808 research outputs found
Between umbra and penumbra
International audienceComputing shadow boundaries is a difficult problem in the case of non-point light sources. A point is in the umbra if it does not see any part of any light source; it is in full light if it sees entirely all the light sources; otherwise, it is in the penumbra. While the common boundary of the penumbra and the full light is well understood, less is known about the boundary of the umbra. In this paper we prove various bounds on the complexity of the umbra and the penumbra cast by a segment or polygonal light source on a plane in the presence of polygon or polytope obstacles. In particular, we show that a single segment light source may cast on a plane, in the presence of two triangles, four connected components of umbra and that two fat convex obstacles of total complexity n can engender Omega(n) connected components of umbra. In a scene consisting of a segment light source and k disjoint polytopes of total complexity n, we prove an Omega(nk^2+k^4) lower bound on the maximum number of connected components of the umbra and a O(nk^3) upper bound on its complexity. We also prove that, in the presence of k disjoint polytopes of total complexity n, some of which being light sources, the umbra cast on a plane may have Omega(n^2k^3 + nk^5) connected components and has complexity O(n^3k^3). These are the first bounds on the size of the umbra in terms of both k and n. These results prove that the umbra, which is bounded by arcs of conics, is intrinsically much more intricate than the full light/penumbra boundary which is bounded by line segments and whose worst-case complexity is in Omega(n alpha(k) +km +k^2) and O(n alpha(k) +km alpha(k) +k^2), where m is the complexity of the polygonal light source
Flow patterns around old sunspots and flare activity
New magnetic flux emerges significantly more probably in already
existing solar active regions. Based on the the Debrecen Observatory
photographic observations, several active regions are collected, where at least
one large, X-class flare was recorded, and emergence of new activity, birth
and quick motion of new umbrae was observed in the vicinity of old spots, the
new activity emerged in the center of the old active region.
Newly emerging
magnetic flux in older sunspot groups can be distinguished by its quicker and
generally westward proper motions. Umbrae of the new activity do not coalesce
with older umbrae of the same polarity, but both elastic and inelastic
collisions between them can be observed. Spots of the emerging new activity can
flow around old unipolar spots (presumably shallower structures,
“ω-loops”) westward, like a hydrodynamic flow around a cylinder, forming
a wake behind it. Collision of different polarities in the wake can lead to
large flares. The presence of old spots disturbs the normal emergence of the
new activity, so motions of the new spots are distorted by the flow, the new
emerging “Ω-loop” can be stuck between the umbrae of the old, tight
dipole, the orientation of the new dipole can be distorted by as much as 180◦.
The general direction of the flow around the old spots seems to depend on
the latitude, the angle between the motion axis and the E-W direction grows
with the latitude.
The intensive flare activity seems to be connected strongly
with the newly emerging magnetic flux; interacting of differently oriented
dipoles and the difference of the orientation of the emerging new dipole from
the ordinary Hale-Nicholson orientation is also significant. Simply large
gradients of magnetic fields (δ-configuration) are not enough, dynamical
processes (emergence of new flux, shearing or colliding motions of umbrae of
different magnetic polarity) must also be present for large flares
High Resolution Helioseismic Imaging of Subsurface Structures and Flows of A Solar Active Region Observed by Hinode
We analyze a solar active region observed by the Hinode CaII H line using the
time-distance helioseismology technique, and infer wave-speed perturbation
structures and flow fields beneath the active region with a high spatial
resolution. The general subsurface wave-speed structure is similar to the
previous results obtained from SOHO/MDI observations. The general subsurface
flow structure is also similar, and the downward flows beneath the sunspot and
the mass circulations around the sunspot are clearly resolved. Below the
sunspot, some organized divergent flow cells are observed, and these structures
may indicate the existence of mesoscale convective motions. Near the light
bridge inside the sunspot, hotter plasma is found beneath, and flows divergent
from this area are observed. The Hinode data also allow us to investigate
potential uncertainties caused by the use of phase-speed filter for short
travel distances. Comparing the measurements with and without the phase-speed
filtering, we find out that inside the sunspot, mean acoustic travel times are
in basic agreement, but the values are underestimated by a factor of 20-40%
inside the sunspot umbra for measurements with the filtering. The initial
acoustic tomography results from Hinode show a great potential of using
high-resolution observations for probing the internal structure and dynamics of
sunspots.Comment: accepted for publication in Ap
The pre-penumbral magnetic canopy in the solar atmosphere
Penumbrae are the manifestation of magnetoconvection in highly inclined (to the vertical direction)
magnetic field. The penumbra of a sunspot tends to form, initially, along the arc of the umbra
antipodal to the main region of flux emergence. The question of how highly inclined magnetic field
can concentrate along the antipodal curves of umbrae, at least initially, remains to be answered.
Previous observational studies have suggested the existence of some form of overlying magnetic canopy
which acts as the progenitor for penumbrae. We propose that such overlying magnetic canopies are
a consequence of how the magnetic field emerges into the atmosphere and are, therefore, part of the
emerging region. We show, through simulations of twisted flux tube emergence, that canopies of
highly inclined magnetic field form preferentially at the required locations above the photosphere
Wave dynamics in a sunspot umbra
The high spatial and time resolution data obtained with SDO/AIA for the
sunspot in active region NOAA 11131 on 08 December 2010 were analysed with the
time-distance plot technique and the pixelised wavelet filtering method.
Oscillations in the 3 min band dominate in the umbra. The integrated spectrum
of umbral oscillations contains distinct narrowband peaks at 1.9 min, 2.3 min,
and 2.8 min. The power significantly varies in time, forming distinct
oscillation trains. The oscillation power distribution over the sunspot in the
horizontal plane reveals that the enhancements of the oscillation amplitude, or
wave fronts, have a distinct structure consisting of an evolving two-armed
spiral and a stationary circular patch at the spiral origin, situated near the
umbra centre. This structure is seen from the temperature minimum to the
corona. In time, the spiral rotates anti-clockwise. The wave front spirality is
most pronounced during the maximum amplitude phases of the oscillations. In the
low-amplitude phases the spiral breaks into arc-shaped patches. The 2D
cross-correlation function shows that the oscillations at higher atmospheric
levels occur later than at lower layers. The phase speed is estimated to be
about 100 km/s. The fine spectral analysis shows that the central patch
corresponds to the high-frequency oscillations, while the spiral arms highlight
the lower-frequency oscillations in the 3-min band. The vertical and horizontal
radial structure of the oscillations is consistent with the model that
interprets umbral oscillations as slow magnetoacoustic waves filtered by the
atmospheric temperature non-uniformity in the presence of the magnetic field
inclination from the vertical. The mechanism for the polar-angle structure of
the oscillations, in particular the spirality of the wave fronts, needs to be
revealed.Comment: 8 pages, 9 figures, Astronomy and Astrophysics, 201
Stable Umbral Chromospheric Structures
Aims. To understand the morphology of the chromosphere in sunspot umbra. We
investigate if the horizontal structures observed in the spectral core of the
Ca II H line are ephemeral visuals caused by the shock dynamics of more stable
structures, and examine their relationship with observables in the H-alpha
line. Methods. Filtergrams in the core of the Ca II H and H-alpha lines as
observed with the Swedish 1-m Solar Telescope are employed. We utilise a
technique that creates composite images and tracks the flash propagation
horizontally. Results. We find 0"15 wide horizontal structures, in all of the
three target sunspots, for every flash where the seeing was moderate to good.
Discrete dark structures are identified that are stable for at least two umbral
flashes, as well as systems of structures that live for up to 24 minutes. We
find cases of extremely extended structures with similar stability, with one
such structure showing an extent of 5". Some of these structures have a
correspondence in H-alpha but we were unable to find a one to one
correspondence for every occurrence. If the dark streaks are formed at the same
heights as umbral flashes then there are systems of structures with strong
departures from the vertical for all three analysed sunspots. Conclusions.
Long-lived Ca II H filamentary horizontal structures are a common and likely
ever-present feature in the umbra of sunspots. If the magnetic field in the
chromosphere of the umbra is indeed aligned with the structures, then the
present theoretical understanding of the typical umbra needs to be revisited.Comment: Accepted to Astronomy and Astrophysics. Online material (Fig3.mov and
Fig4.mov) will be available at A&
Image patch analysis and clustering of sunspots: a dimensionality reduction approach
Sunspots, as seen in white light or continuum images, are associated with
regions of high magnetic activity on the Sun, visible on magnetogram images.
Their complexity is correlated with explosive solar activity and so classifying
these active regions is useful for predicting future solar activity. Current
classification of sunspot groups is visually based and suffers from bias.
Supervised learning methods can reduce human bias but fail to optimally
capitalize on the information present in sunspot images. This paper uses two
image modalities (continuum and magnetogram) to characterize the spatial and
modal interactions of sunspot and magnetic active region images and presents a
new approach to cluster the images. Specifically, in the framework of image
patch analysis, we estimate the number of intrinsic parameters required to
describe the spatial and modal dependencies, the correlation between the two
modalities and the corresponding spatial patterns, and examine the phenomena at
different scales within the images. To do this, we use linear and nonlinear
intrinsic dimension estimators, canonical correlation analysis, and
multiresolution analysis of intrinsic dimension.Comment: 5 pages, 7 figures, accepted to ICIP 201
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