Three-dimensional magnetic structure of a sunspot : Comparison of the photosphere and upper chromosphere
TL;DR: In this article, the magnetic field of a sunspot in the upper chromosphere was investigated and compared with the photospheric properties of the field in the Tenerife Infrared Polarimeter-2 (TIP-2).
Abstract: Aims. We investigate the magnetic field of a sunspot in the upper chromosphere and compare it to the photospheric properties of the field. Methods. We observed the main leading sunspot of the active region NOAA 11124 during two days with the Tenerife Infrared Polarimeter-2 (TIP-2) mounted at the German Vacuum Tower Telescope (VTT). Through inversion of Stokes spectra of the He i triplet at 10 830 A, we obtained the magnetic field vector of the upper chromosphere. For comparison with the photosphere, we applied height-dependent inversions of the Si i 10 827.1 A and Ca i 10 833.4 A lines. Results. We found that the umbral magnetic field strength in the upper chromosphere is lower by a factor of 1.30–1.65 compared to the photosphere. The magnetic field strength of the umbra decreases from the photosphere toward the upper chromosphere by an average rate of 0.5–0.9 G km -1 . The difference in the magnetic field strength between both atmospheric layers steadily decreases from the sunspot center to the outer boundary of the sunspot; the field, in particular its horizontal component, is stronger in the chromopshere outside the spot and this is suggestive of a magnetic canopy. The sunspot displays a twist that on average is similar in the two layers. However, the differential twist between the photosphere and chromosphere increases rapidly toward the outer penumbral boundary. The magnetic field vector is more horizontal with respect to the solar surface by roughly 5–20° in the photosphere compared to the upper chromosphere. Above a lightbridge, the chromospheric magnetic field is equally strong as that in the umbra, whereas the field of the lightbridge is weaker than its surroundings in the photosphere by roughly 1 kG. This suggests a cusp-like magnetic field structure above the lightbridge.
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TL;DR: In this article, high-spatial resolution spectropolarimetric observations from the Solar Optical Telescope on-board the Hinode spacecraft were employed to investigate the fine structure of the penumbral magnetic fields.
Abstract: We employ high-spatial resolution spectropolarimetric observations from the Solar Optical Telescope on-board the Hinode spacecraft to investigate the fine structure of the penumbral magnetic fields. The Stokes vector of two neutral iron lines at 630 nm is inverted at every spatial pixel to retrieve the depth-dependence of the magnetic field vector, line-of-sight velocity and thermodynamic parameters. We show that the azimuthal angle of the magnetic field vector has opposite sign on both sides above the penumbral filaments. This is consistent with the wrapping of an inclined field around the horizontal filaments. The wrapping effect is stronger for filaments with larger horizontal extensions. In addition, we find that the external magnetic field can penetrate into the intraspines, leading to non-radial magnetic fields inside them. These findings shed some light on the controversial small-scale structure of the sunspot penumbra.
40 citations
TL;DR: Umbral flashes and running penumbral waves (RPWs) in sunspot chromospheres leave a dramatic imprint in the intensity profile of the Can 8542 angstrom line as discussed by the authors.
Abstract: Context. Umbral flashes (UF) and running penumbral waves (RPWs) in sunspot chromospheres leave a dramatic imprint in the intensity profile of the Can 8542 angstrom line. Recent studies have focusse ...
35 citations
TL;DR: In this article, the authors investigate the origin of periodic variations of the magnetic field strength by analyzing a time-series of high temporal cadence observations acquired in the Ca II line with the CRISP instrument at the Swedish 1-m Solar Telescope.
Abstract: Umbral flashes (UF) and running penumbral waves (RPWs) in sunspot chromospheres leave a dramatic imprint in the intensity profile of the Ca II 854.2 nm line. Recent studies have focussed on also explaining the observed polarization profiles, that show even more dramatic variations during the passage of these shock fronts. While most of these variations can be explained with an almost constant magnetic field as a function of time, several studies have reported changes in the inferred magnetic field strength during UF phases. In this study we investigate the origin of these periodic variations of the magnetic field strength by analyzing a time-series of high temporal cadence observations acquired in the Ca II line with the CRISP instrument at the Swedish 1-m Solar Telescope. In particular, we analyze how the inferred geometrical height scale changes between quiescent and UF phases, and whether those changes are enough to explain the observed changes in $B$. We have performed non-LTE data inversions with the NICOLE code of a time-series of very high spatio-temporal resolution observations in the Ca II and Fe I 630.15\630.25 nm lines. Our results indicate that the Ca II line in sunspots is greatly sensitive to magnetic fields at $\log\tau_{500}=-5$ during UFs and quiescence. However, this optical depth value does not correspond to the same geometrical height during the two phases. Our results indicate that during UFs and RPWs the $\log\tau=-5$ is located at a higher geometrical height than during quiescence. Additionally, the inferred magnetic field values are higher in UFs (~270 G) and in RPWs (~100 G). Our results suggest that opacity changes caused by UFs and RPWs cannot explain the observed temporal variations in the magnetic field, as the line seems to form at higher geometrical heights where the field is expected to be lower.
21 citations
TL;DR: In this article, the authors studied the physical parameters of the penumbra in a large and fully developed sunspot, one of the largest over the last two solar cycles, by using full-Stokes measurements taken at the photospheric Fe I 617.3 nm and chromospheric Ca II 854.2 nm lines with the Interferometric Bidimensional Spectrometer.
Abstract: We studied the physical parameters of the penumbra in a large and fully-developed sunspot, one of the largest over the last two solar cycles, by using full-Stokes measurements taken at the photospheric Fe I 617.3 nm and chromospheric Ca II 854.2 nm lines with the Interferometric Bidimensional Spectrometer. Inverting measurements with the NICOLE code, we obtained the three-dimensional structure of the magnetic field in the penumbra from the bottom of the photosphere up to the middle chromosphere. We analyzed the azimuthal and vertical gradient of the magnetic field strength and inclination. Our results provide new insights on the properties of the penumbral magnetic fields in the chromosphere at atmospheric heights unexplored in previous studies. We found signatures of the small-scale spine and intra-spine structure of both the magnetic field strength and inclination at all investigated atmospheric heights. In particular, we report typical peak-to-peak variations of the field strength and inclination of $\approx 300$ G and $\approx 20^{\circ}$, respectively, in the photosphere, and of $\approx 200$ G and $\approx 10^{\circ}$ in the chromosphere. Besides, we estimated the vertical gradient of the magnetic field strength in the studied penumbra: we find a value of $\approx 0.3$ G km$^{-1}$ between the photosphere and the middle chromosphere. Interestingly, the photospheric magnetic field gradient changes sign from negative in the inner to positive in the outer penumbra.
18 citations
TL;DR: In this article, the authors determined the direction and strength of the photospheric and lower chromospheric magnetic field in the umbra and penumbra of a sunspot from inversions of spectropolarimetric observations of photosphere lines at 617,nm and 1565,nm, respectively.
Abstract: We determined the direction and strength of the photospheric and lower chromospheric magnetic field in the umbra and penumbra of a sunspot from inversions of spectropolarimetric observations of photospheric lines at 617\,nm and 1565\,nm, and the chromospheric \ion{Ca}{ii} IR line at 854\,nm, respectively. We compare the magnetic field vector with the direction of 75 flow channels that harbor the chromospheric inverse Evershed effect (IEF) near their downflow points (DFPs) in the sunspot's penumbra. The azimuth and inclination of the IEF channels to the line of sight (LOS) were derived from spatial maps of the LOS velocity and line-core intensity of the \ion{Ca}{ii} IR line and a thermal inversion of the \ion{Ca}{ii} IR spectra to obtain temperature cubes. We find that the flow direction of the IEF near the DFPs is aligned with the photospheric magnetic field to within about $\pm$\,15\,deg. The IEF flow fibrils make an angle of 30--90\,deg to the local vertical with an average value of about 65\,deg. The average field strength at the DFPs is about 1.3\,kG. Our findings suggest that the IEF in the lower chromosphere is a field-aligned siphon flow, where the larger field strength at the inner footpoints together with the lower temperature in the penumbra causes the necessary gas pressure difference relative to the outer footpoints in the hotter quiet Sun with lower magnetic field strength. The IEF connects to magnetic field lines that are not horizontal like for the regular photospheric Evershed flow, but which continue upwards into the chromosphere indicating an "uncombed" penumbral structure.
17 citations
References
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TL;DR: The solar optical telescope (SOT) as discussed by the authors is a 50-cm diffraction-limited Gregorian telescope with the Stokes Spectro-Polarimeter (SP) attached to it.
Abstract: The Solar Optical Telescope (SOT) aboard the Hinode satellite (formerly called Solar-B) consists of the Optical Telescope Assembly (OTA) and the Focal Plane Package (FPP). The OTA is a 50-cm diffraction-limited Gregorian telescope, and the FPP includes the narrowband filtergraph (NFI) and the broadband filtergraph (BFI), plus the Stokes Spectro-Polarimeter (SP). The SOT provides unprecedented high-resolution photometric and vector magnetic images of the photosphere and chromosphere with a very stable point spread function and is equipped with an image-stabilization system with performance better than 0.01 arcsec rms. Together with the other two instruments on Hinode (the X-Ray Telescope (XRT) and the EUV Imaging Spectrometer (EIS)), the SOT is poised to address many fundamental questions about solar magnetohydrodynamics. This paper provides an overview; the details of the instrument are presented in a series of companion papers.
1,448 citations
TL;DR: The Solar Optical Telescope (SOT) as mentioned in this paper is a 50 cm diffraction-limited Gregorian telescope, and includes the narrow-band (NFI) and wideband (BFI) filtergraphs.
Abstract: The Solar Optical Telescope (SOT) aboard the Hinode satellite (formerly called Solar-B) consists of the Optical Telescope Assembly (OTA) and the Focal Plane Package (FPP). The OTA is a 50 cm diffraction-limited Gregorian telescope, and the FPP includes the narrow-band (NFI) and wide-band (BFI) filtergraphs, plus the Stokes spectro-polarimeter (SP). SOT provides unprecedented high resolution photometric and vector magnetic images of the photosphere and chromosphere with a very stable point spread function, and is equipped with an image stabilization system that reduces the error to less than 0.01 arcsec rms. Together with the other two instruments on Hinode (the X-Ray Telescope (XRT) and EUV Imaging Spectrometer (EIS)), SOT is poised to address many fundamental questions about solar magneto-hydrodynamics. Note that this is an overview, and the details of the instrument are presented in a series of companion papers.
1,229 citations
TL;DR: Sunspots are the most readily visible manifestations of solar magnetic field concentrations and their interaction with the Sun's plasma as mentioned in this paper, and their internal structure has been extensively studied for almost 400 years and their magnetic nature has been known since 1908.
Abstract: Sunspots are the most readily visible manifestations of solar magnetic field concentrations and of their interaction with the Sun's plasma. Although sunspots have been extensively studied for almost 400 years and their magnetic nature has been known since 1908, our understanding of a number of their basic properties is still evolving, with the last decades producing considerable advances. In the present review I outline our current empirical knowledge and physical understanding of these fascinating structures. I concentrate on the internal structure of sunspots, in particular their magnetic and thermal properties and on some of their dynamical aspects.
748 citations
01 Jan 1994
TL;DR: Magnetic Elements R.H. Muller, L.A. Title, T.D.W. Smith, Z.I. Sheeley Jr., Y.-M.
Abstract: Context C. Zwaan. Techniques E. Landi Degl'Innocenti, C.U. Keller, W. Schmidt, H. Balthasar, E. Wiehr. Magnetic Elements R. Muller, L.H. Strous, S.K. Solanki, J.H.M.J. Bruls, O. Steiner, T. Ayres, W. Livington, H. Uitenbroek, A. Skumanich, B.W. Lites, V. Martinez Pillet, K. Muglach, V. Gaizauskas, B. Schmieder, P. Heinzel, G. Tsiropoula, C.E. Alessandrakis, F.-L. Deubner, J. Hofmann, E. Kossack, B. Fleck, R.J. Rutten, J.H. Thomas, G. Severino, M.-T. Gomez, B. Caccin, P. Maltby, J. Staude, R.A. Shine, A.M. Title, T.D. Tarbell, K. Smith, Z.A. Frank, G. Scharmer, P.C. Martens, N. Hurlburt, A.M. Title, L.A. Acton, C.A.P. Montavon. Magnetic Patterns P.N. Brandt, R.J. Rutten, R.A. Shine, J. Trujillo Bueno, G.W. Simon, L.J. November, G.B. Scharmer, C.J. Schrijver, J.K. Lawrence, A.C. Cadavid, A.A. Ruzmaikin, N.O. Weiss, R.F. Howard, S.F. Martin, R. Bilimoria, P.W. Tracadas, Ch.R. Echols, K.L. Harvey, J.O. Stenflo, N.R. Sheeley Jr., Y.-M. Wang. Theory of Magnetoconvection P. Hoyng, F. Moreno-Insertis, M. Schuessler, P. Caligari, K. Petrovay, O. Steiner, M. Knoelker. A. Nordlund, K. Galsgaard, R.F. Stein. Prospects J.M. Beckers, J. Rayrole, P. Mein, F. Cavallini, M. Semel, B. Fleck, V. Domingo, A.I. Poland.
292 citations
TL;DR: In this article, the vector magnetic field structure of a small, symmetric sunspot observed very close to disk center was explored using data from the High Altitude Observatory/National Solar Observatory Advanced Stokes Polarimeter (ASP).
Abstract: The vector magnetic field structure of a small, symmetric sunspot observed very close to disk center has been explored using data from the High Altitude Observatory/National Solar Observatory Advanced Stokes Polarimeter (ASP). This instrument provides, for the first time, quantitative information on sunspot photospheric vector magnetic fields with high angular resolution, as derived from full Stokes profiles of the Zeeman-sensitive Fe I line pair at 630 nm
220 citations