11 research outputs found
Evolution of the flow field in decaying active regions, Transition from a moat flow to a supergranular flow
We investigate the evolution of the horizontal flow field around sunspots
during their decay by analysing its extension and horizontal velocity around
eight spots using SDO/HMI Doppler maps. By assuming a radially symmetrical flow
field, the applied analysis method determines the radial dependence of the
azimuthally averaged flow field. For comparison, we studied the flow in
supergranules using the same technique. All investigated, fully fledged
sunspots are surrounded by a flow field whose horizontal velocity profile
decreases continuously from 881 m s at 1.1 Mm off the spot boundary,
down to 199 m s at a mean distance of 11.9 Mm to that boundary. Once the
penumbra is fully dissolved, however, the velocity profile of the flow changes:
The horizontal velocity increases with increasing distance to the spot boundary
until a maximum value of about 398 m s is reached. Then, the horizontal
velocity decreases for farther distances to the spot boundary. In
supergranules, the horizontal velocity increases with increasing distance to
their centre up to a mean maximum velocity of 355 m s. For larger
distances, the horizontal velocity decreases. We thus find that the velocity
profile of naked sunspots resembles that of supergranular flows. The evolution
of the flow field around individual sunspots is influenced by the way the
sunspot decays and by the interplay with the surrounding flow areas.
Observations of the flow around eight decaying sunspots suggest that as long as
penumbrae are present, sunspots with their moat cell are embedded in network
cells. The disappearance of the penumbra (and consequently the moat flow) and
the competing surrounding supergranular cells, both have a significant role in
the evolution of the flow field: The moat cell transforms into a supergranule,
which hosts the remaining naked spot.Comment: accepted for publication in A&A, 11 pages, 6 figures, 3 tables;
appendix with 9 figures and 8 online movie
Magnetic properties of a long-lived sunspot - Vertical magnetic field at the umbral boundary
Context. In a recent statistical study of sunspots in 79 active regions, the
vertical magnetic field component averaged along the umbral
boundary is found to be independent of sunspot size. The authors of that study
conclude that the absolute value of at the umbral boundary is
the same for all spots.
Aims. We investigate the temporal evolution of averaged along
the umbral boundary of one long-lived sunspot during its stable phase.
Methods. We analysed data from the HMI instrument on-board SDO. Contours of
continuum intensity at , whereby
refers to the average over the quiet sun areas, are used to extract the
magnetic field along the umbral boundary. Projection effects due to different
formation heights of the Fe I 617.3 nm line and continuum are taken into
account. To avoid limb artefacts, the spot is only analysed for heliocentric
angles smaller than .
Results. During the first disc passage, NOAA AR 11591, remains
constant at 1693 G with a root-mean-square deviation of 15 G, whereas the
magnetic field strength varies substantially (mean 2171 G, rms of 48 G) and
shows a long term variation. Compensating for formation height has little
influence on the mean value along each contour, but reduces the variations
along the contour when away from disc centre, yielding a better match between
the contours of G and .
Conclusions. During the disc passage of a stable sunspot, its umbral boundary
can equivalently be defined by using the continuum intensity or
the vertical magnetic field component . Contours of fixed
magnetic field strength fail to outline the umbral boundary.Comment: accepted for publication in A&A; v2 minor edit, correcting statement
regarding one citatio
Chromospheric impact of an exploding solar granule
Observations of multi-wavelength and therefore height-dependent information
following events throughout the solar atmosphere and unambiguously assigning a
relation between these rapidly evolving layers are rare and difficult to
obtain. Yet, they are crucial for our understanding of the physical processes
that couple the different regimes in the solar atmosphere. We characterize the
exploding granule event with simultaneous observations of Hinode
spectroplarimetric data in the solar photosphere and Hinode broadband CaIIH
images combined with Interface Region Imaging Spectrograph (IRIS) slit spectra.
We follow the evolution of an exploding granule and its connectivity throughout
the atmosphere and analyze the dynamics of a magnetic element that has been
affected by the abnormal granule. In addition to magnetic flux maps we use a
local correlation tracking method to infer the horizontal velocity flows in the
photosphere and apply a wavelet analysis on several IRIS chromospheric emission
features such as MgIIk2v and MgIIk3 to detect oscillatory phenomena indicating
wave propagation. During the vigorous expansion of the abnormal granule we
detect radially outward horizontal flows, causing, together with the horizontal
flows from the surrounding granules, the magnetic elements in the bordering
intergranular lanes to be squeezed and elongated. In reaction to the squeezing,
we detect a chromospheric intensity and velocity oscillation pulse which we
identify as an upward traveling hot shock front propagating clearly through the
IRIS spectral line diagnostics of MgIIh&k. Conclusion: Exploding granules can
trigger upward-propagating shock fronts that dissipate in the chromosphere.Comment: 5 pages (3 figures)+1 page movie snapshots(2 figures), accepted in
A&A letters, movies can be found at http://www.science-media.org/216 and
http://www.science-media.org/21
Spectropolarimetry of sunspot penumbrae
Die vorliegende Arbeit beschäftigt sich mit der
Beobachtung von Feinstrukturen der Penumbren von
Sonnenflecken mittels hoher räumlicher Auflösung und
Vorwärtsmodellierung. Unter Verwendung von
Bildrekonstruktionstechniken in Kombination mit
zweidimensionalen spektropolarimetrischen Bildern
können Magnetogramme und Geschwindigkeitsverläufe mit
einer Auflösung im Bereich von 0".4 erstellt werden.
Die daraus erhaltenen Ergebnisse stimmen überein mit
der Annahme tief liegender Strömungkanäle entlang des
horizontalen magnetsichen Felds oder möglicherweise
Strömungkanäle, die aus den sub-photosphärischen
Schichten austreten und wieder eintauchen, vergleichbar
einer "Meereschlange".Darüberhinaus werden weitere
dynamische Phänomene, welche den Reichturm der in
Penumbren ablaufenden Prozesse zeigen, diskutiert und
einige neue nicht erwartete Eigenschaften
aufgezeigt
The magnetic nature of umbra-penumbra boundary in sunspots
Sunspots are the longest-known manifestation of solar activity, and their
magnetic nature has been known for more than a century. Despite this, the
boundary between umbrae and penumbrae, the two fundamental sunspot regions, has
hitherto been solely defined by an intensity threshold. Here, we aim at
studying the magnetic nature of umbra-penumbra boundaries in sunspots of
different sizes, morphologies, evolutionary stages, and phases of the solar
cycle. We used a sample of 88 scans of the Hinode/SOT spectropolarimeter to
infer the magnetic field properties in at the umbral boundaries. We defined
these umbra-penumbra boundaries by an intensity threshold and performed a
statistical analysis of the magnetic field properties on these boundaries. We
statistically prove that the umbra-penumbra boundary in stable sunspots is
characterised by an invariant value of the vertical magnetic field component:
the vertical component of the magnetic field strength does not depend on the
umbra size, its morphology, and phase of the solar cycle. With the statistical
Bayesian inference, we find that the strength of the vertical magnetic field
component is, with a likelihood of 99\%, in the range of 1849-1885 G with the
most probable value of 1867 G. In contrast, the magnetic field strength and
inclination averaged along individual boundaries are found to be dependent on
the umbral size: the larger the umbra, the stronger and more horizontal the
magnetic field at its boundary. The umbra and penumbra of sunspots are
separated by a boundary that has hitherto been defined by an intensity
threshold. We now unveil the empirical law of the magnetic nature of the
umbra-penumbra boundary in stable sunspots: it is an invariant vertical
component of the magnetic field.Comment: accepted as A&A lette
Critical Science Plan for the Daniel K. Inouye Solar Telescope (DKIST)
Open Access funding provided by the National Solar Observatory (NSO). The NSO is operated by the Association of Universities for Research in Astronomy, Inc., and is funded by the National Science Foundation.The National Science Foundation’s Daniel K. Inouye Solar Telescope (DKIST) will revolutionize our ability to measure, understand, and model the basic physical processes that control the structure and dynamics of the Sun and its atmosphere. The first-light DKIST images, released publicly on 29 January 2020, only hint at the extraordinary capabilities that will accompany full commissioning of the five facility instruments. With this Critical Science Plan (CSP) we attempt to anticipate some of what those capabilities will enable, providing a snapshot of some of the scientific pursuits that the DKIST hopes to engage as start-of-operations nears. The work builds on the combined contributions of the DKIST Science Working Group (SWG) and CSP Community members, who generously shared their experiences, plans, knowledge, and dreams. Discussion is primarily focused on those issues to which DKIST will uniquely contribute.Publisher PDFPeer reviewe