676 research outputs found
MHD wave modes resolved in fine-scale chromospheric magnetic structures
Within the last decade, due to significant improvements in the spatial and
temporal resolution of chromospheric data, magnetohydrodynamic (MHD) wave
studies in this fascinating region of the Sun's atmosphere have risen to the
forefront of solar physics research. In this review we begin by reviewing the
challenges and debates that have manifested in relation to MHD wave mode
identification in fine-scale chromospheric magnetic structures, including
spicules, fibrils and mottles. Next we go on to discuss how the process of
accurately identifying MHD wave modes also has a crucial role to play in
estimating their wave energy flux. This is of cardinal importance for
estimating what the possible contribution of MHD waves is to solar atmospheric
heating. Finally, we detail how such advances in chromospheric MHD wave studies
have also allowed us, for the first time, to implement cutting-edge
magnetoseismological techniques that provide new insight into the
sub-resolution plasma structuring of the lower solar atmosphere.Comment: 16 pages, 6 figures, to appear as a chapter in the upcoming AGU/Wiley
book "Low-frequency Waves in Space Plasmas
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
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
The Area Distribution of Solar Magnetic Bright Points
Magnetic Bright Points (MBPs) are among the smallest observable objects on
the solar photosphere. A combination of G-band observations and numerical
simulations is used to determine their area distribution. An automatic
detection algorithm, employing 1-dimensional intensity profiling, is utilized
to identify these structures in the observed and simulated datasets. Both
distributions peak at an area of 45000 km, with a sharp decrease
towards smaller areas. The distributions conform with log-normal statistics,
which suggests that flux fragmentation dominates over flux convergence.
Radiative magneto-convection simulations indicate an independence in the MBP
area distribution for differing magnetic flux densities. The most commonly
occurring bright point size corresponds to the typical width of intergranular
lanes.Comment: Astrophysical Journal, accepte
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
Departure of high temperature iron lines from the equilibrium state in flaring solar plasmas
The aim of this study is to clarify if the assumption of ionization
equilibrium and a Maxwellian electron energy distribution is valid in flaring
solar plasmas. We analyze the 2014 December 20 X1.8 flare, in which the
\ion{Fe}{xxi} 187~\AA, \ion{Fe}{xxii} 253~\AA, \ion{Fe}{xxiii} 263~\AA\ and
\ion{Fe}{xxiv} 255~\AA\ emission lines were simultaneously observed by the EUV
Imaging Spectrometer onboard the Hinode satellite. Intensity ratios among these
high temperature Fe lines are compared and departures from isothermal
conditions and ionization equilibrium examined. Temperatures derived from
intensity ratios involving these four lines show significant discrepancies at
the flare footpoints in the impulsive phase, and at the looptop in the gradual
phase. Among these, the temperature derived from the
\ion{Fe}{xxii}/\ion{Fe}{xxiv} intensity ratio is the lowest, which cannot be
explained if we assume a Maxwellian electron distribution and ionization
equilibrium, even in the case of a multi-thermal structure. This result
suggests that the assumption of ionization equilibrium and/or a Maxwellian
electron energy distribution can be violated in evaporating solar plasma around
10~MK.Comment: 10 pages, 4 figures, to appear in Ap
Solar feature tracking in both spatial and temporal domains
A new method for automated coronal loop tracking, in both spatial and temporal
domains, is presented. The reliability of this technique was tested with TRACE 171A observations.
The application of this technique to a flare-induced kink-mode oscillation, revealed a
3500 km spatial periodicity which occur along the loop edge. We establish a reduction in oscillatory
power, for these spatial periodicities, of 45% over a 322 s interval. We relate the reduction
in oscillatory power to the physical damping of these loop-top 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
Time-dependent suppression of oscillatory power in evolving solar magnetic fields
Oscillation amplitudes are generally smaller within magnetically active regions like sunspots and plage when compared to their surroundings. Such magnetic features, when viewed in spatially resolved power maps, appear as regions of suppressed power due to reductions in the oscillation amplitudes. Employing high spatial- and temporal-resolution observations from the Dunn Solar Telescope (DST) in New Mexico, we study the power suppression in a region of evolving magnetic fields adjacent to a pore. By utilizing wavelet analysis, we study for the first time how the oscillatory properties in this region change as the magnetic field evolves with time. Image sequences taken in the blue continuum, G-band, Ca ii K, and Hα filters were used in this study. It is observed that the suppression found in the chromosphere occupies a relatively larger area, confirming previous findings. Also, the suppression is extended to structures directly connected to the magnetic region, and is found to get enhanced as the magnetic field strength increased with time. The dependence of the suppression on the magnetic field strength is greater at longer periods and higher formation heights. Furthermore, the dominant periodicity in the chromosphere was found to be anti-correlated with increases in the magnetic field strength
The Source of Three-minute Magneto-acoustic Oscillations in Coronal Fans
We use images of high spatial, spectral and temporal resolution, obtained
using both ground- and space-based instrumentation, to investigate the coupling
between wave phenomena observed at numerous heights in the solar atmosphere.
Intensity oscillations of 3 minutes are observed to encompass photospheric
umbral dot structures, with power at least three orders-of-magnitude higher
than the surrounding umbra. Simultaneous chromospheric velocity and intensity
time series reveal an 87 \pm 8 degree out-of-phase behavior, implying the
presence of standing modes created as a result of partial wave reflection at
the transition region boundary. An average blue-shifted Doppler velocity of
~1.5 km/s, in addition to a time lag between photospheric and chromospheric
oscillatory phenomena, confirms the presence of upwardly-propagating slow-mode
waves in the lower solar atmosphere. Propagating oscillations in EUV intensity
are detected in simultaneous coronal fan structures, with a periodicity of 172
\pm 17 s and a propagation velocity of 45 \pm 7 km/s. Numerical simulations
reveal that the damping of the magneto-acoustic wave trains is dominated by
thermal conduction. The coronal fans are seen to anchor into the photosphere in
locations where large-amplitude umbral dot oscillations manifest. Derived
kinetic temperature and emission measure time-series display prominent
out-of-phase characteristics, and when combined with the previously established
sub-sonic wave speeds, we conclude that the observed EUV waves are the coronal
counterparts of the upwardly-propagating magneto-acoustic slow-modes detected
in the lower solar atmosphere. Thus, for the first time, we reveal how the
propagation of 3 minute magneto-acoustic waves in solar coronal structures is a
direct result of amplitude enhancements occurring in photospheric umbral dots.Comment: Accepted into ApJ (13 pages and 10 figures
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