34 research outputs found
Do quasi-regular structures really exist in the solar photosphere? I. Observational evidence
Two series of solar-granulation images -- the La Palma series of 5 June 1993
and the SOHO MDI series of 17--18 January 1997 -- are analysed both
qualitatively and quantitatively. New evidence is presented for the existence
of long-lived, quasi-regular structures (first reported by Getling and Brandt
(2002)), which no longer appear unusual in images averaged over 1--2-h time
intervals. Such structures appear as families of light and dark concentric
rings or families of light and dark parallel strips (``ridges'' and
``trenches'' in the brightness distributions). In some cases, rings are
combined with radial ``spokes'' and can thus form ``web'' patterns. The
characteristic width of a ridge or trench is somewhat larger than the typical
size of granules. Running-average movies constructed from the series of images
are used to seek such structures. An algorithm is developed to obtain, for
automatically selected centres, the radial distributions of the azimuthally
averaged intensity, which highlight the concentric-ring patterns. We also
present a time-averaged granulation image processed with a software package
intended for the detection of geological structures in aerospace images. A
technique of running-average-based correlations between the brightness
variations at various points of the granular field is developed and indications
are found for a dynamical link between the emergence and sinking of hot and
cool parcels of the solar plasma. In particular, such a correlation analysis
confirms our suggestion that granules -- overheated blobs -- may repeatedly
emerge on the solar surface. Based on our study, the critical remarks by Rast
(2002) on the original paper by Getling and Brandt (2002) can be dismissed.Comment: 21 page, 8 figures; accepted by "Solar Physics
Comparison of large-scale flows on the Sun measured by time-distance helioseismology and local correlation tracking technique
We present a direct comparison between two different techniques time-distance
helioseismology and a local correlation tracking method for measuring mass
flows in the solar photosphere and in a near-surface layer: We applied both
methods to the same dataset (MDI high-cadence Dopplergrams covering almost the
entire Carrington rotation 1974) and compared the results. We found that after
necessary corrections, the vector flow fields obtained by these techniques are
very similar. The median difference between directions of corresponding vectors
is 24 degrees, and the correlation coefficients of the results for mean zonal
and meridional flows are 0.98 and 0.88 respectively. The largest discrepancies
are found in areas of small velocities where the inaccuracies of the computed
vectors play a significant role. The good agreement of these two methods
increases confidence in the reliability of large-scale synoptic maps obtained
by them.Comment: 14 pages, 6 figures, just before acceptance in Solar Physic
Acoustic Events in the Solar Atmosphere from Hinode/SOT NFI observations
We investigate the properties of acoustic events (AEs), defined as spatially
concentrated and short duration energy flux, in the quiet sun using
observations of a 2D field of view (FOV) with high spatial and temporal
resolution provided by the Solar Optical Telescope (SOT) onboard
\textit{Hinode}. Line profiles of Fe \textsc{i} 557.6 nm were recorded by the
Narrow band Filter Imager (NFI) on a FOV during 75 min with a
time step of 28.75 s and 0.08 pixel size. Vertical velocities were computed
at three atmospheric levels (80, 130 and 180 km) using the bisector technique
allowing the determination of energy flux in the range 3-10 mHz using two
complementary methods (Hilbert transform and Fourier power spectra). Horizontal
velocities were computed using local correlation tracking (LCT) of continuum
intensities providing divergences.
The net energy flux is upward. In the range 3-10 mHz, a full FOV space and
time averaged flux of 2700 W m (lower layer 80-130 km) and 2000 W
m (upper layer 130-180 km) is concentrated in less than 1% of the solar
surface in the form of narrow (0.3) AE. Their total duration (including rise
and decay) is of the order of s. Inside each AE, the mean flux is W m (lower layer) and W m (upper). Each
event carries an average energy (flux integrated over space and time) of J (lower layer) to J (upper). More than events
could exist permanently on the Sun, with a birth and decay rate of 3500
s. Most events occur in intergranular lanes, downward velocity regions,
and areas of converging motions.Comment: 18 pages, 10 figure
On Signatures of Twisted Magnetic Flux Tube Emergence
Recent studies of NOAA active region 10953, by Okamoto {\it et al.} ({\it
Astrophys. J. Lett.} {\bf 673}, 215, 2008; {\it Astrophys. J.} {\bf 697}, 913,
2009), have interpreted photospheric observations of changing widths of the
polarities and reversal of the horizontal magnetic field component as
signatures of the emergence of a twisted flux tube within the active region and
along its internal polarity inversion line (PIL). A filament is observed along
the PIL and the active region is assumed to have an arcade structure. To
investigate this scenario, MacTaggart and Hood ({\it Astrophys. J. Lett.} {\bf
716}, 219, 2010) constructed a dynamic flux emergence model of a twisted
cylinder emerging into an overlying arcade. The photospheric signatures
observed by Okamoto {\it et al.} (2008, 2009) are present in the model although
their underlying physical mechanisms differ. The model also produces two
additional signatures that can be verified by the observations. The first is an
increase in the unsigned magnetic flux in the photosphere at either side of the
PIL. The second is the behaviour of characteristic photospheric flow profiles
associated with twisted flux tube emergence. We look for these two signatures
in AR 10953 and find negative results for the emergence of a twisted flux tube
along the PIL. Instead, we interpret the photospheric behaviour along the PIL
to be indicative of photospheric magnetic cancellation driven by flows from the
dominant sunspot. Although we argue against flux emergence within this
particular region, the work demonstrates the important relationship between
theory and observations for the successful discovery and interpretation of
signatures of flux emergence.Comment: 14 pages, 8 figures, accepted for publication in Solar Physic
Two-Dimensional Spectroscopy of Photospheric Shear Flows in a Small delta Spot
In recent high-resolution observations of complex active regions,
long-lasting and well-defined regions of strong flows were identified in major
flares and associated with bright kernels of visible, near-infrared, and X-ray
radiation. These flows, which occurred in the proximity of the magnetic neutral
line, significantly contributed to the generation of magnetic shear. Signatures
of these shear flows are strongly curved penumbral filaments, which are almost
tangential to sunspot umbrae rather than exhibiting the typical radial
filamentary structure. Solar active region NOAA 10756 was a moderately complex,
beta-delta sunspot group, which provided an opportunity to extend previous
studies of such shear flows to quieter settings. We conclude that shear flows
are a common phenomenon in complex active regions and delta spots. However,
they are not necessarily a prerequisite condition for flaring. Indeed, in the
present observations, the photospheric shear flows along the magnetic neutral
line are not related to any change of the local magnetic shear. We present
high-resolution observations of NOAA 10756 obtained with the 65-cm vacuum
reflector at Big Bear Solar Observatory (BBSO). Time series of
speckle-reconstructed white-light images and two-dimensional spectroscopic data
were combined to study the temporal evolution of the three-dimensional vector
flow field in the beta-delta sunspot group. An hour-long data set of consistent
high quality was obtained, which had a cadence of better than 30 seconds and
sub-arcsecond spatial resolution.Comment: 23 pages, 6 gray-scale figures, 4 color figures, 2 tables, submitted
to Solar Physic
Comparisons of Supergranule Characteristics During the Solar Minima of Cycles 22/23 and 23/24
Supergranulation is a component of solar convection that manifests itself on
the photosphere as a cellular network of around 35 Mm across, with a turnover
lifetime of 1-2 days. It is strongly linked to the structure of the magnetic
field. The horizontal, divergent flows within supergranule cells carry local
field lines to the cell boundaries, while the rotational properties of
supergranule upflows may contribute to the restoration of the poloidal field as
part of the dynamo mechanism that controls the solar cycle. The solar minimum
at the transition from cycle 23 to 24 was notable for its low level of activity
and its extended length. It is of interest to study whether the convective
phenomena that influences the solar magnetic field during this time differed in
character to periods of previous minima. This study investigates three
characteristics (velocity components, sizes and lifetimes) of solar
supergranulation. Comparisons of these characteristics are made between the
minima of cycles 22/23 and 23/24 using MDI Doppler data from 1996 and 2008,
respectively. It is found that whereas the lifetimes are equal during both
epochs (around 18 h), the sizes are larger in 1996 (35.9 +/- 0.3 Mm) than in
2008 (35.0 +/- 0.3 Mm), while the dominant horizontal velocity flows are weaker
(139 +/- 1 m/s in 1996; 141 +/- 1 m/s in 2008). Although numerical differences
are seen, they are not conclusive proof of the most recent minimum being
inherently unusual.Comment: 22 pages, 5 figures. Solar Physics, in pres
An Automated Algorithm to Distinguish and Characterize Solar Flares and Associated Sequential Chromospheric Brightenings
We present a new automated algorithm to identify, track, and characterize
small-scale brightening associated with solar eruptive phenomena observed in
H{\alpha}. The temporal spatially-localized changes in chromospheric
intensities can be separated into two categories: flare ribbons and sequential
chromospheric brightenings (SCBs). Within each category of brightening we
determine the smallest resolvable locus of pixels, a kernel, and track the
temporal evolution of the position and intensity of each kernel. This tracking
is accomplished by isolating the eruptive features, identifying kernels, and
linking detections between frames into trajectories of kernels. We fully
characterize the evolving intensity and morphology of the flare ribbons by
observing the tracked flare kernels in aggregate. With the location of SCB and
flare kernels identified, they can easily be overlaid on top of complementary
data sets to extract Doppler velocities and magnetic field intensities
underlying the kernels. This algorithm is adaptable to any dataset to identify
and track solar features.Comment: 22 pages, 9 figure
Multiscale magnetic underdense regions on the solar surface: Granular and Mesogranular scales
The Sun is a non-equilibrium dissipative system subjected to an energy flow
which originates in its core. Convective overshooting motions create
temperature and velocity structures which show a temporal and spatial
evolution. As a result, photospheric structures are generally considered to be
the direct manifestation of convective plasma motions. The plasma flows on the
photosphere govern the motion of single magnetic elements. These elements are
arranged in typical patterns which are observed as a variety of multiscale
magnetic patterns. High resolution magnetograms of quiet solar surface revealed
the presence of magnetic underdense regions in the solar photosphere, commonly
called voids, which may be considered a signature of the underlying convective
structure. The analysis of such patterns paves the way for the investigation of
all turbulent convective scales from granular to global. In order to address
the question of magnetic structures driven by turbulent convection at granular
and mesogranular scales we used a "voids" detection method. The computed voids
distribution shows an exponential behavior at scales between 2 and 10 Mm and
the absence of features at 5-10 Mm mesogranular scales. The absence of
preferred scales of organization in the 2-10 Mm range supports the multiscale
nature of flows on the solar surface and the absence of a mesogranular
convective scale
Can we Determine Electric Fields and Poynting Fluxes from Vector Magnetograms and Doppler Measurements?
The availability of vector magnetogram sequences with sufficient accuracy and
cadence to estimate the time derivative of the magnetic field allows us to use
Faraday's law to find an approximate solution for the electric field in the
photosphere, using a Poloidal-Toroidal Decomposition (PTD) of the magnetic
field and its partial time derivative. Without additional information, however,
the electric field found from this technique is under-determined -- Faraday's
law provides no information about the electric field that can be derived the
gradient of a scalar potential. Here, we show how additional information in the
form of line-of-sight Doppler flow measurements, and motions transverse to the
line-of-sight determined with ad-hoc methods such as local correlation
tracking, can be combined with the PTD solutions to provide much more accurate
solutions for the solar electric field, and therefore the Poynting flux of
electromagnetic energy in the solar photosphere. Reliable, accurate maps of the
Poynting flux are essential for quantitative studies of the buildup of magnetic
energy before flares and coronal mass ejections.Comment: Solar Physics, in press. 14 pages, 3 figure
DOT Tomography of the Solar Atmosphere VII. Chromospheric Response to Acoustic Events
We use synchronous movies from the Dutch Open Telescope sampling the
G band, Ca II and Halpha with five-wavelength profile sampling to study the
response of the chromosphere to acoustic events in the underlying photosphere.
We first compare the visibility of the chromosphere in Ca II H and Halpha,
demonstrate that studying the chromosphere requires Halpha data, and summarize
recent developments in understanding why this is so. We construct divergence
and vorticity maps of the photospheric flow field from the G-band images and
locate specific events through the appearance of bright Ca II H grains. The
reaction of the Halpha chromosphere is diagnosed in terms of brightness and
Doppler shift. We show and discuss three particular cases in detail: a regular
acoustic grain marking shock excitation by granular dynamics, a persistent
flasher which probably marks magnetic-field concentration, and an exploding
granule. All three appear to buffet overlying fibrils, most clearly in
Dopplergrams. Although our diagnostic displays to dissect these phenomena are
unprecedentedly comprehensive, adding even more information (photospheric
Doppler tomography and magnetograms, chromospheric imaging and Doppler mapping
in the ultraviolet) is warranted.Comment: accepted by Solar Physic