1,403 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
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
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 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
Tracking magnetic bright point motions through the solar atmosphere
High-cadence, multiwavelength observations and simulations are employed for the analysis of solar photospheric magnetic bright points (MBPs) in the quiet Sun. The observations were obtained with the Rapid Oscillations in the Solar Atmosphere (ROSA) imager and the Interferometric Bidimensional Spectrometer at the Dunn Solar Telescope. Our analysis reveals that photospheric MBPs have an average transverse velocity of approximately 1 km s−1, whereas their chromospheric counterparts have a slightly higher average velocity of 1.4 km s−1. Additionally, chromospheric MBPs were found to be around 63 per cent larger than the equivalent photospheric MBPs. These velocity values were compared with the output of numerical simulations generated using the MURAM code. The simulated results were similar, but slightly elevated, when compared to the observed data. An average velocity of 1.3 km s−1 was found in the simulated G-band images and an average of 1.8 km s−1 seen in the velocity domain at a height of 500 km above the continuum formation layer. Delays in the change of velocities were also analysed. Average delays of ∼4 s between layers of the simulated data set were established and values of ∼29 s observed between G-band and Ca II K ROSA observations. The delays in the simulations are likely to be the result of oblique granular shock waves, whereas those found in the observations are possibly the result of a semi-rigid flux tube
A Search for High-Frequency Coronal Brightness Variations in the 21 August 2017 Total Solar Eclipse
We report on a search for short-period intensity variations in the green-line
FeXIV 530.3 nm emission from the solar corona during the 21 August 2017 total
eclipse viewed from Idaho in the United States. Our experiment was performed
with a much more sensitive detection system, and with better spatial
resolution, than on previous occasions (1999 and 2001 eclipses), allowing fine
details of quiet coronal loops and an active-region loop system to be seen. A
guided 200-mm-aperture Schmidt-Cassegrain telescope was used with a
state-of-the-art CCD camera having 16-bit intensity discrimination and a
field-of-view 0.43 degree x 0.43 degree that encompassed approximately one
third of the visible corona. The camera pixel size was 1.55 arcseconds, while
the seeing during the eclipse enabled features of approx. 2 arcseconds (1450 km
on the Sun) to be resolved. A total of 429 images were recorded during a 122.9
second portion of the totality at a frame rate of 3.49 images per second. In
the analysis, we searched particularly for short-period intensity oscillations
and travelling waves, since theory predicts fast-mode magneto-hydrodynamic
(MHD) waves with short periods may be important in quiet coronal and
active-region heating. Allowing first for various instrumental and photometric
effects, we used a wavelet technique to search for periodicities in some 404
000 pixels in the frequency range 0.5-1.6 Hz (periods: 2 second to 0.6 second).
We also searched for travelling waves along some 65 coronal structures.
However, we found no statistically significant evidence in either. This
negative result considerably refines the limit that we obtained from our
previous analyses, and it indicates that future searches for short-period
coronal waves may be better directed towards Doppler shifts as well as
intensity oscillations
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
Erratum: Temporal evolution of small-scale internetwork magnetic fields in the solar photosphere
While the longitudinal field that dominates photospheric network regions has
been studied extensively, small scale transverse fields have recently been
found to be ubiquitous in the quiet internetwork photosphere. Few observations
have captured how this field evolves. We aim to statistically characterise the
magnetic properties and observe the temporal evolution of small scale magnetic
features. We present two high spatial/temporal resolution observations that
reveal the dynamics of two disk centre internetwork regions taken by the new
GRIS/IFU (GREGOR Infrared Spectrograph Integral Field Unit) with the highly
magnetically sensitive Fe I line pair at 15648.52 {\AA} and 15652.87 {\AA}.
With the SIR code, we consider two inversion schemes: scheme 1 (S1), where a
magnetic atmosphere is embedded in a field free medium, and scheme 2 (S2), with
two magnetic models and a fixed stray light component. S1 inversions returned a
median magnetic field strength of 200 and 240 G for the two datasets,
respectively. We consider the median transverse (horizontal) component, among
pixels with Stokes Q or U, and the median unsigned longitudinal (vertical)
component, among pixels with Stokes V, above a noise threshold. We determined
the former to be 263 G and 267 G, and the latter to be 131 G and 145 G, for the
two datasets, respectively. We present three regions of interest (ROIs),
tracking the dynamics of small scale magnetic features. We apply S1 and S2
inversions to specific profiles, and find S2 produces better approximations
when there is evidence of mixed polarities. We find patches of linear
polarization with magnetic flux density between 130 and 150 G, appearing
preferentially at granule/intergranular lane (IGL) boundaries. The weak hG
magnetic field appears to be organised in terms of complex loop structures,
with transverse fields often flanked by opposite polarity longitudinal fields.Comment: Accepted for publication in A&A, 22 pages, 17 figures. Abstract
abridged for mailing list, full abstract included in PD
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