SOLAR VARIABILITY FROM 240 TO 1750 nm IN TERMS OF FACULAE BRIGHTENING AND SUNSPOT DARKENING FROM SCIAMACHY

Abstract

The change of spectral decomposition of the total radiative output on various timescales of solar magnetic activity is of large interest to terrestrial and solar-stellar atmosphere studies. Starting in 2002, SCIAMACHY was the first satellite instrument to observe daily solar spectral irradiance (SSI) continuously from 230 nm (UV) to 1750 nm (nearinfrared; near-IR). In order to address the question of how much UV, visible (vis), and IR spectral regions change on 27 day and 11 year timescales, we parameterize short-term SSI variations in terms of faculae brightening (Mg ii index) and sunspot darkening (photometric sunspot index) proxies. Although spectral variations above 300 nm are below 1% and, therefore, well below the accuracy of absolute radiometric calibration, relative accuracy for short-term changes is shown to be in the per mill range. This enables us to derive short-term spectral irradiance variations from the UV to the near-IR. During Halloween solar storm in 2003 with a record high sunspot area, we observe a reduction of 0.3% in the near-IR to 0.5% in the vis and near-UV. This is consistent with a 0.4% reduction in total solar irradiance (TSI). Over an entire 11 year solar cycle, SSI variability covering simultaneously the UV, vis, and IR spectral regions have not been directly observed so far. Using variations of solar proxies over solar cycle 23, solar cycle spectral variations have been estimated using scaling factors that best matched short-term variations of SCIAMACHY. In the 300-400 nm region, which strongly contributes to TSI solar cycle change, a contribution of 34% is derived from SCIAMACHY observations, which is lower than the reported values from SUSIM satellite data and the empirical SATIRE model. The total UV contribution (below 400 nm) to TSI solar cycle variations is estimated to be 55%.

Intercomparison of SCIAMACHY and SIM vis-IR irradiance over several solar rotational timescales

“…The knowledge on how it varies as a function of wavelength is a key in understanding solar-stellar (Hudson 1988;Berdyugina 2005;Nandy & Martens 2007;Hall 2008;Priest 2009), and solar-terrestrial connections (Hoyt & Schatten 1997;Lean 1997;Lean & Rind 2001;Arnold 2002;Fröhlich & Lean 2004;Haigh 2003Haigh , 2007Rind et al 2008;Domingo et al 2009;de Wit & Watermann 2010;Gray et al 2010). Our present understanding of solar spectral irradiance (SSI) variability is based on direct SSI measurements in the UV (Lean 1987;Woods & Rottman 2002;Rottman et al 2004) and visible-infrared (vis-IR) regions (Harder et al 2005a,b;Pagaran et al 2009). Regular daily UV measurements from space began in the late 1970s, while vis-IR measurements just started in the 2000s with the launch of SCIAMACHY and SIM (see Fig.…”

“…Since the launch of SCIAMACHY aboard ENVISAT (2002-present) (Bovensmann et al 1999;Skupin et al 2005a,b;Gottwald et al 2006) and SIM aboard SORCE (2003-present) (Harder et al 2000(Harder et al , 2005a, first quantitative statements on the variability of solar output in the visible and near-IR could be made. See for example, Fontenla et al (2004); Unruh et al (2008); Harder et al (2009) ;Pagaran et al (2009). Despite the limited time coverage of these direct SSI measurements, they provide the starting point for reconstructing SSI in the pre-satellite, telescopic and even pre-telescopic era Tobiska et al 2000;Lean et al 2005;Krivova et al 2006;Tobiska & Bouwer 2006;Pagaran et al 2009;Krivova et al 2009Krivova et al , 2011 these SSI reconstructions are nevertheless used, for example, as "realistic" solar input to general circulation models (GCMs) in assessing the overall role of the changing sun in a changing terrestrial climate (Haigh 2003(Haigh , 2007de Wit & Watermann 2010).…”

“…During an 11-year solar cycle, TSI varies by 0.1% between solar maximum and minimum or during a 27-day solar rotation by up to 0.2-0.3% depending on the level of sunspot activity (Fröhlich & Lean 2004;Rottman 2006). About half of TSI variability comes from the UV, i.e., about 30-60% Krivova et al 2006;Pagaran et al 2009), the remainder from vis-IR. However, not all spectral regions vary in the same phase with TSI as shown by SCIAMACHY (Pagaran et al 2009) and SIM (Harder et al 2009) observations.…”

“…About half of TSI variability comes from the UV, i.e., about 30-60% Krivova et al 2006;Pagaran et al 2009), the remainder from vis-IR. However, not all spectral regions vary in the same phase with TSI as shown by SCIAMACHY (Pagaran et al 2009) and SIM (Harder et al 2009) observations. Even though the radiometric calibration in SCIAMACHY, primarily an earth atmosphere sounder, has a somewhat lower priority than A&A 528, A67 (2011) Fig.…”

“…for dedicated solar satellite spectrometry like SIM, its stability is sufficient to detect changes, e.g., at the per-mill level in vis-IR, over brief periods like 27-day solar rotation (Pagaran et al 2009). As an advanced version of GOME Weber et al 1998) aboard ERS-2, SCIAMACHY is a multichannel grating spectrometer, whose primary aim is the retrieval of trace gases in the Earth's atmosphere on a global scale.…”