149 research outputs found
Temporal relation between quiet-Sun transverse fields and the strong flows detected by IMaX/SUNRISE
Localized strongly Doppler-shifted Stokes V signals were detected by
IMaX/SUNRISE. These signals are related to newly emerged magnetic loops that
are observed as linear polarization features. We aim to set constraints on the
physical nature and causes of these highly Doppler-shifted signals. In
particular, the temporal relation between the appearance of transverse fields
and the strong Doppler shifts is analyzed in some detail. We calculated the
time difference between the appearance of the strong flows and the linear
polarization. We also obtained the distances from the center of various
features to the nearest neutral lines and whether they overlap or not. These
distances were compared with those obtained from randomly distributed points on
observed magnetograms. Various cases of strong flows are described in some
detail. The linear polarization signals precede the appearance of the strong
flows by on average 84+-11 seconds. The strongly Doppler-shifted signals are
closer (0.19") to magnetic neutral lines than randomly distributed points
(0.5"). Eighty percent of the strongly Doppler-shifted signals are close to a
neutral line that is located between the emerging field and pre-existing
fields. That the remaining 20% do not show a close-by pre-existing field could
be explained by a lack of sensitivity or an unfavorable geometry of the
pre-existing field, for instance, a canopy-like structure. Transverse fields
occurred before the observation of the strong Doppler shifts. The process is
most naturally explained as the emergence of a granular-scale loop that first
gives rise to the linear polarization signals, interacts with pre-existing
fields (generating new neutral line configurations), and produces the observed
strong flows. This explanation is indicative of frequent small-scale
reconnection events in the quiet Sun.Comment: 11 pages, 8 figure
Influence of phase-diversity image reconstruction techniques on circular polarization asymmetries
Full Stokes filter-polarimeters are key instruments for investigating the
rapid evolution of magnetic structures on the solar surface. To this end, the
image quality is routinely improved using a-posteriori image reconstruction
methods. We analyze the robustness of circular polarization asymmetries to
phase-diversity image reconstruction techniques. We use snapshots of
magneto-hydrodynamical simulations carried out with different initial
conditions to synthesize spectra of the magnetically sensitive Fe I line at
5250.2 A. We degrade the synthetic profiles spatially and spectrally to
simulate observations with the IMaX full Stokes filter-polarimeter. We also
simulate the focused/defocused pairs of images used by the phase-diversity
algorithm for reconstruction and the polarimetric modulation scheme. We assume
that standard optimization methods are able to infer the projection of the
wavefront on the Zernike polynomials with 10% precision. We also consider the
less favorable case of 25% precision. We obtain reconstructed monochromatic
modulated images that are later demodulated and compared with the original
maps. Although asymmetries are often difficult to define in the quiet Sun due
to the complexity of the Stokes V profiles, we show how asymmetries are
degraded with spatial and spectral smearing. The results indicate that,
although image reconstruction techniques reduce the spatial smearing, they can
modify the asymmetries of the profiles, mainly caused by the appearance of
spatially-correlated noise.Comment: 10 pages, accepted for publication in A&
Properties of sunspots in cycle 23: I. Dependence of brightness on sunspot size and cycle phase
In this paper we investigate the dependence of umbral core brightness, as
well as the mean umbral and penumbral brightness on the phase of the solar
cycle and on the size of the sunspot. Albregtsen & Maltby (1978) reported an
increase in umbral core brightness from the early to the late phase of solar
cycle from the analysis of 13 sunspots which cover solar cycles 20 and 21. Here
we revisit this topic by analysing continuum images of more than 160 sunspots
observed by the MDI instrument on board the SOHO spacecraft for the period
between 1998 March to 2004 March, i.e. a sizable part of solar cycle 23. The
advantage of this data set is its homogeneity, with no seeing fluctuations. A
careful stray light correction, which is validated using the Mercury transit of
7th May, 2003, is carried out before the umbral and penumbral intensities are
determined. The influence of the Zeeman splitting of the nearby NiI spectral
line on the measured 'continuum' intensity is also taken into account. We did
not observe any significant variation in umbral core, mean umbral and mean
penumbral intensities with solar cycle, which is in contrast to earlier
findings for the umbral core intensity. We do find a strong and clear
dependence of the umbral brightness on sunspot size, however. The penumbral
brightness also displays a weak dependence. The brightness-radius relationship
has numerous implications, some of which, such as those for the energy
transport in umbrae, are pointed out.Comment: 16 pages, 21 postscript figures, accepted for publication in A&
Comparison of solar photospheric bright points between SUNRISE observations and MHD simulations
Bright points (BPs) in the solar photosphere are radiative signatures of
magnetic elements described by slender flux tubes located in the darker
intergranular lanes. They contribute to the ultraviolet (UV) flux variations
over the solar cycle and hence may influence the Earth's climate. Here we
combine high-resolution UV and spectro-polarimetric observations of BPs by the
SUNRISE observatory with 3D radiation MHD simulations. Full spectral line
syntheses are performed with the MHD data and a careful degradation is applied
to take into account all relevant instrumental effects of the observations. It
is demonstrated that the MHD simulations reproduce the measured distributions
of intensity at multiple wavelengths, line-of-sight velocity, spectral line
width, and polarization degree rather well. Furthermore, the properties of
observed BPs are compared with synthetic ones. These match also relatively
well, except that the observations display a tail of large and strongly
polarized BPs not found in the simulations. The higher spatial resolution of
the simulations has a significant effect, leading to smaller and more numerous
BPs. The observation that most BPs are weakly polarized is explained mainly by
the spatial degradation, the stray light contamination, and the temperature
sensitivity of the Fe I line at 5250.2 \AA{}. The Stokes asymmetries of the
BPs increase with the distance to their center in both observations and
simulations, consistent with the classical picture of a production of the
asymmetry in the canopy. This is the first time that this has been found also
in the internetwork. Almost vertical kilo-Gauss fields are found for 98 % of
the synthetic BPs. At the continuum formation height, the simulated BPs are on
average 190 K hotter than the mean quiet Sun, their mean BP field strength is
1750 G, supporting the flux-tube paradigm to describe BPs.Comment: Accepted for publication in Astronomy & Astrophysics on May 30 201
Supersonic Evershed flow outside Sunspots
We report on the discovery of mostly horizontal field channels just outside
sunspot penumbrae (in the so-called `moat' region) that are seen to sustain
supersonic flows (line-of-sight component of 6 km s{-1}). The spectral
signature of these supersonic flows corresponds to circular polarization
profiles with an additional, satellite, third lobe of the same sign as the
parent sunspot' Stokes V blue lobe, for both downflows and upflows. This is
consistent with an outward directed flow that we interpret as the continuation
of the magnetized Evershed flow outside sunspots at supersonic speeds. In
Stokes Q and U, a clear signature of a transverse field connecting the two flow
streams is observed. Such an easily detectable spectral signature should allow
for a clear identification of these horizontal field channels in other
spectropolarimetric sunspot data. For the spot analyzed in this paper, a total
of 5 channels with this spectral signature have been unambiguously found
The formation and disintegration of magnetic bright points observed by Sunrise/IMaX
The evolution of the physical parameters of magnetic bright points (MBPs)
located in the quiet Sun (mainly in the interwork) during their lifetime is
studied. First we concentrate on the detailed description of the magnetic field
evolution of three MBPs. This reveals that individual features follow
different, generally complex, and rather dynamic scenarios of evolution. Next
we apply statistical methods on roughly 200 observed MBP evolutionary tracks.
MBPs are found to be formed by the strengthening of an equipartition field
patch, which initially exhibits a moderate downflow. During the evolution,
strong downdrafts with an average velocity of 2.4 km/s set in. These flows,
taken together with the concurrent strengthening of the field, suggest that we
are witnessing the occurrence of convective collapses in these features,
although only 30% of them reach kG field strengths. This fraction might turn
out to be larger when the new 4 m class solar telescopes are operational as
observations of MBPs with current state of the art instrumentation could still
be suffering from resolution limitations. Finally, when the bright point
disappears (although the magnetic field often continues to exist) the magnetic
field strength has dropped to the equipartition level and is generally somewhat
weaker than at the beginning of the MBP's evolution. Noteworthy is that in
about 10% of the cases we observe in the vicinity of the downflows small-scale
strong (exceeding 2 km/s) intergranular upflows related spatially and
temporally to these downflows.Comment: 19 pages, 13 figures; final version published in "The Astrophysical
Journal
Centre-to-limb properties of small, photospheric quiet Sun jets
Strongly Doppler-shifted Stokes profiles have been detected in the quiet
Sun with the IMaX instrument on-board the SUNRISE stratospheric balloon-borne
telescope. High velocities are required in order to produce such signals, hence
these events have been interpreted as jets, although other sources are also
possible. We aim to characterize the variation of the main properties of these
events (occurrence rate, lifetime, size and velocities) with their position on
the solar disk between disk centre and the solar limb. These events have been
identified in Sunrise/IMaX data according to the same objective criteria at all
available positions on the solar disk. Their properties were determined using
standard techniques. Our study yielded a number of new insights into this
phenomenon. Most importantly, the number density of these events is independent
of the heliocentric angle, i.e. the investigated supersonic flows are nearly
isotropically distributed. Size and lifetime are also nearly independent of the
heliocentric angle, while their intensity contrast increases towards the solar
limb. The Stokes jets are associated with upflow velocities deduced from
Stokes , which are stronger towards the limb. Their intensity decreases with
time, while their line-of-sight (LOS) velocity does not display a clear
temporal evolution. Their association with linear polarization signals
decreases towards the limb. The density of events appears to be independent of
heliocentric angle, establishing that they are directed nearly randomly. If
these events are jets triggered by magnetic reconnection between emerging
magnetic flux and the ambient field, then our results suggest that there is no
preferred geometry for the reconnection process.Comment: 9 pages, 9 figure
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