Measurements

1.3.1.1 Measurements of TSI

Before the satellite era, the solar irradiance was measured as a constant value with a wide range from 1338 W/m²to 1428 W/m²(see reviews bySmith and Gottlieb 1974;Froehlich and Brusa 1981). The TSI measurements were so inaccurate that the subtle changes in the solar irradiance could not be detected.

Figure 1.9: Space-based TSI measurements covering the period of 1978 – 2018. Indivi-dual measurements of different satellites are shown by different colours (labelled in the plot). The monthly mean sunspot numbers are represented by dots in the bottom part of the plot. Courtesy of G. Kopp⁴.

Direct measurements of TSI by space-based instruments began with a series of satel-lites missions in 1978 (Willson et al. 1981; Rottman 1988; Floyd et al. 2003; Fröhlich 2012; Kopp 2014). Fig. 1.9 summarizes the space-based TSI measurements over time, with different colours indicating different satellites. The corresponding sunspot number is plotted at the bottom. It is noticeable that all the measurements share similar traits in the solar irradiance, such as the presence of the 11-year solar cycle and the short-term peaks and dips due to the passage of surface magnetic features across the solar disc (Will-son et al. 1981;Hudson et al. 1982;Foukal and Lean 1986). However, it is obvious that the absolute levels of these measurements (especially at early times) do not match each other. The TSI values measured by the SORCE/TIM⁵ are≈5 W/m²lower than other the measurements by other contemporaneous instruments, which themselves disagree by a few W/m².

These differences came from the optical (aperture) design. It has been demonstrated by the TSI radiometer Facility (TRF, Kopp et al. 2007) that the other instruments (e.g., ACRIMSAT/ACRIM⁶ and SoHO/VIRGO⁷) had a different arrangement of two apertures

4http://spot.colorado.edu/~koppg/TSI/

5Total Irradiance Monitor onboard the SOlar Radiation and Climate Experiment satellite.

6Active Cavity Radiometer Irradiance Monitor onboard the Active Cavity Radiometer Irradiance Moni-tor SATellite.

7Variability of SOlar Irradiance and Gravity Oscillations onboard the Solar Heliospheric Observatory.

1.3 Solar total and spectral irradiance (view-limiting and precision aperture), which allowed radiation to be deflected inside the cavity (Kopp et al. 2007; Fröhlich 2012). The optical design of TIM has successfully reduced the effects of scattered light and diffraction in the instrument. Following this, the measurements from other instruments have been calibrated and adjusted downwards to the TIM measurements. This lower level of TSI measurements has been validated by the recently launched Picard/PREMOS⁸ instrument (Schmutz et al. 2009, 2013). It was the only TSI radiometer, which was calibrated in vacuum at full TSI power levels prior to its launch. Therefore, the lower TSI level measured by the SORCE/TIM is likely accurate. In this thesis, we take the value of 1360.52 W/m²for the 2008 minimum (averaged over the period November 2008 to January 2009) based on the SORCE/TIM measurement (Kopp and Lawrence 2005;Kopp et al. 2005a,b).

Due to the limited lifetime of the instruments, all the measurements lasted not lon-ger than one full solar cycle, which makes it difficult to assess the long-term secular change. Due to the change in the instrumental sensitivity and their degradation over time, it is not trivial to calibrate all the measured records and to determine the long-term solar variability. To meet this need, three composites were, therefore, constructed: (1) PMOD (Fröhlich 2000,2003,2006,2009) (2) IRMB (also called RMIB,Dewitte et al. 2004), and (3) ACRIM (Willson and Hudson 1991;Willson 1997;Willson and Mordvinov 2003), as shown in Fig.1.10.

These three different composites agree with each other well in short-term changes while the difference in the long-term trend (see Fig. 2 in Solanki et al. 2013) is more critical, especially in many climate studies. For instance, the PMOD composite shows a continuous increasing trend from the solar activity minimum in 1986 to the 2008 mi-nimum. The ACRIM composite shows an increasing trend from the solar minimum in 1986 to the 1996 minimum and a decreasing trend from the solar minimum in 1996 to the 2008 minimum. The IRMB composite shows a continuous increasing trend from the solar minimum in 1986 to the 2008 minimum (opposite to the PMOD). A large part of the difference in the long-term trend among the three TSI composites is due to the correction of the early series measured by Hickey-Frieden (HF) radiometer onboard Nimbus-7/ERB⁹ (Fröhlich 2006,2012). Only PMOD composite record applies such correction while both ACRIM and IRMB composites do not.

1.3.1.2 Measurements of SSI

The SSI has been observed by various space-borne instruments over the last four decades.

Each instrument covers different observation periods and spectral ranges, as summari-zed in Fig. 1.11. The early measurements of the SSI were limited to the UV wave-length range below 400 nm. As with TSI (Sect. 1.3.1.1), UV radiation (120 – 400 nm) was monitored almost without interruption since 1978, started with the measurement by

8Precision Monitor Sensor on board the Picard satellite.

9Earth Radiation Budget instrument onboard the Nimbus-7 satellite

1980 1985 1990 1995 2000 2005 2010 2015 1364

1366 1368

1980 1985 1990 1995 2000 2005 2010 2015 a) PMOD Composite

1980 1985 1990 1995 2000 2005 2010 2015 1358

1980 1985 1990 1995 2000 2005 2010 2015 1364

1366

1368 c) IRMB Composite

Total Solar Irradiance (Wm−2 )

Figure 1.10: Three composite records of TSI since 1978: (a) PMOD, (b) ACRIM, (c) IRMB. Courtesy of PMOD/C. Fröhlich¹⁰.

Nimbus-7/SBUV¹¹(Cebula et al. 1992). Since 1991, UARS/SOLSTICE¹²(Rottman et al.

1993) and UARS/SUSIM¹³(Brueckner et al. 1993) provided the two main UV observa-tion records (e.g.,Floyd et al. 2003). Later, ERS-2/GOME¹⁴ (launched in 1996;Weber et al. 1998;Munro et al. 2006) and ENVISAT/SCIAMACHY¹⁵ (launched in 2002; Sku-pin et al. 2005) started to measure a wider range of solar spectrum, in the 240 – 790 nm and 240 – 2380 nm, respectively. However, these two instruments were focused on at-mospheric sounding and lacked of in-flight degradation tracking, making them unsuitable for the study of solar SSI variation. With the launch of the SORCE satellite in 2003, a broad spectral range of SSI (from Lyman-αto 2 400 nm) with a high temporal resolution was available since 2004. The regular measurements were made by two instruments: the SORCE/SOLSTICE (Snow et al. 2005) and the SORCE/SIM (Harder et al. 2005,2009).

Consistent phenomena have been observed among the SSI measurements, revealing that the relative SSI variation is strongly wavelength-dependent and increases towards shorter wavelengths (Fig. 3 inSolanki et al. 2013). The visible and IR ranges have the

10https://www.pmodwrc.ch/en/research-development/solar-physics/tsi-composite/

11Solar Backscatter Ultraviolet Radiometer.

12Solar Stellar Irradiance Comparison Experiment onboard the Upper Atmosphere Research Satellite.

13Solar Ultraviolet Spectral Irradiance Monitor.

14Global Ozone Monitoring Experiment onboard the second European Remote Sensing satellite.

15SCanning Imaging Absoption spectroMeter for Atmospheric CHartographY on board the ENViron-mental SATellite.

1.3 Solar total and spectral irradiance

Figure 1.11: Timeline (X-axis) and spectral range (Y-axis, above 100 nm) of the SSI observations since 1965. Taken fromErmolli et al.(2013).

least variability (≈0.1%), whereas the variability of 1% to 100% are observed in the UV wavelengths (e.g.,Floyd et al. 2003). For extent details of the SSI measurements, we refer to the reviews byDomingo et al.(2009);Fröhlich(2012);Ermolli et al. (2013); Solanki et al.(2013) andYeo et al.(2014a).