2,272 research outputs found
Nanoflare Activity in the Solar Chromosphere
We use ground-based images of high spatial and temporal resolution to search
for evidence of nanoflare activity in the solar chromosphere. Through close
examination of more than 10^9 pixels in the immediate vicinity of an active
region, we show that the distributions of observed intensity fluctuations have
subtle asymmetries. A negative excess in the intensity fluctuations indicates
that more pixels have fainter-than-average intensities compared with those that
appear brighter than average. By employing Monte Carlo simulations, we reveal
how the negative excess can be explained by a series of impulsive events,
coupled with exponential decays, that are fractionally below the current
resolving limits of low-noise equipment on high-resolution ground-based
observatories. Importantly, our Monte Carlo simulations provide clear evidence
that the intensity asymmetries cannot be explained by photon-counting
statistics alone. A comparison to the coronal work of Terzo et al. (2011)
suggests that nanoflare activity in the chromosphere is more readily occurring,
with an impulsive event occurring every ~360s in a 10,000 km^2 area of the
chromosphere, some 50 times more events than a comparably sized region of the
corona. As a result, nanoflare activity in the chromosphere is likely to play
an important role in providing heat energy to this layer of the solar
atmosphere.Comment: 7 pages, 3 figures, accepted into Ap
Chromospheric Velocities of a C-class Flare
We use high spatial and temporal resolution observations from the Swedish
Solar Telescope to study the chromospheric velocities of a C-class flare
originating from active region NOAA 10969. A time-distance analysis is employed
to estimate directional velocity components in H-alpha and Ca II K image
sequences. Also, imaging spectroscopy has allowed us to determine flare-induced
line-of-sight velocities. A wavelet analysis is used to analyse the periodic
nature of associated flare bursts. Time-distance analysis reveals velocities as
high as 64 km/s along the flare ribbon and 15 km/s perpendicular to it. The
velocities are very similar in both the H-alpha and Ca II K time series.
Line-of-sight H-alpha velocities are red-shifted with values up to 17 km/s. The
high spatial and temporal resolution of the observations have allowed us to
detect velocities significantly higher than those found in earlier studies.
Flare bursts with a periodicity of approximately 60 s are also detected. These
bursts are similar to the quasi-periodic oscillations observed at hard X-ray
and radio wavelength data. Some of the highest velocities detected in the solar
atmosphere are presented. Line-of-sight velocity maps show considerable mixing
of both the magnitude and direction of velocities along the flare path. A
change in direction of the velocities at the flare kernel has also been
detected which may be a signature of chromospheric evaporation.Comment: Accepted for publication in Astronomy and Astrophysics, 5 figure
Propagating Wave Phenomena Detected in Observations and Simulations of the Lower Solar Atmosphere
We present high-cadence observations and simulations of the solar
photosphere, obtained using the Rapid Oscillations in the Solar Atmosphere
imaging system and the MuRAM magneto-hydrodynamic code, respectively. Each
dataset demonstrates a wealth of magneto-acoustic oscillatory behaviour,
visible as periodic intensity fluctuations with periods in the range 110-600 s.
Almost no propagating waves with periods less than 140s and 110s are detected
in the observational and simulated datasets, respectively. High concentrations
of power are found in highly magnetised regions, such as magnetic bright points
and intergranular lanes. Radiative diagnostics of the photospheric simulations
replicate our observational results, confirming that the current breed of
magneto-hydrodynamic simulations are able to accurately represent the lower
solar atmosphere. All observed oscillations are generated as a result of
naturally occurring magnetoconvective processes, with no specific input driver
present. Using contribution functions extracted from our numerical simulations,
we estimate minimum G-band and 4170 Angstrom continuum formation heights of 100
km and 25 km, respectively. Detected magneto-acoustic oscillations exhibit a
dominant phase delay of -8 degrees between the G-band and 4170 Angstrom
continuum observations, suggesting the presence of upwardly propagating waves.
More than 73% of MBPs (73% from observations, 96% from simulations) display
upwardly propagating wave phenomena, suggesting the abundant nature of
oscillatory behaviour detected higher in the solar atmosphere may be traced
back to magnetoconvective processes occurring in the upper layers of the Sun's
convection zone.Comment: 13 pages, 9 figures, accepted into Ap
On the effect of oscillatory phenomena on Stokes inversion results
Stokes inversion codes are crucial in returning properties of the solar
atmosphere, such as temperature and magnetic field strength. However, the
success of such algorithms to return reliable values can be hindered by the
presence of oscillatory phenomena within magnetic wave guides. Returning
accurate parameters is crucial to both magnetohydrodynamics studies and solar
physics in general. Here, we employ a simulation featuring propagating MHD
waves within a flux tube with a known driver and atmospheric parameters. We
invert the Stokes profiles for the 6301 \unicode{0xc5} and 6302
\unicode{0xc5} line pair emergent from the simulations using the well-known
Stokes Inversions from Response functions (SIR) code to see if the atmospheric
parameters can be returned for typical spatial resolutions at ground-based
observatories. The inversions return synthetic spectra comparable to the
original input spectra, even with asymmetries introduced in the spectra from
wave propagation in the atmosphere. The output models from the inversions match
closely to the simulations in temperature, line-of-sight magnetic field and
line-of-sight velocity within typical formation heights of the inverted lines.
Deviations from the simulations are seen away from these height regions. The
inversion results are less accurate during passage of the waves within the line
formation region. The original wave period could be recovered from the
atmosphere output by the inversions, with empirical mode decomposition
performing better than the wavelet approach in this task.Comment: Accepted for publication in Phil. Trans. R. Soc. A, 20 pages, 4
figure
The Velocity Distribution of Solar Photospheric Magnetic Bright Points
We use high spatial resolution observations and numerical simulations to
study the velocity distribution of solar photospheric magnetic bright points.
The observations were obtained with the Rapid Oscillations in the Solar
Atmosphere instrument at the Dunn Solar Telescope, while the numerical
simulations were undertaken with the MURaM code for average magnetic fields of
200 G and 400 G. We implemented an automated bright point detection and
tracking algorithm on the dataset, and studied the subsequent velocity
characteristics of over 6000 structures, finding an average velocity of
approximately 1 km/s, with maximum values of 7 km/s. Furthermore, merging
magnetic bright points were found to have considerably higher velocities, and
significantly longer lifetimes, than isolated structures. By implementing a new
and novel technique, we were able to estimate the background magnetic flux of
our observational data, which is consistent with a field strength of 400 G.Comment: Accepted for publication in ApJL, 12 pages, 2 figure
Molecular random tilings as glasses
We have recently shown [Blunt et al., Science 322, 1077 (2008)] that
p-terphenyl-3,5,3',5'-tetracarboxylic acid adsorbed on graphite self-assembles
into a two-dimensional rhombus random tiling. This tiling is close to ideal,
displaying long range correlations punctuated by sparse localised tiling
defects. In this paper we explore the analogy between dynamic arrest in this
type of random tilings and that of structural glasses. We show that the
structural relaxation of these systems is via the propagation--reaction of
tiling defects, giving rise to dynamic heterogeneity. We study the scaling
properties of the dynamics, and discuss connections with kinetically
constrained models of glasses.Comment: 5 pages, 5 figure
The arctic circle boundary and the Airy process
We prove that the, appropriately rescaled, boundary of the north polar region
in the Aztec diamond converges to the Airy process. The proof uses certain
determinantal point processes given by the extended Krawtchouk kernel. We also
prove a version of Propp's conjecture concerning the structure of the tiling at
the center of the Aztec diamond.Comment: Published at http://dx.doi.org/10.1214/009117904000000937 in the
Annals of Probability (http://www.imstat.org/aop/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Predesign study for a modern 4-bladed rotor for the NASA rotor systems research aircraft
Trade-off study results and the rationale for the final selection of an existing modern four-bladed rotor system that can be adapted for installation on the Rotor Systems Research Aircraft (RSRA) are reported. The results of the detailed integration studies, parameter change studies, and instrumentation studies and the recommended plan for development and qualification of the rotor system is also given. Its parameter variants, integration on the RSRA, and support of ground and flight test programs are also discussed
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