946 research outputs found
Vigorous convection in a sunspot granular light bridge
Light bridges are the most prominent manifestation of convection in sunspots.
The brightest representatives are granular light bridges composed of features
that appear to be similar to granules. An in-depth study of the convective
motions, temperature stratification, and magnetic field vector in and around
light bridge granules is presented with the aim of identifying similarities and
differences to typical quiet-Sun granules. Spectropolarimetric data from the
Hinode Solar Optical Telescope were analyzed using a spatially coupled
inversion technique to retrieve the stratified atmospheric parameters of light
bridge and quiet-Sun granules. Central hot upflows surrounded by cooler fast
downflows reaching 10 km/s clearly establish the convective nature of the light
bridge granules. The inner part of these granules in the near surface layers is
field free and is covered by a cusp-like magnetic field configuration. We
observe hints of field reversals at the location of the fast downflows. The
quiet-Sun granules in the vicinity of the sunspot are covered by a low-lying
canopy field extending radially outward from the spot. The similarities between
quiet-Sun and light bridge granules point to the deep anchoring of granular
light bridges in the underlying convection zone. The fast, supersonic downflows
are most likely a result of a combination of invigorated convection in the
light bridge granule due to radiative cooling into the neighboring umbra and
the fact that we sample deeper layers, since the downflows are immediately
adjacent to the slanted walls of the Wilson depression.Comment: 10 pages, 11 figure
Measuring the Wilson depression of sunspots using the divergence-free condition of the magnetic field vector
Context: The Wilson depression is the difference in geometric height of unit
continuum optical depth between the sunspot umbra and the quiet Sun. Measuring
the Wilson depression is important for understanding the geometry of sunspots.
Current methods suffer from systematic effects or need to make assumptions on
the geometry of the magnetic field. This leads to large systematic
uncertainties of the derived Wilson depressions.
Aims: We aim at developing a robust method for deriving the Wilson depression
that only requires the information about the magnetic field that is accessible
from spectropolarimetry, and that does not rely on assumptions on the geometry
of sunspots or on their magnetic field.
Methods: Our method is based on minimizing the divergence of the magnetic
field vector derived from spectropolarimetric observations. We focus on large
spatial scales only in order to reduce the number of free parameters.
Results: We test the performance of our method using synthetic Hinode data
derived from two sunspot simulations. We find that the maximum and the umbral
averaged Wilson depression for both spots determined with our method typically
lies within 100 km of the true value obtained from the simulations. In
addition, we apply the method to Hinode observations of a sunspot. The derived
Wilson depression (about 600 km) is consistent with results typically obtained
from the Wilson effect. We also find that the Wilson depression obtained from
using horizontal force balance gives 110 - 180 km smaller Wilson depressions
than both, what we find and what we deduce directly from the simulations. This
suggests that the magnetic pressure and the magnetic curvature force contribute
to the Wilson depression by a similar amount.Comment: 12 pages, 8 figures. Accepted for publication in Astronomy &
Astrophysic
Vertical flows and mass flux balance of sunspot umbral dots
A new Stokes inversion technique that greatly reduces the effect of the
spatial point spread function of the telescope is used to constrain the
physical properties of umbral dots (UDs). The depth-dependent inversion of the
Stokes parameters from a sunspot umbra recorded with Hinode SOT/SP revealed
significant temperature enhancements and magnetic field weakenings in the core
of the UDs in deep photospheric layers. Additionally, we found upflows of
around 960 m/s in peripheral UDs (i.e., UDs close to the penumbra) and
600 m/s in central UDs. For the first time, we also detected
systematic downflows for distances larger than 200 km from the UD center that
balance the upflowing mass flux. In the upper photosphere, we found almost no
difference between the UDs and their diffuse umbral background.Comment: Accepted for publication in Astronomy and Astrophysic
Structure of sunspot penumbral filaments: a remarkable uniformity of properties
The sunspot penumbra comprises numerous thin, radially elongated filaments
that are central for heat transport within the penumbra, but whose structure is
still not clear. To investigate the fine-scale structure of these filaments, we
perform a depth-dependent inversion of spectropolarimetric data of a sunspot
very close to solar disk center obtained by Hinode (SOT/SP). We have used a
recently developed spatially coupled 2D inversion scheme which allows us to
analyze the fine structure of individual penumbral filaments up to the
diffraction limit of the telescope. Filaments of different sizes in all parts
of penumbra display very similar magnetic field strengths, inclinations and
velocity patterns. The similarities allowed us to average all these filaments
and to extract the physical properties common to all of them. This average
filament shows upflows associated with an upward pointing field at its inner,
umbral end and along its axis, downflows along the lateral edge and strong
downflows in the outer end associated with a nearly vertical, strong and
downward pointing field. The upflowing plasma is significantly hotter than the
downflowing plasma. The hot, tear-shaped head of the averaged filament can be
associated with a penumbral grain. The central part of the filament shows
nearly horizontal fields with strengths of ~1kG. The field above the filament
converges, whereas a diverging trend is seen in the deepest layers near the
head of the filament. We put forward a unified observational picture of a
sunspot penumbral filament. It is consistent with such a filament being a
magneto-convective cell, in line with recent MHD simulations. The uniformity of
its properties over the penumbra sets constraints on penumbral models and
simulations. The complex and inhomogeneous structure of the filament provides a
natural explanation for a number of long-running controversies in the
literature.Comment: 19 pages; 12 figures; accepted for publication in A&
Inclinations of small quiet-Sun magnetic features based on a new geometric approach
High levels of horizontal magnetic flux have been reported in the quiet-Sun
internetwork, often based on Stokes profile inversions. Here we introduce a new
method for deducing the inclination of magnetic elements and use it to test
magnetic field inclinations from inversions. We determine accurate positions of
a set of small, bright magnetic elements in high spatial resolution images
sampling different photospheric heights obtained by the Sunrise balloon-borne
solar observatory. Together with estimates of the formation heights of the
employed spectral bands, these provide us with the inclinations of the magnetic
features. We also compute the magnetic inclination angle of the same magnetic
features from the inversion of simultaneously recorded Stokes parameters. Our
new, geometric method returns nearly vertical fields (average inclination of
around 14 deg with a relatively narrow distribution having a standard deviation
of 6 deg). In strong contrast to this, the traditionally used inversions give
almost horizontal fields (average inclination of 75+-8 deg) for the same small
magnetic features, whose linearly polarised Stokes profiles are adversely
affected by noise. The almost vertical field of bright magnetic features from
our geometric method is clearly incompatible with the nearly horizontal
magnetic fields obtained from the inversions. This indicates that the amount of
magnetic flux in horizontal fields deduced from inversions is overestimated in
the presence of weak Stokes signals, in particular if Stokes Q and U are close
to or under the noise level. By combining the proposed method with inversions
we are not just improving the inclination, but also the field strength. This
technique allows us to analyse features that are not reliably treated by
inversions, thus greatly extending our capability to study the complete
magnetic field of the quiet Sun.Comment: 12 pages, 9 figures, 1 table; Accepted for publication in Astronomy &
Astrophysic
No universal connection between the vertical magnetic field and the umbra-penumbra boundary in sunspots
Context. It has been reported that the boundary between the umbra and the
penumbra of sunspots occurs at a canonical value of the strength of the
vertical magnetic field, independently of the size of the spot. This critical
field strength is interpreted as to be the threshold for the onset of
magnetoconvection.
Aims. Here we investigate the reasons why this criterion, also called the
Jur\v{c}\'ak criterion in the literature, does not always identify the boundary
between umbra and penumbra.
Methods. We perform a statistical analysis of 23 sunspots observed with
Hinode/SOT. We compare the properties of the continuum intensity and the
vertical magnetic field between filaments and spines and how they vary between
spots of different sizes.
Results. We find that the inner boundary of the penumbra is not related to a
universal value of the vertical magnetic field. The properties of spines and
filaments vary between spots of different sizes. Both components are darker in
larger spots and the spines exhibit stronger vertical magnetic field. These
variations of the properties of filaments and spines with spot size are also
the reason for the reported invariance of the averaged vertical magnetic field
at 50% of the mean continuum intensity.
Conclusions. The formation of filaments and the onset of magnetoconvection
are not related to a canonical value of the strength of the vertical magnetic
field. Such a seemingly unique magnetic field strength is rather an effect of
the filling factor of spines and penumbral filaments.Comment: 15 pages, 11 figures. Accepted for publication in Astronomy and
Astrophysic
Like or dislike? Adolescents’ responses to personalized social network site advertising
Increasingly, personal data posted by users of social network sites (SNSs) can be used to personalize advertising. The present study investigates how adolescents respond to personalized ads in terms of attitude toward the ad, brand engagement and intention to forward, and whether privacy concerns moderate their responses. According to pretest results, a medium level of personalization was expected to be optimal in terms of advertising effectiveness. A within-subjects experiment involving 40 participants aged 14–18 years was performed. Three conditions of personalized advertising were designed with, respectively, low, medium, and high levels of personalization. The study found that the highest personalization condition generated the most positive response and that privacy concerns did not moderate the effects of personalization. The privacy paradox is discussed as an alternative explanation, along with other implications of the results
Influence of NLTE effects in Fe I lines on inverted atmosphere II. 6301 A and 6302 A lines formed in 3DNLTE
This paper forms the second part of our study on how the neglect of NLTE
conditions in the formation of Fe I 6301.5 A and the 6302.5 A lines influences
the atmosphere obtained by inverting their profiles in LTE. The main cause of
NLTE effects is the line opacity deficit due to the excess ionization of the Fe
I atoms by the UV photons in the Sun. In the first paper, the above
photospheric lines were assumed to have formed in 1DNLTE and the effects of
horizontal radiation transfer (RT) were neglected. In the present paper, the
iron lines are computed in 3DNLTE. We investigate the influence of horizontal
RT on the inverted atmosphere and how it can enhance or reduce the errors due
to the neglect of 1DNLTE effects. The iron lines are computed in LTE, 1DNLTE
and 3DNLTE. They all are inverted using an LTE inversion code. The atmosphere
from the inversion of LTE profiles is taken as the reference model. The test
atmospheres from the inversion of 1DNLTE and 3DNLTE profiles are compared with
it. The differences between models are analysed and correspondingly attributed
to NLTE and 3D effects. The effects of horizontal RT are evident in regions
surrounded by strong horizontal gradients in temperature. In some regions, the
3D effects enhance the 1DNLTE effects while in some, they weaken. The errors
due to neglecting the 3D effects are less than 5% in temperature while the
errors are mostly less than 20% in both velocity and magnetic field strength.
These errors are found to survive spatial and spectral degradation. The neglect
of horizontal RT is found to introduce errors in the derived atmosphere. How
large the errors are depends on how strong the local horizontal gradients are
in temperature. Compared to the 1DNLTE effect, the 3D effects are more
localised to specific regions in the atmosphere and overall less dominant.Comment: Accepted for publication in A&A. Abstract abridged for arxiv
submissio
Detection of the strongest magnetic field in a sunspot light bridge
Traditionally, the strongest magnetic fields on the Sun have been measured in
sunspot umbrae. More recently, however, much stronger fields have been measured
at the ends of penumbral filaments carrying the Evershed and counter-Evershed
flows. Superstrong fields have also been reported within a light bridge
separating two umbrae of opposite polarities. We aim to accurately determine
the strengths of the strongest fields in a light bridge using an advanced
inversion technique and to investigate their detailed structure. We analyze
observations from the spectropolarimeter on board the Hinode spacecraft of the
active region AR 11967. The thermodynamic and magnetic configurations are
obtained by inverting the Stokes profiles using an inversion scheme that allows
multiple height nodes. Both the traditional 1D inversion technique and the
so-called 2D coupled inversions, which take into account the point spread
function of the Hinode telescope, are used. We find a compact structure with an
area of 32.7 arcsec within a bipolar light bridge with field strengths
exceeding 5 kG, confirming the strong fields in this light bridge reported in
the literature. Two regions associated with downflows of 5 km s
harbor field strengths larger than 6.5 kG, covering a total area of 2.97
arcsec. The maximum field strength found is 8.2 kG, which is the largest
ever observed field in a bipolar light bridge up to now.Comment: ApJ 2020, 895:12
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