229 research outputs found
The dark side of solar photospheric G-band bright points
Bright small-scale magnetic elements found mainly in intergranular lanes at
the solar surface are named bright points (BPs). They show high contrasts in
Fraunhofer G-band observations and are described by nearly vertical slender
flux tubes or sheets. A recent comparison between BP observations in the
ultraviolet (UV) and visible spectral range recorded with the balloon-borne
observatory SUNRISE and state-of-the-art magnetohydrodynamical (MHD)
simulations revealed a kiloGauss magnetic field for 98% of the synthetic BPs.
Here we address the opposite question, namely which fraction of pixels hosting
kiloGauss fields coincides with an enhanced G-band brightness. We carried out
3D radiation MHD simulations for three magnetic activity levels (corresponding
to the quiet Sun, weak and strong plage) and performed a full spectral line
synthesis in the G-band. Only 7% of the kiloGauss pixels in our quiet-Sun
simulation coincide with a brightness lower than the mean quiet-Sun intensity,
while 23% of the pixels in the weak-plage simulation and even 49% in the
strong-plage simulation are associated with a local darkening. Dark
strong-field regions are preferentially found in the cores of larger flux
patches that are rare in the quiet Sun, but more common in plage regions, often
in the vertices of granulation cells. The significant brightness shortfall in
the core of larger flux patches coincide with a slight magnetic field
weakening. KiloGauss elements in the quiet Sun are on average brighter than
similar features in plage regions. Almost all strong-field pixels display a
more or less vertical magnetic field orientation. Hence in the quiet Sun,
G-band BPs correspond almost one-to-one with kiloGauss elements. In weak plage
the correspondence is still very good, but not perfect.Comment: Accepted for publication in Astronomy & Astrophysic
Brightness of Solar Magnetic Elements as a Function of Magnetic Flux at High Spatial Resolution
We investigate the relationship between the photospheric magnetic field of
small-scale magnetic elements in the quiet Sun (QS) at disc centre, and the
brightness at 214 nm, 300 nm, 313 nm, 388 nm, 397 nm, and at 525.02 nm. To this
end we analysed spectropolarimetric and imaging time series acquired
simultaneously by the IMaX magnetograph and the SuFI filter imager on-board the
balloon-borne observatory Sunrise during its first science flight in 2009, with
high spatial and temporal resolution.
We find a clear dependence of the contrast in the near ultraviolet (NUV) and
the visible on the line-of-sight component of the magnetic field, , which is best described by a logarithmic model. This function represents
well the relationship between the Ca II H-line emission and , and
works better than a power-law fit adopted by previous studies. This, along with
the high contrast reached at these wavelengths, will help with determining the
contribution of small-scale elements in the QS to the irradiance changes for
wavelengths below 388 nm. At all wavelengths including the continuum at 525.40
nm the intensity contrast does not decrease with increasing . This
result also strongly supports that Sunrise has resolved small strong magnetic
field elements in the internetwork, resulting in constant contrasts for large
magnetic fields in our continuum contrast at 525.40 nm vs.
scatterplot, unlike the turnover obtained in previous observational studies.
This turnover is due to the intermixing of the bright magnetic features with
the dark intergranular lanes surrounding them
Investigations of small-scale magnetic features on the solar surface
Solar activity is controlled by the magnetic field, which also causes the
variability of the solar irradiance that in turn is thought to influence the
climate on Earth. The magnetic field manifests itself in the form of structures
of largely different sizes. This thesis concentrates on two types of the
smallest known magnetic features: The first part studies the properties of
umbral dots, dot-like bright features in the dark umbra of a sunspot. The
obtained umbral dot properties provide a remarkable confirmation of the results
of recent magneto-hydrodynamical simulations. Observations as well as
simulations show that umbral dots differ from their surroundings mainly in the
lowest photospheric layers, where the temperature is enhanced and the magnetic
field is weakened. In addition, the interior of the umbral dots displays strong
upflow velocities which are surrounded by weak downflows. This qualitative
agreement further strengthens the interpretation of umbral dots as localized
columns of overturning convection. The second part of the thesis investigates
bright points, which are small-scale brightness enhancements in the darker
intergranular lanes of the quiet Sun produced by magnetic flux concentrations.
Observational data obtained by the balloon-borne solar telescope SUNRISE are
used in this thesis. For the first time contrasts of bright points in the
important ultraviolet spectral range are determined. A comparison of
observational data with magneto-hydrodynamical simulations revealed a close
correspondence, but only after effects due to the limited spectral and spatial
resolution were carefully included. 98% of the synthetic bright points are
found to be associated with a nearly vertical kilo-Gauss field.Comment: PhD thesis, Braunschweig University, 209 pages; ISBN
978-3-942171-73-1, uni-edition GmbH 201
The small-scale structure of photospheric convection retrieved by a deconvolution technique applied to Hinode/SP data
Solar granules are bright patterns surrounded by dark channels called
intergranular lanes in the solar photosphere and are a manifestation of
overshooting convection. Observational studies generally find stronger upflows
in granules and weaker downflows in intergranular lanes. This trend is,
however, inconsistent with the results of numerical simulations in which
downflows are stronger than upflows through the joint action of gravitational
acceleration/deceleration and pressure gradients. One cause of this discrepancy
is the image degradation caused by optical distortion and light diffraction and
scattering that takes place in an imaging instrument. We apply a deconvolution
technique to Hinode/SP data in an attempt to recover the original solar scene.
Our results show a significant enhancement in both, the convective upflows and
downflows, but particularly for the latter. After deconvolution, the up- and
downflows reach maximum amplitudes of -3.0 km/s and +3.0 km/s at an average
geometrical height of roughly 50 km, respectively. We found that the velocity
distributions after deconvolution match those derived from numerical
simulations. After deconvolution the net LOS velocity averaged over the whole
FOV lies close to zero as expected in a rough sense from mass balance.Comment: 32 pages, 13 figures, accepted for publication in Ap
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
Linear Polarization Features in the Quiet-Sun Photosphere: Structure and Dynamics
We present detailed characteristics of linear polarization features (LPFs) in
the quiet-Sun photosphere from high resolution observations obtained with
Sunrise/IMaX. We explore differently treated data with various noise levels in
linear polarization signals, from which structure and dynamics of the LPFs are
studied. Physical properties of the detected LPFs are also obtained from the
results of Stokes inversions. The number of LPFs, as well as their sizes and
polarization signals, are found to be strongly dependent on the noise level,
and on the spatial resolution. While the linear polarization with
signal-to-noise ratio covers about 26% of the entire area in the
least noisy data in our study (with a noise level of in the
unit of Stokes continuum), the detected (spatially resolved) LPFs cover
about 10% of the area at any given time, with an occurrence rate on the order
of s arcsec. The LPFs were found to be short
lived (in the range of s), relatively small structures (radii of
arcsec), highly inclined, posing hG fields, and move with an
average horizontal speed of 1.2 km s. The LPFs were observed (almost)
equally on both upflow and downflow regions, with intensity contrast always
larger than that of the the average quiet-Sun.Comment: 20 pages, 9 figures, Accepted for publication in the Solar Physics
journa
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
Evidence of convective rolls in a sunspot penumbra
aims: We study the recently discovered twisting motion of bright penumbral
filaments with the aim of constraining their geometry and the associated
magnetic field. methods: A large sunspot located 40\degr from disk center was
observed at high resolution with the 1-m Swedish Solar Telescope. Inversions of
multi-wavelength polarimetric data and speckle reconstructed time series of
continuum images were used to determine proper motions, as well as the velocity
and magnetic structure in penumbral filaments. results: The continuum movie
reveals apparent lateral motions of bright and dark structures inside bright
filaments oriented parallel to the limb, confirming recent Hinode results. In
these filaments we measure upflows of on their
limbward side and weak downflows on their centerward side. The magnetic field
in them is significantly weaker and more horizontal than in the adjacent dark
filaments. conclusions: The data indicate the presence of vigorous convective
rolls in filaments with a nearly horizontal magnetic field. These are separated
by filaments harbouring stronger, more vertical fields. Because of reduced gas
pressure, we see deeper into the latter. When observed near the limb, the
disk-centerward side of the horizontal-field filaments appear bright due to the
\textit{hot wall} effect known from faculae. We estimate that the convective
rolls transport most of the energy needed to explain the penumbral radiative
flux.Comment: 4 pages, 4 figures, letter to Astronomy & Astrophysic
First high-resolution images of the Sun in the 2796 \AA{} Mg II k line
We present the first high-resolution solar images in the Mg II k 2796 \AA{}
line. The images, taken through a 4.8 \AA{} broad interference filter, were
obtained during the second science flight of SUNRISE in June 2013 by the SuFI
instrument. The Mg II k images display structures that look qualitatively very
similar to images taken in the core of Ca II H. The Mg II images exhibit
reversed granulation (or shock waves) in the internetwork regions of the quiet
Sun, at intensity contrasts that are similar to those found in Ca II H. Very
prominent in Mg II are bright points, both in the quiet Sun and in plage
regions, particularly near disk center. These are much brighter than at other
wavelengths sampled at similar resolution. Furthermore, Mg II k images also
show fibril structures associated with plage regions. Again, the fibrils are
similar to those seen in Ca II H images, but tend to be more pronounced,
particularly in weak plage.Comment: Accepted for publication in The Astrophysical Journal Letter
Brightness, distribution, and evolution of sunspot umbral dots
We present a 106-minute TiO (705.7nm) time series of high spatial and
temporal resolution that contains thousands of umbral dots (UDs) in a mature
sunspot in the active region NOAA 10667 at =0.95. The data were acquired
with the 1-m Swedish Solar Telescope on La Palma. With the help of a multilevel
tracking (MLT) algorithm the sizes, brightnesses, and trajectories of 12836
umbral dots were found and analyzed. The MLT allows UDs with very low contrast
to be reliably identified. Inside the umbra we determine a UD filling factor of
11%. The histogram of UD lifetimes is monotonic, i.e. a UD does not have a
typical lifetime. Three quarters of the UDs lived for less than 150s and showed
no or little motion. The histogram of the UD diameters exhibits a maximum at
225km, i.e. most of the UDs are spatially resolved. UDs display a typical
horizontal velocity of 420m/s and a typical peak intensity of 51% of the mean
intensity of the quiet photosphere, making them on average 20% brighter than
the local umbral background. Almost all mobile UDs (large birth-death distance)
were born close to the umbra-penumbra boundary, move towards the umbral center,
and are brighter than average. Notably bright and mobile UDs were also observed
along a prominent UD chain, both ends of which are located at the
umbra-penumbra boundary. Their motion started primarily at either of the ends
of the chain, continued along the chain, and ended near the chain's center. We
observed the splitting and merging of UDs and the temporal succession of both.
For the first time the evolution of brightness, size, and horizontal speed of a
typical UD could be determined in a statistically significant way. Considerable
differences between the evolution of central and peripheral UDs are found,
which point to a difference in origin
- …