Solar limb faculae

Hirzberger, J.; Wiehr, E.

doi:10.1051/0004-6361:20052789

Cited by 44 publications

(53 citation statements)

References 30 publications

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“…18). This decline in the line core residual intensity contrast CLV profiles is also consistent with the spectroscopic observations of Stellmacher & Wiehr (1979) and Hirzberger & Wiehr (2005). Comparing line core intensity and residual intensity contrast (Figs.…”

Section: Line Core Intensity Contrastsupporting

confidence: 89%

Intensity contrast of solar network and faculae

Yeo

Solanki

Krivova

2013

A&A

105

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Aims. This study aims at setting observational constraints on the continuum and line core intensity contrast of network and faculae, specifically, their relationship with magnetic field and disc position. Methods. Full-disc magnetograms and intensity images by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) were employed. Bright magnetic features, representing network and faculae, were identified and the relationship between their intensity contrast at continuum and line core with magnetogram signal and heliocentric angle examined. Care was taken to minimize the inclusion of the magnetic canopy and straylight from sunspots and pores as network and faculae.Results. In line with earlier studies, network features, on a per unit magnetic flux basis, appeared brighter than facular features. Intensity contrasts in the continuum and line core differ considerably, most notably, they exhibit opposite centre-to-limb variations. We found this difference in behaviour to likely be due to the different mechanisms of the formation of the two spectral components. From a simple model based on bivariate polynomial fits to the measured contrasts we confirmed spectral line changes to be a significant driver of facular contribution to variation in solar irradiance. The discrepancy between the continuum contrast reported here and in the literature was shown to arise mainly from differences in spatial resolution and treatment of magnetic signals adjacent to sunspots and pores. Conclusions. HMI is a source of accurate contrasts and low-noise magnetograms covering the full solar disc. For irradiance studies it is important to consider not just the contribution from the continuum but also from the spectral lines. In order not to underestimate long-term variations in solar irradiance, irradiance models should take the greater contrast per unit magnetic flux associated with magnetic features with low magnetic flux into account.

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Section: Line Core Intensity Contrastsupporting

confidence: 89%

Intensity contrast of solar network and faculae

Yeo

Solanki

Krivova

2013

A&A

105

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“…Spruit 1976;Keller et al 2004) and the optical depth shifts upward where magnetic elements have a larger temperature excess due to the shallower temperature gradient inside than outside the flux tubes (Steiner 2005). Signatures of these hot granular walls ("faculae") can be seen directly in the high resolution continuum and G-band images of Lites et al (2004), Kobel et al (2009) Hirzberger & Wiehr (2005) and Berger et al (2007), with results of contrast at various heliocentric angles being presented in the last two papers. While the present paper only deals with disk center data, the center-to-limb variation of the continuum contrast of magnetic elements will be presented in a forthcoming one (Kobel et al, Paper III of this series, in prep.).…”

Section: Introductionmentioning

confidence: 95%

The continuum intensity as a function of magnetic field

Kobel

Solanki

Borrero

2011

A&A

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Context. Small-scale magnetic fields are major contributors to the solar irradiance variations. Hence, the continuum intensity contrast of magnetic elements in the quiet Sun (QS) network and in active region (AR) plage is an essential quantity that needs to be measured reliably. Aims. By using Hinode/SP disk center data at a constant, high spatial resolution, we aim at updating results of earlier ground-based studies of contrast vs. magnetogram signal, and to look for systematic differences between AR plages and QS network. Methods. The field strength, filling factor and inclination of the field was retrieved by means of a Milne-Eddington inversion (VFISV code). As in earlier studies, we then performed a pixel-by-pixel study of 630.2 nm continuum contrast vs. apparent (i.e. averaged over a pixel) longitudinal magnetic field over large fields of view in ARs and in the QS. Results. The continuum contrast of magnetic elements reaches larger values in the QS (on average 3.7%) than in ARs (on average 1.3%). This could not be attributed to any systematic difference in the chosen contrast references, so that it mainly reflects an intrinsic brightness difference. The larger contrasts in the QS are in agreement with earlier, lower resolution results, although our values are larger due to our better spatial resolution. At Hinode's spatial resolution, moreover, the relationship between contrast and apparent longitudinal field strength exhibits a peak at around 700 G in both the QS and ARs, whereas earlier lower resolution studies only found a peak in the QS and a monotonic decrease in ARs. We attribute this discrepancy both to our careful removal of the pores and their close surroundings affected by the telescope diffraction, as well as to the enhanced spatial resolution and very low scattered light of the Hinode Solar Optical Telescope. We verified that the magnetic elements producing the peak in the contrast curve are rather vertical in the AR and in the QS, so that the larger contrasts in the QS cannot be explained by larger inclinations, as had been proposed earlier.The opposite polarities in ARs do not exhibit any noticeable difference in inclination either, although they reach different contrasts when the amount of flux is significantly unbalanced between the polarities. Conclusions. According to our inversions, the magnetic elements producing the peak of the contrast curves have similar properties (field strength, inclination, filling factor) in ARs and in the QS, so that the larger brightness of magnetic elements in the QS remains unexplained. Indirect evidence suggests that the contrast difference is not primarily due to any difference in average size of the magnetic elements. A possible explanation lies in the different efficiencies of convective energy transport in the QS and in ARs, which will be the topic of a second paper.

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“…There, a partial evacuation of the atmosphere and the related shift of the optical depth scale cause the excess emission. In other words, magnetic bright points and faculae are (quasi-stationary) bright because of the Wilson depression (Keller et al 2004;Steiner 2005;Hirzberger & Wiehr 2005). The enhanced skewness at magnetic locations at intermediate wavelengths therefore could be caused by the spatial variation in the optical depth scale in the presence of magnetic fields of variable total magnetic flux.…”

Section: Energy Sources For the Chromospheric Radiative Lossesmentioning

confidence: 99%

The energy of waves in the photosphere and lower chromosphere

Beck

Rezaei

Puschmann

2012

A&A

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Context. The energy source powering the solar chromosphere is still undetermined, but leaves its traces in observed intensities. Aims. We investigate the statistics of the intensity distributions as a function of the wavelength for Ca ii H and the Ca ii IR line at 854.2 nm to estimate the energy content in the observed intensity fluctuations. Methods. We derived the intensity variations at different heights of the solar atmosphere, as traced by the line wings and line cores of the two spectral lines. We converted the observed intensities to absolute energy units employing reference profiles calculated in non-local thermal equilibrium (NLTE). We also converted the intensity fluctuations to corresponding brightness temperatures assuming LTE. Results. The root-mean-square (rms) fluctuations of the emitted intensity are about 0.6 (1.2) W m −2 ster −1 pm −1 near the core of the Ca ii IR line at 854.2 nm (Ca ii H), corresponding to relative intensity fluctuations of about 20% (30%). For the line wing, we find rms values of about 0.3 W m −2 ster −1 pm −1 for both lines, corresponding to relative fluctuations below 5%. The relative rms values show a local minimum for wavelengths forming at a height of about 130 km, but otherwise increase smoothly from the wing to the core, i.e., from photosphere to chromosphere. The corresponding rms brightness temperature fluctuations are below 100 K for the photosphere and up to 500 K in the chromosphere. The skewness of the intensity distributions is close to zero in the outer line wing and positive throughout the rest of the line spectrum, owing to the frequent occurrence of high-intensity events. The skewness shows a pronounced local maximum at locations with photospheric magnetic fields for wavelengths in-between those of the line wing and the line core (z ≈ 150−300 km), and a global maximum at the very core (z ≈ 1000 km) for both magnetic and field-free locations. Conclusions. The energy content of the intensity fluctuations is insufficient to create a chromospheric temperature rise that would be similar to the one in most reference models of the solar atmosphere. The increase in the rms fluctuations with height indicates the presence of upwardly propagating acoustic waves of increasing oscillation amplitude. The intensity and temperature variations indicate that there is a clear increase in dynamical activity from photosphere towards the chromosphere, but the variations fall short of the magnitude predicted by fully dynamical chromospheric models by a factor of about five. The enhanced skewness between the photosphere and lower solar chromosphere at magnetic locations is indicative of a mechanism that acts solely on magnetized plasma.

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Solar limb faculae

Cited by 44 publications

References 30 publications

Intensity contrast of solar network and faculae

Intensity contrast of solar network and faculae

The continuum intensity as a function of magnetic field

The energy of waves in the photosphere and lower chromosphere

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