Multi-color photometric observations of facular contrasts

Lawrence, J. K.

doi:10.1007/bf00171712

Cited by 26 publications

(13 citation statements)

References 15 publications

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“…This is in contrast to earlier measurements (e.g. Lawrence 1988;Auffret & Muller 1991;Sánchez Cuberes et al 2002;Okunev & Kneer 2004), but confirms the findings by de Boer et al (1997) and by Sütterlin et al (1999) extending them from θ ≤ 82…”

Section: Statistical Behaviorsupporting

confidence: 89%

Solar limb faculae

Hirzberger¹,

Wiehr²

2005

A&A

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Abstract. We observe solar limb faculae at an unprecedented spatial resolution with the new 1 m Swedish Solar Telescope SST on La Palma. Speckle-reconstructed images are used to study 4475 limb facular grains simultaneously in the 430 nm G-band and the 587.5 ± 1.5 continuum up to only 1 from the limb (cos θ = 0.05). No systematic contrast decrease is found even a few arcsec from the solar limb. The facular grains appear to be "projected" on the limb-side neighboring granules; approaching the disc center, the corresponding features occur as inter-granular G-band bright points. Independently, we took spectra with the French-Italian THEMIS telescope on Tenerife and find that the known "line-gap effect" from disc center disappears near the limb. Here, the facular continuum is enhanced whereas the normalized profiles are unchanged with respect to the undisturbed neighborhood.

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Section: Statistical Behaviorsupporting

confidence: 89%

Solar limb faculae

Hirzberger¹,

Wiehr²

2005

A&A

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“…There is also a large temperature difference from one structure to the next. It is therefore difficult to directly use the published contrasts found in the literature (for example Frazier 1971;Lawrence 1988;Ermolli et al 2007), because they are very sensitive to the way plages are defined. The large dispersion between individual plages, as well as the differences between plages and network structures (Foukal et al 1991;Worden et al 1998;Ortiz et al 2002Ortiz et al , 2006) also adds to the uncertainty of these measurements.…”

Section: Spot and Plage Observed Contrastsmentioning

confidence: 99%

Using the Sun to estimate Earth-like planets detection capabilities

Meunier¹,

Desort²,

Lagrange³

2010

A&A

270

394

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Aims. Stellar activity produced by spots and plages affects the radial velocity (RV) signatures. Because even low activity stars would produce such a signal, it is crucial to determine how it influences our ability to detect small planetary signals such as those produced by Earth-mass planets in the habitable zone (HZ). In a recent paper, we investigated the impact of sunlike spots. We aim here to investigate the additional impact of plages. Methods. We used the spot and plage properties over a solar cycle to derive the RV that would be observed if the Sun was seen edgeon. The RV signal comes from the photometric contribution of spots and plages and from the attenuation of the convective blueshift in plages. We analyzed the properties of the RV signal at different activity levels and compared it with commonly used activity indicators such as photometry and the Ca index. We also compared it with the signal that would be produced by an Earth-mass planet in the HZ. Results. We find that the photometric contributions of spots and plages to the RV signal partially balance each other out, so that the residual signal is comparable to the spot signal. However, the plage contribution due to the convective blueshift attenuation dominates the total signal, with an amplitude over the solar cycle of about 8-10 m/s. Short-term variations are also significantly greater than the spot and plage photometric contribution. This contribution is very strongly correlated with the Ca index on the long term, which may be a way to distinguish between stellar activity and a planet. Conclusions. Providing a very good temporal sampling and signal-to-noise ratio, the photometric contribution of plages and spots should not prevent detection of Earth-mass planets in the HZ. However, the convection contribution makes such a direct detection impossible, unless its effect can be corrected for by methods that still need to be found. We show that it is possible to identify the convection contribution if the sensitivity is good enough, for example, by using activity indicators.

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“…they have a positive "contrast". This is particularly pronounced when observing in the core of spectral lines and in molecular bands (Chapman & Sheeley 1968;Sheeley 1969;Muller & Roudier 1984), but also holds at continuum wavelengths or broader wavelength bands, even at disk center, if the magnetic elements are sufficiently resolved (see Muller & Keil 1983;Foukal & Fowler 1984;Lawrence 1988;Riethmüller et al 2010;Schüssler & Solanki 1988;Vögler & Schüssler 2003, the two last citations refering to radiation-MHD simulations). Note that the contrast A&A 542, A96 (2012) of magnetic features further increases from the disk center to the limb in a way that is still a matter of debate (Steiner 2007) and shall be treated in a forthcoming paper.…”

Section: Introductionmentioning

confidence: 97%

The continuum intensity as a function of magnetic field

Kobel

Solanki

Borrero

2012

A&A

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Context. To deepen our understanding of the role of small-scale magnetic fields in active regions (ARs) and in the quiet Sun (QS) on the solar irradiance, it is fundamental to investigate the physical processes underlying their continuum brightness. Previous results showed that magnetic elements in the QS reach larger continuum intensities than in ARs at disk center, but left this difference unexplained. Aims. We use Hinode/SP disk center data to study the influence of the local amount of magnetic flux on the vigour of the convective flows and the continuum intensity contrasts. Methods. The apparent (i.e. averaged over a pixel) longitudinal field strength and line-of-sight (LOS) plasma velocity were retrieved by means of Milne-Eddington inversions (VFISV code). We analyzed a series of boxes taken over AR plages and the QS, to determine how the continuum intensity contrast of magnetic elements, the amplitude of the vertical flows and the box-averaged contrast were affected by the mean longitudinal field strength in the box (which scales with the total unsigned flux in the box).Results. Both the continuum brightness of the magnetic elements and the dispersion of the LOS velocities anti-correlate with the mean longitudinal field strength. This can be attributed to the "magnetic patches" (here defined as areas where the longitudinal field strength is above 100 G) carrying most of the flux in the boxes. There the velocity amplitude and the spatial scale of convection are reduced. Due to this hampered convective transport, these patches appear darker than their surroundings. Consequently, the average brightness of a box decreases as the the patches occupy a larger fraction of it and the amount of embedded flux thereby increases. Conclusions. Our results suggest that as the magnetic flux increases locally (e.g. from weak network to strong plage), the heating of the magnetic elements is reduced by the intermediate of a more suppressed convective energy transport within the larger and stronger magnetic patches. This, together with the known presence of larger magnetic features, could explain the previously found lower contrasts of the brightest magnetic elements in ARs compared to the QS. The inhibition of convection also affects the average continuum brightness of a photospheric region, so that at disk center, an area of photosphere in strong network or plage appears darker than a purely quiet one. This is qualitatively consistent with the predictions of 3D MHD simulations.

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Multi-color photometric observations of facular contrasts

Cited by 26 publications

References 15 publications

Solar limb faculae

Solar limb faculae

Using the Sun to estimate Earth-like planets detection capabilities

The continuum intensity as a function of magnetic field

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