[see, for example, Fox, 2004; Domingo et al., 2009]
Fλ(t) = (1−fF −fS)FλQ+fFFλF +fSFλS, (2.6) where superscripts Q, F, and S refer to the quiet sun, faculae, and sunspots, respectively; f stand for fractional area of faculae (fF) or sunspots (fS), Fλ for flux spectral distribution of the quiet sun (FλQ), faculae (FλF), and sunspots (FλS).
Examples of this model are Kurucz [Kurucz, 1993, 1995; Chance and Kurucz, 2010] and SATIRE (Spectral And Total Irradiance REconstructions) [Unruh et al., 1999; Krivova et al., 2003, 2006] models.
2.) Intensity/Emission spectra. Radiances are calculated based upon emissions assuming some temperature and pressure profiles of the solar atmosphere, which are characteristic for certain surface structures like sunspots. The flux is calcu-lated using the formula [see, for example, Fox, 2004; Domingo et al., 2009]
F(λ, t) =FQ(λ)
1 +
structures
∆Fstructures(λ) FQ
, (2.7)
whereFQis the quiet sun intensity expressed among other quantities the center-to-limb variation, ∆Fstructures is similar to FQ except it is a function of intensity contrast for all surface structures. In the computation of irradiances, the exact details can be found in Fox [2004].
Examples of this model are NRLEUV (Naval Research Laboratory Extreme Ultra-violet) [Lean et al., 2003] and its extension NRLEUV 2 [Warren, 2006].
3.) Intensity/Radiance spectra. The spectral irradiance is calculated [see, for ex-ample, Fox, 2004; Domingo et al., 2009]
I(λ, t) =
structures
µ
Istructures(λ, µ), (2.8)
as a sum of all contributing structures, each having individual intensities of fea-tures, whereIstructures(λ, µ)depends on the source function.
Examples of this model are SunRISE (Radiative Inputs from the Sun to the Earth) [Fontenla et al., 1999, e.g.] and SRPM (Solar Radiation Physical Modeling) [Fontenla et al., 2006, 2007, 2009].
spectral irradiances requires assumptions, for example, variations over several 27-day solar rotations can be linearly extrapolated to decadal 11-year solar cycle time scales;
this assumes that the physical mechanisms underlying irradiance variability over solar rotation and solar cycle timescales are very similar, and so on.
The following models have been used to reconstruct the past spectral irradiances. The time coverage of the reconstruction goes beyond the satellite era (1978–present). The satellite era covers three solar cycles 21 to 23.
• The Solar2000 (S2K), SIP (Solar Irradiance Platform) is an empirical irradiance model described in Tobiska et al. [2000] with its subsequent improvements de-scribed in Tobiska and Bouwer [2006].8 The model uses several observed irra-diances from a variety of sources, i.e., from rocket, aircraft, ground, and space-borne platforms. Among the many models the SIP model offers is the model S2K+VUV2002 in version SOLAR2000 Research Grade V2.33. VUV2002 (1–
420 nm) is based on FUV and UV irradiances from UARS beginning in 1991 as published in Woods and Rottman [2002] and TIMED/SORCE measurements beginning in 2002 that are modeled using daily F10.7 cm flux as proxy. Above 420 nm, the ASTM E-490 reference spectrum is used, whose integrated total irra-diance is scaled to agree with TSI [Fr ¨ohlich and Lean, 1998b]. No solar variability is modeled in the spectral region above 420 nm.
• The UV-vis-IR irradiance model by Lean et al. [1997, 2005], which is also called NRLSSI, calculates SSI empirically on a per-wavelength basis.9 Observed irra-diances are parametrized in terms of solar proxies of sunspot area and facular brightening. The NRLSSI solar proxy model has been adjusted to SEE/TIMED and SOLSTICE/UARS data in the 0–120 nm and 120–300 nm wavelength ranges, respectively. Above 300 nm, SSI is a composite of SOLSPEC up to 900 nm and the Kurucz spectrum at longer wavelengths. In this region model results of sunspot and facular contrasts from Unruh model are used [Lean, 2000b]. Fur-thermore, its integrated SSI from 120 to 105nm is constrained to agree with bolo-metric TSI.
• The SATIRE (Spectral And Total Irradiance REconstructions) model from Krivova et al. calculates solar irradiances (TSI and SSI) based on the assumption that variations are caused by magnetic fields at the surface [Solanki and Krivova, 2004b]. The model superposes representative model irradiances for quiet sun, sunspot umbrae and penumbrae, and networks that are based on magnetic sur-face observations from MDI (Michelson Doppler Imager) continuum images and ground-based observations [Kurucz, 1993; Unruh et al., 1999; Krivova et al., 2003, 2006]. Below 300 nm, a semi-empirical approach [Krivova et al., 2006]
8http://www.spacewx.com/solar2000.html
9http://lasp.colorado.edu/LISIRD/NRLSSI/NRLSSI.html
is used to extend to shorter wavelengths (down to 115 nm). The approach uses SUSIM/UARS and Mg II ctw ratio to obtain an improved estimate of solar cycle variations between 240 and 400 nm.
• The SCIA proxy model from Pagaran et al. calculates SSI empirically with the help of solar proxies. It is like NRLSSI but the basis of the modeled spectral irra-diances are selected timeseries measurements from SCIAMACHY. About 2/3 of the present dissertation deals with proxy-based spectral irradiance modeling, see Chapters 4 (Published Manuscript II and Pagaran et al. [2009]) for the details in developing the model and 5 (Published Manuscript III and Pagaran et al. [2011b]) for applications in reconstructing SSI variability over the recent solar cycles 21 to 23.
Chapter 3
SCIAMACHY solar measurements
3.1 Introduction and Motivation
Since 1978, solar spectral irradiance has been measured from different platforms in space and therefore without the influence of the Earth’s atmosphere. These measure-ments have been made for the most part in the UV range. In 1995, they then have been extended to the vis-IR spectral regions by GOME instrument [Weber et al., 1998;
Burrows et al., 1999]; in 2002 up to the SWIR region with SCIAMACHY aboard EN-VISAT [Bovensmann et al., 1999; Skupin et al., 2005a,b; Gottwald et al., 2006] and in 2003 with SIM aboard SORCE [Harder et al., 2000, 2005a,b]. These measurements show that while the UV variations are relatively large the vis-IR variations are relatively tiny, varying between about 0.2% and 0.4%. To observe these tiny vis-IR variations the instrument has to have a relative uncertainty of a few parts in 104 over its lifetime [Rottman et al., 1998]. How the solar measurements from SCIAMACHY and SIM com-pare to each other and to existing solar data especially in the vis-IR spectral regions needs to be investigated. The comparison is important on the quality of continuity and homogeneity of SSI datasets as obtained from different spectrometers.
SCIAMACHY is the first atmospheric sounder instrument to observe the sun on a daily basis from 240 to 1700 nm at a moderately high spectral resolution of 0.2 to 1.5 nm.
SIM, on the other hand, is the first solar-dedicated instrument to perform the same routine but at a lower spectral resolution of 0.25 to 33 nm. While SIM employs one optical element, the F `ery prism, SCIAMACHY uses a combination of predispersing prism and gratings in eight spectral channels. The latter combination ensures that certain spectral absorption features can be adequately resolved, allowing trace atmo-spheric constituents to be retrieved, for example, by the Differential Optical Absorption Spectroscopy (DOAS) method. The ratio of the upwelling radiance and SSI, the sun-normalised radiance, which is inverted to provide information about the amounts and
distribution of important earth atmospheric constituents, does not require absolutely calibrated spectral irradiance to first-order.
In general, direct solar measurements provide the starting point for reconstruction of past spectral irradiances [Lean et al., 1997; Tobiska et al., 2000; Lean et al., 2005;
Krivova et al., 2006; Tobiska and Bouwer, 2006; Krivova et al., 2009, 2011]. Vali-dation by comparison with other data is required in order to assess the accuracy of the solar spectral irradiance data and to assess how calibration and degradation with time may impact its use for reconstruction and long-term trends. Before using SCIA-MACHY irradiances for modeling purposes, validation with SIM and other existing data is performed. Reconstruction of past irradiances starting from SCIAMACHY data is presented in Chapters 4 and 5, Published Manuscripts II and III, respectively.
In the following sections, the objective, method, results, and my contributions to Pub-lished Manuscript I are briefly summarized. With kind permission from the publisher, European Southern Observatory (ESO), Published Manuscript I is then reproduced in full at the end of this chapter as published inAstronomy and Astrophysicsjournal.