Bright fibrils in Ca II K

Pietarila, A.; Hirzberger, J.; Zakharov, V.; Solanki, S. K.

doi:10.1051/0004-6361/200811155

Cited by 47 publications

(76 citation statements)

References 42 publications

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“…These structures might be closely related to the "straws" described by Rutten (2007) and the Ca ii K fibrils recently discovered by Pietarila et al (2009). This appearance is significantly different from the normal penumbra at the limb-side of the sunspot.…”

Section: Ca II H Observationssupporting

confidence: 64%

High-resolution spectro-polarimetry of a flaring sunspot penumbra

Hirzberger¹,

Riethmüller²,

Lagg³

et al. 2009

A&A

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We present simultaneous photospheric and chromospheric observations of the trailing sunspot in NOAA 10904 during a weak flare eruption (GOES magnitude B7.8), obtained with the Swedish Solar Telescope (SST) in La Palma, Canary Islands. High-resolutionCa ii H images show a typical two-ribbon structure that has been hitherto only known for larger flares, and the flare appears in a confined region that is discernible by a bright border. The underlying photosphere shows a disturbed penumbral structure with intersecting branches of penumbral filaments. High-resolution Doppler-and vector-magnetograms exhibit oppositely directed Evershed flows and magnetic field vectors in the individual penumbral branches, resulting in several regions of magnetic azimuth discontinuity and several islands where the vertical magnetic field is reversed. The discontinuity regions are co-spatial with the locations of the onset of the flare ribbons. From the results, we conclude that the confined flare region is detached from the global magnetic field structure by a separatrix marked by the bright border visible in Ca ii H. We further conclude that the islands of reversed vertical field appear because of flux emergence and that the strong magnetic shear appearing in the regions of magnetic azimuth discontinuity triggers the flare.

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Section: Ca II H Observationssupporting

confidence: 64%

High-resolution spectro-polarimetry of a flaring sunspot penumbra

Hirzberger¹,

Riethmüller²,

Lagg³

et al. 2009

A&A

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“…In reality the network is dynamic and likely composed of individual flux concentrations which merge at some height. Observations of chromospheric lines, such as H-α (e.g., de Pontieu et al 2007), the Ca K (e.g., Zirin 1974, Pietarila et al 2009) and Ca II infrared triplet lines around 850 nm (e.g., Vecchio et al 2007), exhibit fibril structures which probably outline magnetic field lines that are more heated than others, i.e., the fibrillar canopy is thermally inhomogeneous. The magnetic field in contrast is likely quite smooth as indicated by the homogeneous appearance of both the uni-and bipolar patches in the SOT data.…”

Section: Discussionmentioning

confidence: 99%

Expansion of magnetic flux concentrations: a comparison of Hinode SOT data and models

Pietarila

Cameron

Solanki

2010

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Context. The expansion of network magnetic fields with height is a fundamental property of flux tube models. A rapid expansion is required to form a magnetic canopy. Aims. We characterize the observed expansion properties of magnetic network elements and compare them with the thin flux tube and sheet approximations, as well as with magnetoconvection simulations. Methods. We used data from the Hinode SOT NFI NaD 1 channel and spectropolarimeter to study the appearance of magnetic flux concentrations seen in circular polarization as a function of position on the solar disk. We compared the observations with synthetic observables from models based on the thin flux tube approximation and magnetoconvection simulations with two different upper boundary conditions for the magnetic field (potential and vertical). Results. The observed circular polarization signal of magnetic flux concentrations changes from unipolar at disk center to bipolar near the limb, which implies an expanding magnetic field. The observed expansion agrees with expansion properties derived from the thin flux sheet and tube approximations. Magnetoconvection simulations with a potential field as the upper boundary condition for the magnetic field also produce bipolar features near the limb while a simulation with a vertical field boundary condition does not. Conclusions. The near-limb apparent bipolar magnetic features seen in high-resolution Hinode observations can be interpreted using a simple flux sheet or tube model. This lends further support to the idea that magnetic features with vastly varying sizes have similar relative expansion rates. The numerical simulations presented here are less useful in interpreting the expansion since the diagnostics we are interested in are strongly influenced by the choice of the upper boundary condition for the magnetic field in the purely photospheric simulations.

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“…As a corollary, on-disk images that don't show evidence of such structuring, even in the quiet Sun, cannot be said to realistically portray the chromospheric conditions. We expect that images in Ca II H and K lines taken with higher spatial and spectral resolution will show a silmilar wealth of fibrillar structures (Pietarila et al 2008). In the absence of such data, and given the observational advantages of working at longer wavelengths (with the one drawback of decreased spatial resolution), the Ca II 854.2 nm line represents an excellent chromospheric indicator.…”

Section: Discussionmentioning

confidence: 99%

The solar chromosphere at high resolution with IBIS

Reardon

Uitenbroek

Cauzzi

2009

A&A

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Aims. Filtergrams obtained in Ca II H, Ca II K, and Hα are often employed as diagnostics of the solar chromosphere. The vastly disparate appearance between the typical filtergrams in these different lines calls into question the nature of what is actually being observed. We investigate the lack of obvious structures of magnetic origin such as fibrils and mottles in on-disk Ca II H and K images. Methods. We directly compare a temporal sequence of classical Ca II K filtergrams with a co-spatial and co-temporal sequence of spectrally resolved Ca II 854.2 images obtained with the Interferometric Bidimensional Spectrometer (IBIS), considering the effect of both the spectral and spatial smearing. We analyze the temporal behavior of the two series by means of Fourier analysis.Results. The lack of fine magnetic structuring in Ca II K filtergrams, even with the narrowest available filters, is due to observational effects, primarily contributions from the bright, photospheric wings of the line that swamp the small and dark chromospheric structures. Signatures of fibrils remain, however, in the temporal evolution of the filtergrams, in particular with the evidence of magnetic shadows around the network elements. The Ca II K filtergrams do not appear, however, to properly reflect the high-frequency behavior of the chromosphere. Using the same analysis, we find no significant chromospheric signature in the Hinode/SOT Ca II H quiet-Sun filtergrams.Conclusions. The picture provided by Hα and Ca II 854.2, which show significant portions of the chromosphere dominated by magnetic structuring, appears to reflect the true and essential nature of the solar chromosphere. Data that do not resolve this aspect, whether spatially or spectrally, may misrepresent the behavior the chromosphere.

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Bright fibrils in Ca II K

Cited by 47 publications

References 42 publications

High-resolution spectro-polarimetry of a flaring sunspot penumbra

High-resolution spectro-polarimetry of a flaring sunspot penumbra

Expansion of magnetic flux concentrations: a comparison of Hinode SOT data and models

The solar chromosphere at high resolution with IBIS

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