176 research outputs found
Spatially-resolved Energetic Electron Properties for the 21 May 2004 Flare from Radio Observations and 3D Simulations
We investigate in detail the 21 May 2004 flare using simultaneous
observations of the Nobeyama Radioheliograph, Nobeyama Radiopolarimeters,
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and Solar and
Heliospheric Observatory (SOHO). The flare images in different spectral ranges
reveal the presence of a well-defined single flaring loop in this event. We
have simulated the gyrosynchrotron microwave emission using the recently
developed interactive IDL tool GX Simulator. By comparing the simulation
results with the observations, we have deduced the spatial and spectral
properties of the non-thermal electron distribution. The microwave emission has
been found to be produced by the high-energy electrons ( keV) with a
relatively hard spectrum (); the electrons were strongly
concentrated near the loop top. At the same time, the number of high-energy
electrons near the footpoints was too low to be detected in the RHESSI images
and spatially unresolved data. The SOHO Extreme-ultraviolet Imaging Telescope
images and the low-frequency microwave spectra suggest the presence of an
extended "envelope" of the loop with lower magnetic field. Most likely, the
energetic electron distribution in the considered flare reflects the localized
(near the loop top) particle acceleration (injection) process accompanied by
trapping and scattering.Comment: Accepted for publication in Solar Physic
RHESSI and SDO/AIA observations of the chromospheric and coronal plasma parameters during a solar flare
X-ray and EUV observations are an important diagnostic of various plasma
parameters of the solar atmosphere during solar flares. Soft X-ray and EUV
observations often show coronal sources near the top of flaring loops, while
hard X-ray emission is mostly observed from chromospheric footpoints. Combining
RHESSI with simultaneous SDO/AIA observations, it is possible for the first
time to determine the density, temperature, and emission profile of the solar
atmosphere over a wide range of heights during a flare, using two independent
methods. Here we analyze a near limb event during the first of three hard X-ray
peaks. The emission measure, temperature, and density of the coronal source is
found using soft X-ray RHESSI images while the chromospheric density is
determined using RHESSI visibility analysis of the hard X-ray footpoints. A
regularized inversion technique is applied to AIA images of the flare to find
the differential emission measure (DEM). Using DEM maps we determine the
emission and temperature structure of the loop, as well as the density, and
compare it with RHESSI results. The soft X-ray and hard X-ray sources are
spatially coincident with the top and bottom of the EUV loop, but the bulk of
the EUV emission originates from a region without co-spatial RHESSI emission.
The temperature analysis along the loop indicates that the hottest plasma is
found near the coronal loop top source. The EUV observations suggest that the
density in the loop legs increases with increasing height while the temperature
remains constant within uncertainties.Comment: 23 pages, 6 figures, accepted for publication in Ap
Solar type III radio burst time characteristics at LOFAR frequencies and the implications for electron beam transport
Context. Solar type III radio bursts contain a wealth of information about the dynamics of electron beams in the solar corona and
the inner heliosphere; this information is currently unobtainable through other means. However, the motion of different regions of an
electron beam (front, middle, and back) have never been systematically analysed before.
Aims. We characterise the type III burst frequency-time evolution using the enhanced resolution of LOFAR (Low Frequency Array)
in the frequency range 30–70 MHz and use this to probe electron beam dynamics.
Methods. The rise, peak, and decay times with a ∼ 0.2 MHz spectral resolution were defined for a collection of 31 type III bursts.
The frequency evolution was used to ascertain the apparent velocities of the front, middle, and back of the type III sources, and the
trends were interpreted using theoretical and numerical treatments.
Results. The type III time profile was better approximated by an asymmetric Gaussian profile and not an exponential, as was used
previously. Rise and decay times increased with decreasing frequency and showed a strong correlation. Durations were shorter than
previously observed. Drift rates from the rise times were faster than from the decay times, corresponding to inferred mean electron
beam speeds for the front, middle, and back of 0.2, 0.17, 0.15 c, respectively. Faster beam speeds correlate with shorter type III
durations. We also find that the type III frequency bandwidth decreases as frequency decreases.
Conclusions. The different speeds naturally explain the elongation of an electron beam in space as it propagates through the heliosphere.
The expansion rate is proportional to the mean speed of the exciter; faster beams expand faster. Beam speeds are attributed to
varying ensembles of electron energies at the front, middle, and back of the beam
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
Electron distribution and energy release in magnetic reconnection outflow regions during the pre-impulsive phase of a solar flare
We present observations of electron energization in magnetic reconnection
outflows during the pre-impulsive phase of solar flare SOL2012-07-19T05:58.
During a time-interval of about 20 minutes, starting 40 minutes before the
onset of the impulsive phase, two X-ray sources were observed in the corona,
one above the presumed reconnection region and one below. For both of these
sources, the mean electron distribution function as a function of time is
determined over an energy range from 0.1~keV up to several tens of keV, for the
first time. This is done by simultaneous forward fitting of X-ray and EUV data.
Imaging spectroscopy with RHESSI provides information on the high-energy tail
of the electron distribution in these sources while EUV images from SDO/AIA are
used to constrain the low specific electron energies. The measured electron
distribution spectrum in the magnetic reconnection outflows is consistent with
a time-evolving kappa-distribution with . The spectral
evolution suggests that electrons are accelerated to progressively higher
energies in the source above the reconnection region, while in the source
below, the spectral shape does not change but an overall increase of the
emission measure is observed, suggesting density increase due to evaporation.
The main mechanisms by which energy is transported away from the source regions
are conduction and free-streaming electrons. The latter dominates by more than
one order of magnitude and is comparable to typical non-thermal energies during
the hard X-ray peak of solar flares, suggesting efficient acceleration even
during this early phase of the event.Comment: 9 pages, 5 figures, accepted for publication in Ap
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
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
Small electron acceleration episodes in the solar corona
We study the energetics of nonthermal electrons produced in small
acceleration episodes in the solar corona. We carried out an extensive survey
spanning 2004--2015 and shortlisted 6 impulsive electron events detected at 1
AU that was not associated with large solar flares(GOES soft X-ray class
C1) or with coronal mass ejections. Each of these events had weak, but
detectable hard Xray (HXR) emission near the west limb, and were associated
with interplanetary type III bursts. In some respects, these events seem like
weak counterparts of "cold/tenuous" flares. The energy carried by the HXR
producing electron population was -- erg, while
that in the corresponding population detected at 1 AU was -- erg. The number of electrons that escape the coronal
acceleration site and reach 1 AU constitute 6 % to 148 % of those that
precipitate downwards to produce thick target HXR emission
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