The center-to-limb behavior of solar active regions at ultraviolet wavelengths

Crane, P. C.; Floyd, L.; Cook, J. W.; Herring, L. C.; Avrett, E. H.; Prinz, D. K.

doi:10.1051/0004-6361:20040012

Cited by 15 publications

(13 citation statements)

References 55 publications

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“…Spectral analysis reveals that the first axis approximately describes the 27-day modulation amplitude while the second axis describes the amplitude of the 13.5-day modulation. The latter is partly caused by centre-to-limb variations, which are indeed known to be wavelength-dependent, with, for example, limb darkening in chromospheric lines (Donnelly & Puga 1990;Crane et al 2004). Here, as in Figure 2a, we have added the location of degraded passbands.…”

Section: à7mentioning

confidence: 99%

New observation strategies for the solar UV spectral irradiance

Cessateur

Lilensten

Wit

et al. 2012

J. Space Weather Space Clim.

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Many applications in space weather and in space situational awareness require continuous solar spectral irradiance measurements in the UV, and to a lesser degree in the visible band. Most space-borne solar radiometers are made out of two different parts: (i) a front filter that selects the passband and (ii) a detector that is usually based on silicon technology. Both are prone to degradation, which may be caused either by the degradation of the filter coating due to local deposition or to structural changes, or by the degradation of the silicon detector by solar radiative and energetic particle fluxes. In this study, we provide a theoretical analysis of the filter degradation that is caused by structural changes such as pinholes; contamination-induced degradation will not be considered. We then propose a new instrumental concept, which is expected to overcome, at least partially, these problems. We show how most of the solar UV spectrum can be reconstructed from the measurement of only five spectral bands. This instrumental concept outperforms present spectrometers in terms of degradation. This new concept in addition overcomes the need for silicon-based detectors, which are replaced by wide band gap material detectors. Front filters, which can contribute to in-flight degradation, therefore are not required, except for the extreme-UV (EUV) range. With a small weight and a low telemetry, this concept may also have applications in solar physics, in astrophysics and in planetology.

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Section: à7mentioning

confidence: 99%

New observation strategies for the solar UV spectral irradiance

Cessateur

Lilensten

Wit

et al. 2012

J. Space Weather Space Clim.

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“…The most conspicuous features are the decaying solar cycle and the solar rotation, which is responsible for a 27-day modulation. A 13.5-day modulation, which is caused by centre-to-limb variations (Crane et al, 2004), is sometimes apparent. The stronger variability of hot coronal lines, such as Fe XVI, only comes out in normalised quantities, whereas standardisation is better suited for comparing fine details in the time evolution.…”

Section: Choice Of the Normalisationmentioning

confidence: 99%

Which solar EUV indices are best for reconstructing the solar EUV irradiance?

Wit

Kretzschmar

Aboudarham

et al. 2008

Advances in Space Research

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The solar EUV irradiance is of key importance for space weather. Most of the time, however, surrogate quantities such as EUV indices have to be used by lack of continuous and spectrally resolved measurements of the irradiance. The ability of such proxies to reproduce the irradiance from different solar atmospheric layers is usually investigated by comparing patterns of temporal correlations. We consider instead a statistical approach. The TIMED/SEE experiment, which has been continuously operating since Feb. 2002, allows for the first time to compare in a statistical manner the EUV spectral irradiance to five EUV proxies: the sunspot number, the f10.7, Ca K, and Mg II indices, and the He I equivalent width.Using multivariate statistical methods, we represent in a single graph the measure of relatedness between these indices and various strong spectral lines. The ability of each index to reproduce the EUV irradiance is discussed; it is shown why so few lines can be effectively reconstructed from them. All indices exhibit comparable performance, apart from the sunspot number, which is the least appropriate. No single index can satisfactorily describe both the level of variability on time scales beyond 27 days, and relative changes of irradiance on shorter time scales.

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“…The variability on a 27-day solar rotation time scale is mostly related to the appearance and disappearance of active regions at the solar surface. The center-to-limb variation also causes a 13.5-day modulation, with an excess of emission for spectral lines that exhibit limb brightening (e.g., coronal lines) and a deficit for wavelengths that exhibit limb darkening, such as the 168-210 nm range (Crane et al 2004).…”

Section: Introduction and Motivationsmentioning

confidence: 99%

Sensitivity of upper atmospheric emissions calculations to solar/stellar UV flux

Barthélemy

Cessateur

2014

J. Space Weather Space Clim.

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The solar UV (UltraViolet) flux, especially the EUV (Extreme UltraViolet) and FUV (Far UltraViolet) components, is one of the main energetic inputs for planetary upper atmospheres. It drives various processes such as ionization, or dissociation which give rise to upper atmospheric emissions, especially in the UV and visible. These emissions are one of the main ways to investigate the upper atmospheres of planets. However, the uncertainties in the flux measurement or modeling can lead to biased estimates of fundamental atmospheric parameters, such as concentrations or temperatures in the atmospheres. We explore the various problems that can be identified regarding the uncertainties in solar/stellar UV flux by considering three examples. The worst case appears when the solar reflection component is dominant in the recorded spectrum as is seen for outer solar system measurements from HST (Hubble Space Telescope). We also show that the estimation of some particular line parameters (intensity and shape), especially Lyman a, is crucial, and that both total intensity and line profile are useful. In the case of exoplanets, the problem is quite critical since the UV flux of their parent stars is often very poorly known.

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The center-to-limb behavior of solar active regions at ultraviolet wavelengths

Cited by 15 publications

References 55 publications

New observation strategies for the solar UV spectral irradiance

New observation strategies for the solar UV spectral irradiance

Which solar EUV indices are best for reconstructing the solar EUV irradiance?

Sensitivity of upper atmospheric emissions calculations to solar/stellar UV flux

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