kinetic simulations, we explore the different
parameters that dictate how far an electron beam can travel before it stops
inducing a significant level of Langmuir waves, responsible for plasma radio
emission. We use the quasilinear approach to model self-consistently the
resonant interaction between electrons and Langmuir waves in inhomogeneous
plasma, and take into consideration the expansion of the guiding magnetic flux
tube and the turbulent density of the interplanetary medium. We find that the
rate of radial expansion has a significant effect on the distance an electron
beam travels before enhanced leves of Langmuir waves, and hence radio waves,
cease. Radial expansion of the guiding magnetic flux tube rarefies the electron
stream to the extent that the density of non-thermal electrons is too low to
drive Langmuir wave production. The initial conditions of the electron beam
have a significant effect, where decreasing the beam density or increasing the
spectral index of injected electrons would cause higher type III stopping
frequencies. We also demonstrate how the intensity of large-scale density
fluctuations increases the highest frequency that Langmuir waves can be driven
by the beam and how the magnetic field geometry can be the cause of type III
bursts only observed at high coronal frequencies.Comment: 11 pages, 8 figures, accepted in Astronomy and Astrophysic
Imaging Spectroscopy of Type U and J Solar Radio Bursts with LOFAR
Radio U-bursts and J-bursts are signatures of electron beams propagating
along magnetic loops confined to the corona. The more commonly observed type
III radio bursts are signatures of electron beams propagating along magnetic
loops that extend into interplanetary space. Given the prevalence of solar
magnetic flux to be closed in the corona, it is an outstanding question why
type III bursts are more frequently observed than U-bursts or J-bursts. We use
LOFAR imaging spectroscopy between 30-80 MHz of low-frequency U-bursts and
J-bursts, for the first time, to understand why electron beams travelling along
coronal loops produce radio emission less often. The different radio source
positions were used to model the spatial structure of the guiding magnetic flux
tube and then deduce the energy range of the exciting electron beams without
the assumption of a standard density model. The radio sources infer a magnetic
loop 1 solar radius in altitude, with the highest frequency sources starting
around 0.6 solar radii. Electron velocities were found between 0.13 c and 0.24
c, with the front of the electron beam travelling faster than the back of the
electron beam. The velocities correspond to energy ranges within the beam from
0.7-11 keV to 0.7-43 keV. The density along the loop is higher than typical
coronal density models and the density gradient is smaller. We found that a
more restrictive range of accelerated beam and background plasma parameters can
result in U-bursts or J-bursts, causing type III bursts to be more frequently
observed. The large instability distances required before Langmuir waves are
produced by some electron beams, and the small magnitude of the background
density gradients make closed loops less facilitating for radio emission than
loops that extend into interplanetary space.Comment: 9 pages, 7 figure
Solar Physics with the Square Kilometre Array
The Square Kilometre Array (SKA) will be the largest radio telescope ever
built, aiming to provide collecting area larger than 1 km. The SKA will
have two independent instruments, SKA-LOW comprising of dipoles organized as
aperture arrays in Australia and SKA-MID comprising of dishes in South Africa.
Currently the phase-1 of SKA, referred to as SKA1, is in its late design stage
and construction is expected to start in 2020. Both SKA1-LOW (frequency range
of 50-350 MHz) and SKA1-MID Bands 1, 2, and 5 (frequency ranges of 350-1050,
950-1760, and 4600-15300 MHz, respectively) are important for solar
observations. In this paper we present SKA's unique capabilities in terms of
spatial, spectral, and temporal resolution, as well as sensitivity and show
that they have the potential to provide major new insights in solar physics
topics of capital importance including (i) the structure and evolution of the
solar corona, (ii) coronal heating, (iii) solar flare dynamics including
particle acceleration and transport, (iv) the dynamics and structure of coronal
mass ejections, and (v) the solar aspects of space weather. Observations of the
Sun jointly with the new generation of ground-based and space-borne instruments
promise unprecedented discoveries.Comment: Accepted for publication in Advances in Space Researc
Fast spectral fitting of hard X-ray bremsstrahlung from truncated power-law electron spectra
<p><b>Context:</b> Hard X-ray bremsstrahlung continuum spectra, such as from solar flares, are commonly described in terms of power-law fits, either to the photon spectra themselves or to the electron spectra responsible for them. In applications various approximate relations between electron and photon spectral indices are often used for energies both above and below electron low-energy cutoffs.</p>
<p><b>Aims:</b> We examine the form of the exact relationships in various situations, and for various cross-sections, showing that empirical relations sometimes used can be highly misleading especially at energies below the low-energy cutoff, and consider how to improve fitting procedures.</p>
<p><b>Methods:</b> We obtain expressions for photon spectra from single, double and truncated power-law electron spectra for a variety of cross-sections and for the thin and thick target models and simple analytic expressions for the non-relativistic Bethe-Heitler case.</p>
<p><b>Results:</b> We show that below the low-energy cutoff Kramers and other constant spectral index forms commonly used are very poor approximations to accurate results, but that our analytical forms are a good match; and that above a low-energy cutoff, the Kramers and non-relativistic Bethe-Heitler results match reasonably well with results for up to energies around 100 keV.</p>
<p><b>Conclusions:</b> Analytical forms of the non-relativistic Bethe-Heitler photon spectra from general power-law electron spectra are good match to exact results for both thin and thick targets and they enable much faster spectral fitting than evaluation of the full spectral integrations.</p>
A Classification Scheme For Turbulent Acceleration Processes In Solar Flares
We establish a classification scheme for stochastic acceleration models
involving low-frequency plasma turbulence in a strongly magnetized plasma. This
classification takes into account both the properties of the accelerating
electromagnetic field, and the nature of the transport of charged particles in
the acceleration region. We group the acceleration processes as either
resonant, non-resonant or resonant-broadened, depending on whether the particle
motion is free-streaming along the magnetic field, diffusive or a combination
of the two. Stochastic acceleration by moving magnetic mirrors and adiabatic
compressions are addressed as illustrative examples. We obtain expressions for
the momentum-dependent diffusion coefficient , both for general forms of
the accelerating force and for the situation when the electromagnetic force is
wave-like, with a specified dispersion relation . Finally,
for models considered, we calculate the energy-dependent acceleration time, a
quantity that can be directly compared with observations of the time profile of
the radiation field produced by the accelerated particles, such as during solar
flares.Comment: 45 pages, submitted to Astrophysical Journa
Suppression of parallel transport in turbulent magnetized plasmas and its impact on the non-thermal and thermal aspects of solar flares
The transport of the energy contained in electrons, both thermal and suprathermal, in solar flares plays a key role in our understanding of many aspects of the flare phenomenon, from the spatial distribution of hard X-ray emission to global energetics. Motivated by recent RHESSI observations that point to the existence of a mechanism that confines electrons to the coronal parts of flare loops more effectively than Coulomb collisions, we here consider the impact of pitch-angle scattering off turbulent magnetic fluctuations on the parallel transport of electrons in flaring coronal loops. It is shown that the presence of such a scattering mechanism in addition to Coulomb collisional scattering can significantly reduce the parallel thermal and electrical conductivities relative to their collisional values. We provide illustrative expressions for the resulting thermoelectric coefficients that relate the thermal flux and electrical current density to the temperature gradient and the applied electric field. We then evaluate the effect of these modified transport coefficients on the flare coronal temperature that can be attained, on the post-impulsive-phase cooling of heated coronal plasma, and on the importance of the beam-neutralizing return current on both ambient heating and the energy loss rate of accelerated electrons. We also discuss the possible ways in which anomalous transport processes have an impact on the required overall energy associated with accelerated electrons in solar flares
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