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Azimuthal variation of penumbral brightness, measured along ellipses with constant distance to the inner penumbral boundary. The six panels correspond to different intervals µ = cos θ of the sunspots. The three curves in each panels show the average behavior for all sunspots within the µ-interval, separately for the three data sets under investigation. The number, n, of sunspots in each interval is indicated. Top curve: TRACE, middle curve: TIP-1, bottom curve: TIP-2. φ = 0 is at center-side (cf. Fig. 2). The dashed line is the 2nd Fourier component of the corresponding curve. The three sets of curves are shifted in intensity by 0.1 units, for clarity.
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We investigate the center-to-limb variation of the brightness of the penumbrae of sunspots. The analysis includes narrow-band and broad-band continuum images of about 80 sunspots observed with the Vacuum Tower Telescope on Tenerife and with the Transition Region And Coronal Explorer (TRACE). We find that the azimuthal intensity variation depends on...
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Aims. This paper describes the Polarimetric and Helioseismic Imager on the Solar Orbiter mission (SO/PHI), the first magnetograph and helioseis- mology instrument to observe the Sun from outside the Sun-Earth line. It is the key instrument meant to address the top-level science question: How does the solar dynamo work and drive connections between the Sun and the heliosphere? SO/PHI will also play an important role in answering the other top-level science questions of Solar Orbiter, as well as hosting the potential of a rich return in further science.
Methods. SO/PHI measures the Zeeman effect and the Doppler shift in the Fe i 617.3 nm spectral line. To this end, the instrument carries out narrow-band imaging spectro-polarimetry using a tunable LiNbO3 Fabry-Perot etalon, while the polarisation modulation is done with liquid crystal variable retarders (LCVRs). The line and the nearby continuum are sampled at six wavelength points and the data are recorded by a 2k × 2k CMOS detector. To save valuable telemetry, the raw data are reduced on board, including being inverted under the assumption of a Milne- Eddington atmosphere, although simpler reduction methods are also available on board. SO/PHI is composed of two telescopes; one, the Full Disc Telescope (FDT), covers the full solar disc at all phases of the orbit, while the other, the High Resolution Telescope (HRT), can resolve structures as small as 200 km on the Sun at closest perihelion. The high heat load generated through proximity to the Sun is greatly reduced by the multilayer-coated entrance windows to the two telescopes that allow less than 4% of the total sunlight to enter the instrument, most of it in a narrow wavelength band around the chosen spectral line.
Results. SO/PHI was designed and built by a consortium having partners in Germany, Spain, and France. The flight model was delivered to Airbus Defence and Space, Stevenage, and successfully integrated into the Solar Orbiter spacecraft. A number of innovations were introduced compared with earlier space-based spectropolarimeters, thus allowing SO/PHI to fit into the tight mass, volume, power and telemetry budgets provided by the Solar Orbiter spacecraft and to meet the (e.g. thermal) challenges posed by the mission’s highly elliptical orbit.
Numerical simulations have by now revealed that the fine scale structure of the penumbra in general and the Evershed effect
in particular is due to overturning convection, mainly confined to gaps with strongly reduced magnetic field strength. The
Evershed flow is the radial component of the overturning convective flow visible at the surface. It is directed outwards –
away from the umbra – because of the broken symmetry due to the inclined magnetic field. The dark penumbral filament cores
visible at high resolution are caused by the “cusps” in the magnetic field that form above the gaps. Still remaining to be
established are the details of what determines the average luminosity of penumbrae, the widths, lengths, and filling factors
of penumbral filaments, and the amplitudes and filling factors of the Evershed flow. These are likely to depend at least partially
also on numerical aspects such as limited resolution and model size, but mainly on physical properties that have not yet been
adequately determined or calibrated, such as the plasma beta profile inside sunspots at depth and its horizontal profile,
the entropy of ascending flows in the penumbra, etc.
Inversions of spectropolarimetric observations of penumbral filaments deliver
the stratification of different physical quantities in an optical depth scale.
However, without establishing a geometrical height scale their
three-dimensional geometrical structure can not be derived. This is crucial in
understanding the correct spatial variation of physical properties in the
penumbral atmosphere and to provide insights into the mechanism capable of
explaining the observed penumbral brightness. The aim of this work is to
determine a global geometrical height scale in the penumbra by minimizing the
divergence of the magnetic field vector and the deviations from static
equilibrium as imposed by a force balance equation that includes pressure
gradients, gravity and the Lorentz force. Optical depth models are derived from
the SIR inversion of spectropolarimetric data of an active region observed with
SOT on-board the Hinode satellite. We use a genetic algorithm to determine the
boundary condition for the inference of geometrical heights. The retrieved
geometrical height scale permits the evaluation of the Wilson depression at
each pixel and the correlation of physical quantities at each height. Our
results fit into the uncombed penumbral scenario, i.e., a penumbra composed of
flux tubes with channelled mass flow and with a weaker and more horizontal
magnetic field as compared with the background field. The ascending material is
hotter and denser than their surroundings. We do not find evidence of
overturning convection or field free regions in the inner penumbral area
analyzed. The penumbral brightness can be explained by the energy transfer of
the ascending mass carried by the Evershed flow, if the physical quantities
below z=-75km are extrapolated from the results of the inversion.
The magnetic, thermal and velocity structure of a sunspot at the solar disk centre ($\theta=2^{\circ}$) is investigated by inverting the full Stokes profiles of three infrared lines. A single magnetic component atmosphere is assumed with height gradients of the physical quantities. Since the line-of-sight (LOS) is perpendicular to the solar surface, differential optical or projection effects do not interfere, as often is the case for the usual observations at oblique LOS. We find a symmetric configuration of the field and flow and the downward motion that increases with radial distance by up to 3 km s$^{-1}$ near the outer penumbral border. The magnetic field is found to be highly axially symmetric without any indication of azimuthal vortices. A tight relation between field strength and inclination is obtained with a gradient of $35^\circ$/1000 G independent of height. The penumbra shows “spines” hosting a pronounced negative correlation between field strength and inclination in the sense that steeper and stronger magnetic fields are related to brightenings in the line cores but not in the continuum. We discuss the dependence of the obtained results on different assumptions of parasitic light, and present indications of its overestimation by the inversion code.
We investigate the properties of the fine structure of a sunspot penumbra based on spectroscopic measurements with high spectral ($\lambda/\delta\lambda=250\,000$) and high spatial ($\approx$$0.5$ arcsec) resolution. The magnetically insensitive Fe I 557.6 nm line is used to probe the penumbral atmosphere. The data was taken at the German Vacuum Tower Telescope with the 2D-spectrometer TESOS, taking advantage of the recently installed Kiepenheuer Adaptive Optics System (KAOS). The field of view covers a sunspot located at 23° off the disk center and its immediate surroundings. The penumbral structure is studied by means of maps computed for the line-of-sight velocity, the line width, the equivalent width and the line depression. Line-of-sight velocities are derived from the Doppler shifts at different bisector levels. From these maps we infer the flow field geometry and study the azimuthal and radial dependences of the line parameters. Our findings can be summarized as follows: (a) the flow pattern has a conspicuous filamentary structure in the deep photospheric layers and is rather diffuse in the high layers. (b) The flow field slightly spreads and fans out with height. (c) The flow geometry confirms the presence of an upflow component in the inner penumbra and a downflow component in the middle and outer penumbra. (d) We find an enhanced brightness of the mid-penumbra (“bright ring”) in the line wings, but not in the continuum or line core. (e) The azimuthal average of the equivalent width, the line width and the absolute flow velocity increase with radial distance within the penumbra. (f) Small-scale variations of the equivalent width and the line width on the center-side penumbra are co-spatial and correlated with (blue-shifted) fluctuations in the line-of-sight velocity. (g) Inner limb-side penumbral grains are associated with blue-shifts of $v\le-400$ m s$^{-1}$, indicating upflows. (h) One umbral dot in our sample is associated with a blue-shift of $v=-200$ m s$^{-1}$.
Context. The topology of penumbral magnetic fields is poorly known. The satellite Hinode has recently revealed penumbral structures of a magnetic polarity that is opposite to the main sunspot polarity. They may be direct confirmation that magnetic field lines and mass flows return to the solar interior throughout the penumbra, a configuration previously inferred from interpretation of observed Stokes profile asymmetries.Aims. We try to point out the relationship between the reverse polarity features found by Hinode, and the model Micro-Structured Magnetic Atmospheres (MISMAs) proposed for sunspots.Methods. The work is based on synthesis and inversion of sunspot Stokes profiles.Results. Existing model MISMAs produce strongly redshifted reverse polarity structures as found by Hinode. Ad hoc model MISMAs also explain the asymmetric Stokes profiles observed by Hinode. The same modeling may be consistent with magnetograms of dark cored penumbral filaments if the dark cores are associated with the reverse polarity. This hypothetical relationship can only be identified in the far red wings of the spectral lines.Conclusions. The reverse polarity patches may result from aligned magnetic field lines and mass flows that bend over and return to the solar interior throughout the penumbra.
Context. The recent discovery of dark-cored penumbral filaments suggests that we are resolving the building blocks of sunspot penumbrae. Their properties are largely unknown but provide important clues to understanding penumbral fine structure. Aims.Our observations provide new constraints for the different scenarios put forward to explain the structure of sunspot penumbrae. Methods.We present an analysis of dark-cored penumbral filaments, based on intensity filtergrams ($G$-band, continuum and $\ion{Ca}{ii}$ H line wing), magnetograms and Dopplergrams, obtained at heliocentric distances between $15\degr$ and $55\degr$. Results.In general, the visibility of dark cores degrades with increasing heliocentric distance. Based on $\ion{Ca}{ii}$ H wing images we conclude that this is due to a geometrical 3D-effect and not due to a simple formation height effect. Only in the center-side penumbra are dark-cored filaments visible at all observed heliocentric distances. We observe that dark-cored filaments frequently split in the umbra, forming a Y-shape that disappears after a few minutes, leaving a shortened filamentary structure and a bright dot in the umbra. The dark-cored filaments have life times $\geq$$ 90$ min. The dark cores are related to a much weaker and a more horizontal magnetic field than their lateral brightenings. Where the dark-cored filaments appear in the umbra, the magnetic field is inclined by 40$\degr$ with respect to the solar surface normal for both the dark core and the bright edges. With increasing distance from the umbra, the magnetic field inclination in the dark cores increases rapidly within a few thousand km. Both the magnetic field strength and inclination in the lateral brightenings show very small variations with spot-center radial distance. The velocity field possesses a strong horizontal component within the dark cores. The absolute line-of-sight (LOS) velocity is larger within the dark cores than in their lateral brightenings. The Evershed flow apparently is present primarily in the dark cores.
We describe a scenario for the topology of the magnetic field in penumbrae that accounts for recent observations showing upflows, downflows, and reverse magnetic polarities. According to our conjecture, short narrow magnetic loops fill the penumbral photosphere. Flows along these arched field lines are responsible for both the Evershed effect and the convective transport. This scenario seems to be qualitatively consistent with most existing observations, including the dark cores in penumbral filaments reported by Scharmer et al. Each bright filament with dark core would be a system of two paired convective rolls with the dark core tracing the common lane where the plasma sinks down. The magnetic loops would have a hot footpoint in one of the bright filament and a cold footpoint in the dark core. The scenario fits in most of our theoretical prejudices (siphon flows along field lines, presence of overturning convection, drag of field lines by downdrafts, etc). If the conjecture turns out to be correct, the mild upward and downward velocities observed in penumbrae must increase upon improving the resolution. This and other observational tests to support or disprove the scenario are put forward.
We interpret penumbral filaments as due to convection in field-free, radially aligned gaps just below the visible surface of the penumbra, intruding into a nearly potential field above. This solves the classical discrepancy between the large heat flux and the low vertical velocities observed in the penumbra. The presence of the gaps causes strong small-scale fluctuations in inclination, azimuth angle and field strength, but without strong forces acting on the gas. The field is nearly horizontal in a region around the cusp-shaped top of the gap, thereby providing an environment for Evershed flows. We identify this region with the recently discovered dark penumbral cores. Its darkness has the same cause as the dark lanes in umbral light-bridges, reproduced in numerical simulations by Nordlund and Stein (2005). We predict that the large vertical and horizontal gradients of the magnetic field inclination and azimuth in the potential field model will produce the net circular polarization seen in observations. The model also explains the significant elevation of bright filaments above their surroundings. It predicts that dark areas in the penumbra are of two different kinds: dark filament cores containing the most inclined (horizontal) fields, and regions between bright filaments, containing the least inclined field lines. Comment: submitted to A&A
