31,564 research outputs found
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
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
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
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
Alfv\'en waves in simulations of solar photospheric vortices
Using advanced numerical magneto-hydrodynamic simulations of the magnetised
solar photosphere, including non-grey radiative transport and a non-ideal
equation of state, we analyse plasma motions in photospheric magnetic vortices.
We demonstrate that apparent vortex-like motions in photospheric magnetic field
concentrations do not exhibit "tornado"-like behaviour or a "bath-tub" effect.
While at each time instance the velocity field lines in the upper layers of the
solar photosphere show swirls, the test particles moving with the
time-dependent velocity field do not demonstrate such structures. Instead, they
move in a wave-like fashion with rapidly changing and oscillating velocity
field, determined mainly by magnetic tension in the magnetised intergranular
downflows. Using time-distance diagrams, we identify horizontal motions in the
magnetic flux tubes as torsional Alfv\'en perturbations propagating along the
nearly vertical magnetic field lines with local Alfv\'en speed.Comment: 5 pages, 4 figures, accepted to ApJ
Nonlinear optical thresholding in a 4-Channel OCDMA system via two-photon absorption
We demonstrate the use of a Two-Photon Absorption based detector in an OCDMA system. This detector provides a significant performance improvement over standard linear detection
A Comparison of Micro-Switch Analytic, Finite element, and Experimental Results
Electrostatically actuated, metal contact, micro-switches depend on having adequate contact force to achieve desired, low contact resistance. In this study, higher contact forces resulted from overdriving cantilever beam style switches, after pull-in or initial contact, until the beam collapsed onto the drive or actuation electrode. The difference between initial contact and beam collapse was defined as the useful contact force range. Micro-switch pull-in voltage, collapse voltage, and contact force predictions, modeled analytically and with the CoventorWare finite element software package, were compared to experimental results. Contact resistance was modeled analytically using Maxwellian spreading resistance theory. Contact resistance and contact force were further investigated by varying the width of the drive electrode. A minimum contact resistance of 0.26 Ω role= presentation style= box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; font-style: normal; font-weight: normal; line-height: normal; font-size: 14.4px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative; \u3eΩ was measured on micro-switches with 150 μm-wide drive electrodes. The useful contact force range for these devices was between 22.7 and 58.3 V. Contributions of this work include: a contact force equation useful for initial micro-switch designs, a detailed pull-in voltage, collapse voltage, and contact force investigation using CoventorWare, a direct comparison of measured results with analytic and finite element predictions, and a means of choosing a micro-switch operating point for optimized contact resistance performance
Pulse pedestal suppression using four-wave mixing in an SOA
Experimental results are presented demonstrating how four-wave mixing in a semiconductor optical amplifier can be used to remove pulse pedestals introduced due to nonlinearities which occur upon pulse propagation in an optical system. Such pedestals would degrade the performance of an optical time-division-multiplexed system due to coherent interaction between channels. An improvement of the temporal pulse suppression ratio to greater than 30 dB is achieved regardless of the level of the pulse pedestal on the input signal. This improvement takes place simultaneously with wavelength conversion and compression of the optical pulse
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