R. Schlichenmaier

Spectropolarimetry in a sunspot penumbra - Spatial dependence of Stokes asymmetries in Fe I 1564.8 nm

Stokes profiles of sunspot penumbrae show distinct asymmetries, which point to gradients in the velocity field and in the magnetic field. We present spectropolarimetric measurements of the Stokes vector in the neutral iron triplet at 1564.8 nm taken with the Tenerife Infrared Polarimeter (TIP) at the German Vacuum Tower Telescope (VTT) in Tenerife. We report on the peculiarities of the profiles of circularly and linearly polarized light for spots at different heliocentric angles. We elaborate on the spatial dependence of Stokes asymmetries within the penumbra and find for profiles of circularly polarized light: (1) In the center-side penumbra the amplitude difference of Stokes-V exhibits a sign reversal on a radial cut, i.e., in the inner (outer) penumbra the red (blue) lobe is broader and shows a smaller amplitude than the blue (red) lobe. (2) In the outer limb-side penumbra (beyond the magnetic neutral line) the red lobe is broader and of less amplitude than the blue lobe. (3) Along the magnetic neutral line we find abnormal Stokes-V profiles, which consist of more than 2 lobes. This indicates the presence of two polarities. For small heliocentric angles abnormal profiles are also seen beyond the magnetic neutral line in the outer penumbra. (4) Maps of the net circular polarization have the tendency to be antisymmetric with respect to the axis that connects disk center with spot center. This finding is striking, because corresponding maps for Fe I 630.25 are symmetric. For linearly polarized profiles we extract the following features: (5) On the center-side penumbra at a heliocentric angle of 56◦ a Doppler-shift as high as 5 km s−1 can be directly measured by the splitting of the π-component of the linearly polarized component. (6) In limbside penumbrae, the profiles of the π-component show the typical asymmetry properties of the Evershed flow as observed in Stokes-I of magnetically insensitive lines. (7) In the outer centerand limb-side penumbrae the center of the π-component is blue-shifted relative to the zero-crossing of the V -profile. Motivated by the moving tube model of Schlichenmaier et al. (1998b), we construct simple model atmospheres featuring hot upflows and cool outflows and calculate corresponding synthetic V -profiles. These profiles are compared with our measured ones and with observed V -profiles in Fe I 630.25 from other authors. We find that the synthetic V -profiles can reproduce all essential characteristics of observed V -profiles for both lines.

Field-aligned Evershed flows in the photosphere of a sunspot penumbra

We determine the inclinations of the vector magnetic field and flow velocity in a sunspot penumbra by interpreting full Stokes profiles of three infrared lines observed with the Tenerife Infrared Polarimeter. It is shown that analyses based on one-component atmospheres deliver flow velocities which are more horizontal than the average magnetic field by up to 10 deg. This apparent violation of the concept of frozen-in magnetic fields is solved as soon as two magnetic atmospheres are allowed to coexist in the resolution element. The magnetic field and velocity in the atmospheric component carrying the Evershed flow are found to be aligned to within

The formation of a sunspot penumbra

\pm 2

A polarization model for the German Vacuum Tower Telescope from in situ and laboratory measurements

deg all the way from the inner to the outer penumbra. This is the first observational confirmation of magnetic fields being frozen into the plasma in sunspots. Our results indicate that sunspot penumbrae can be understood in terms of inclined flux tubes embedded in a more vertical background field. The flux tubes carry most of the Evershed flows and return to the solar surface in the middle penumbra and beyond. The background atmosphere is essentially at rest in the inner penumbra, and harbors small flows in the outer penumbra.

The velocity field of sunspot penumbrae - I. A global view

Context. The formation of a penumbra is crucial for our understanding of solar magnetism, but it has not been observed in detail. Aims. We aim to enhance our knowledge of how a sunspot penumbra forms and how sunspots grow in size. Methods. We present a data set of the active region NOAA 11024 acquired at the German VTT with speckle-reconstructed images in the G-band and Ca ii K. The data set includes spectropolarimetric profiles from GFPI in Fe i 617.3 nm and TIP in Fe i 1089.6 nm. Results. On 2009 July 4, at 08:30 UT, a leading spot without penumbra and pores of opposite polarity were present in the active region. For the next 4:40 h, we observed the formation of a penumbra in the leading spot at a cadence of 5 images per second. We produced speckle reconstructed images of 0. 3 spatial resolution or better, interrupted by one large gap of 35 min and a few more small gaps of about 10 min. The leading spot initially has a size of 230 arcsec 2 with only a few penumbral filaments and then grows to a size of 360 arcsec 2 . The penumbra forms in segments, and it takes about 4 h until it encircles half of the umbra, on the side opposite the following polarity. On the side towards the following polarity, elongated granules mark a region of magnetic flux emergence. Conclusions. This ongoing emergence appears to prevent a steady penumbra from forming on this side. While the penumbra forms, the umbral area is constant; i.e., the increase in the total spot area is caused exclusively by the growth of the penumbra. From this we conclude that the umbra has reached an upper size limit and that any new magnetic flux that joins the spot is linked to the process of penumbral formation.

On the heat transport in a sunspot penumbra

It is essential to properly calibrate the polarimetric properties of telescopes, if one wants to take advantage of the capabilities of high precision spectro-polarimeters. We have constructed a model for the German Vacuum Tower Telescope (VTT) that describes its time-dependent polarization properties. Since the coelostat of the telescope changes the polarization state of the light by introducing cross talk among different polarization states, such a model is necessary to correct the measurements, in order to retrieve the true polarization as emitted from the Sun. The telescope model is quantified by a time-dependent Mueller matrix that depends on the geometry of the light beam through the telescope, and on material properties: the refractive indices of the coelostat mirrors, and the birefringence of the entrance window to the vacuum tube. These material properties were determined experimentally in-situ by feeding the telescope with known states of polarization (including unpolarized light) and by measuring its response, and from measurements of an aluminum-coated sample in the laboratory. Accuracy can in our case be determined only for the combination of telescope and spectro-polarimeter used; for the instrument POLIS at the VTT, we estimate an accuracy of ± 4–

Magnetic properties of G-band bright points in a sunspot moat

5\times 10^{-3}

Penumbral fine structure: Theoretical understanding

for the cross talk correction coefficients.

Relation between photospheric magnetic field and chromospheric emission

Aims. We investigated the vertical penumbral plasma flow on small spatial scales using data recorded by the spectropolarimeter of the solar optical telescope onboard Hinode. Methods. We computed maps of apparent Doppler velocities by comparing the spectral positions of the Fe I 630.15 nm & Fe I 630.25 nm lines with the averaged line profiles of the quiet Sun. To visualize the flow pattern in the low photosphere, we used a bisector of the wing of the absorption lines. The small heliocentric angle (3° ≤ Θ ≤ 9°) of our data sets means that the horizontal component of the Evershed flow (EF) does not contribute significantly to the line shift. Results. We found that in the quiet Sun (QS), the area showing upflows is always larger than the one exhibiting downflows. In the penumbra, upflows dominate only at low velocities |v dop | ≤ 0.4 km s ―1 , while at higher velocities |v dop | ≥ 0.6 km s ―1 downflows prevail. Additionally, the maximal upflow velocity in penumbrae is lower, while the maximal downflow velocity is larger with respect to the QS velocities. Furthermore, on a spatial average, the penumbra shows a redshift, corresponding to a downflow of more than 0.1 km s ―1 . Upflows are elongated and appear predominately in the inner penumbra. Strong downflows with velocities of up to 9 km s ―1 are concentrated at the penumbra-QS boundary. They are magnetized and are rather round. The inner penumbra shows an average upflow, which turns into a mean downflow in the outer penumbra. The upflow patches in the inner penumbra and the downflow locations in the outer penumbra could be interpreted as the sources and the sinks of the EF. We did not find any indication of roll-type convection within penumbral filaments.

The formation of sunspot penumbra - Magnetic field properties

The penumbra radiates an energy flux that is roughly 75% of the quiet-sun value. One mechanism proposed to bring this flux to the surface is interchange convection of magnetic flux tubes according to which hot flux tubes rise to the surface, cool off their heat by radiation and sink down again. Another way to deposit heat in the penumbral photosphere is by steady upflows along magnetic flux tubes. We discuss these two mechanisms and elaborate on consequences that can be compared with and constrained by observations. We estimate the time scales for variations of the intensity and the magnetic field pattern. By comparing them with the corresponding observed time scales, we find that pure interchange convection is unable to account for the observed penumbral heat flux. Heating the penumbra by steady upflows along magnetic flux tubes, however, turns out to be sufficient to explain the penumbral brightness, under the condition that significant magnetic return flux is present within the penumbra. Associated with the magnetic return flux, downflows within the penumbra should be present, in accordance with recent observational findings of such downflows. Exploring other possible heating mechanisms, we find that dissipation of magnetic energy is negligible, while dissipation of the kinetic energy of the Evershed flow could contribute significantly to the brightness of the penumbra.