90 research outputs found
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
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
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
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
The Low-High-Low Trend of Type III Radio Burst Starting Frequencies and Solar Flare Hard X-rays
Using simultaneous X-ray and radio observations from solar flares, we
investigate the link between the type III radio burst starting frequency and
hard X-ray spectral index. For a proportion of events the relation derived
between the starting height (frequency) of type III radio bursts and the
electron beam velocity spectral index (deduced from X-rays) is used to infer
the spatial properties (height and size) of the electron beam acceleration
region. Both quantities can be related to the distance travelled before an
electron beam becomes unstable to Langmuir waves. To obtain a list of suitable
events we considered the RHESSI catalogue of X-ray flares and the Phoenix 2
catalogue of type III radio bursts. From the 200 events that showed both type
III and X-ray signatures, we selected 30 events which had simultaneous emission
in both wavelengths, good signal to noise in the X-ray domain and > 20 seconds
duration. We find that > 50 % of the selected events show a good correlation
between the starting frequencies of the groups of type III bursts and the hard
X-ray spectral indices. A low-high-low trend for the starting frequency of type
III bursts is frequently observed. Assuming a background electron density model
and the thick target approximation for X-ray observations, this leads to a
correlation between starting heights of the type III emission and the beam
electron spectral index. Using this correlation we infer the altitude and
vertical extents of the flare acceleration regions. We find heights from 183 Mm
down to 25 Mm while the sizes range from 13 Mm to 2 Mm. These values agree with
previous work that places an extended flare acceleration region high in the
corona. We analyse the assumptions required and explore possible extensions to
our assumed model. We discuss these results with respect to the acceleration
heights and sizes derived from X-ray observations alone.Comment: 15 pages, 8 figures, Accepted to Astronomy and Astrophysic
X-ray and UV investigation into the magnetic connectivity of a solar flare
We investigate the X-ray and UV emission detected by RHESSI and TRACE in the
context of a solar flare on the 16th November 2002 with the goal of better
understanding the evolution of the flare. We analysed the characteristics of
the X-ray emission in the 12-25 and 25-50 keV energy range while we looked at
the UV emission at 1600 {\AA}. The flare appears to have two distinct phases of
emission separated by a 25-second time delay, with the first phase being
energetically more important. We found good temporal and spatial agreement
between the 25-50 keV X-rays and the most intense areas of the 1600 {\AA} UV
emission. We also observed an extended 100-arcsecond < 25 keV source that
appears coronal in nature and connects two separated UV ribbons later in the
flare. Using the observational properties in X-ray and UV wavelengths, we
propose two explanations for the flare evolution in relation to the spine/fan
magnetic field topology and the accelerated electrons. We find that a
combination of quasi separatrix layer reconnection and null-point reconnection
is required to account for the observed properties of the X-ray and UV
emission.Comment: 8 pages, 8 figures, published in Astronomy and Astrophysic
The spectral content of SDO/AIA 1600 and 1700 \AA\ filters from flare and plage observations
The strong enhancement of the ultraviolet emission during solar flares is
usually taken as an indication of plasma heating in the lower solar atmosphere
caused by the deposition of the energy released during these events. Images
taken with broadband ultraviolet filters by the {\em Transition Region and
Coronal Explorer} (TRACE) and {\em Atmospheric Imaging Assembly} (AIA 1600 and
1700~\AA) have revealed the morphology and evolution of flare ribbons in great
detail. However, the spectral content of these images is still largely unknown.
Without the knowledge of the spectral contribution to these UV filters, the use
of these rich imaging datasets is severely limited. Aiming to solve this issue,
we estimate the spectral contributions of the AIA UV flare and plage images
using high-resolution spectra in the range 1300 to 1900~\AA\ from the Skylab
NRL SO82B spectrograph. We find that the flare excess emission in AIA 1600~\AA\
is { dominated by} the \ion{C}{4} 1550~\AA\ doublet (26\%), \ion{Si}{1}
continua (20\%), with smaller contributions from many other chromospheric lines
such as \ion{C}{1} 1561 and 1656~\AA\ multiplets, \ion{He}{2} 1640~\AA,
\ion{Si}{2} 1526 and 1533~\AA. For the AIA 1700~\AA\ band, \ion{C}{1} 1656~\AA\
multiplet is the main contributor (38\%), followed by \ion{He}{2} 1640 (17\%),
and accompanied by a multitude of other, { weaker} chromospheric lines, with
minimal contribution from the continuum. Our results can be generalized to
state that the AIA UV flare excess emission is of chromospheric origin, while
plage emission is dominated by photospheric continuum emission in both
channels.Comment: Accepted for publication in ApJ Skylab NRL SO82B data used in this
work available at http://dx.doi.org/10.5525/gla.researchdata.68
Fine structure of type III solar radio bursts from Langmuir wave motion in turbulent plasma
The Sun frequently accelerates near-relativistic electron beams that travel
out through the solar corona and interplanetary space. Interacting with their
plasma environment, these beams produce type III radio bursts, the brightest
astrophysical radio sources seen from the Earth. The formation and motion of
type III fine frequency structures is a puzzle but is commonly believed to be
related to plasma turbulence in the solar corona and solar wind. Combining a
theoretical framework with kinetic simulations and high-resolution radio type
III observations using the Low Frequency Array, we quantitatively show that the
fine structures are caused by the moving intense clumps of Langmuir waves in a
turbulent medium. Our results show how type III fine structure can be used to
remotely analyse the intensity and spectrum of compressive density
fluctuations, and can infer ambient temperatures in astrophysical plasma, both
significantly expanding the current diagnostic potential of solar radio
emission.Comment: 22 pages, 9 figure
Solar wind density turbulence and solar flare electron transport from the Sun to the Earth
Solar flare accelerated electron beams propagating away from the Sun can
interact with the turbulent interplanetary media, producing plasma waves and
type III radio emission. These electron beams are detected near the Earth with
a double power-law energy spectrum. We simulate electron beam propagation from
the Sun to the Earth in the weak turbulent regime taking into account the
self-consistent generation of plasma waves and subsequent wave interaction with
density fluctuations from low frequency MHD turbulence. The rate at which
plasma waves are induced by an unstable electron beam is reduced by background
density fluctuations, most acutely when fluctuations have large amplitudes or
small wavelengths. This suppression of plasma waves alters the wave
distribution which changes the electron beam transport. Assuming a 5/3
Kolmogorov-type power density spectrum of fluctuations often observed near the
Earth, we investigate the corresponding energy spectrum of the electron beam
after it has propagated 1 AU. We find a direct correlation between the spectrum
of the double power-law below the break energy and the turbulent intensity of
the background plasma. For an initial spectral index of 3.5, we find a range of
spectra below the break energy between 1.6-2.1, with higher levels of
turbulence corresponding to higher spectral indices.Comment: 9 pages, 9 figures, to be published in Ap
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