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Solar Irradiance Variability: Insights into Long-Term Changes of the Sun

This article discusses the construction of a composite Total Solar Irradiance (TSI) dataset from 1975/1978 to present and explores the variability and uncertainties of TSI by comparing it with simultaneous measurements from other spacecraft. It also examines the role of surface magnetic structures in explaining TSI variability and investigates long-term trends of TSI and spectral irradiance.

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Solar Irradiance Variability: Insights into Long-Term Changes of the Sun

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  1. What can observed solar irradiance variability tell us about possible long-term changes of the Sun? Claus Fröhlich Physikalisch-Meteorologisches Observatorium Davos World Radiation Center CH 7260 Davos Dorf Jürg Beer & Friedhelm Steinhilber Swiss Federal Institute for Environmental Science and Technology (EAWAG), CH 8600 Dübendorf SPD-AAS/AGU Meeting, Fort Lauderdale, May, 27-30 2008

  2. Outline • Construction of a composite TSI from 1975/1978 to today • Discussion of TSI variability and its uncertainties by comparison with simultaneous measurements from other S/C • How much can be explained by models based on surface magnetic structures • What about long-term trends of TSI and spectral irradiance? • Conclusions SPD-AAS/AGU Meeting, Fort Lauderdale, May, 27-30 2008

  3. Construction of a composite TSI • Since 1978 reliable TSI measurements from space are available from the following overlapping missions with electrically-calibrated radiometers (ECR): • HF (Hickey-Frieden) on NIMBUS7 (17/11/1978 - 24/01/1993) • ACRIM-I on SMM (16/02/1980 - 01/06/1989) • ERBE on ERBS (24/10/1984 - 12/03/2003) • ACRIM-II on UARS (06/10/1991 - 27/09/2001) • SOVA on EURECA (11/08/1992 - 15/05/1993) • VIRGO (PMO6V, DIARAD) on SOHO (07/02/1996 - ………) • ACRIM-III on AcrimSat (05/04/2000 – ………) • TIM on SORCE (25/02/2003 - ………) • For all composite results from HF, ACRIM-I and II and during the last cycle VIRGO, ACRIM-III or DIARAD/VIRGO are used. • ERBE data are used as an independent dataset for comparison – mainly during the period of ‘ACRIM Gap’. As its sampling is so sparse (once every 14 days for a few minutes). For the PMOD composite daily data from a proxy model are also used for interpolation between the measured ERBE points. SPD-AAS/AGU Meeting, Fort Lauderdale, May, 27-30 2008

  4. PMOD Composite is used for further discussions SPD-AAS/AGU Meeting, Fort Lauderdale, May, 27-30 2008

  5. Comparison with other TSI • Before we go on we need to be sure that we have data representing solar variability. That is, we need to demonstrate that trend of TSI as observed during cycle 23 is not an instrumental artifact. • We do this by comparison of VIRGO which covers most of cycle 23 with ACRIM II on UARS and TIM on SORCE. • The degradation correction for SORCE has changed from version 7 to 8 and the slope almost doubled from 58 to 108 ppm/dec. • Up to now we have stated that the uncertainty of a change between minima is about 50 ppm. With this new result we will have to increase it to about 70 ppm. SPD-AAS/AGU Meeting, Fort Lauderdale, May, 27-30 2008

  6. Proxy Model of Irradiance Variations • Proxy models based on surface magnetic structures can quite well describe the influence of sunspots, faculae and network on TSI and with great success also the variation of the UV irradiance. • The darkening due to sunspots is described by PSI which is derived from the SOON database describing the sunspots with their area and position on the disk. With some assumptions about the contrast the darkening can be calculated quite accurately. One can also use white light images to directly measure the areas of the umbra and penumbra. • For faculae and network the direct observation of the areas is difficult. So, they are derived from plage areas as observed in Ca images or from magneto grams. The MgII Index is not basically different from e.g. plage areas from Ca images, but is available as daily values from space measurements which started at the same time as the TSI observations on NIMBUS-7. • No proxy shows a long-term trend by e.g. changing its values of the minima. They stay within a few percent of the cycle amplitude. And such a behaviour is confirmed back to the early 20th century with CaK images from Mt. Willson (see session ‘A century of CaK’ on Friday). SPD-AAS/AGU Meeting, Fort Lauderdale, May, 27-30 2008

  7. Proxy Model of Irradiance Variations • The Mg index can be divided into short and long-term parts representing the faculae and network within and outside active regions. • It is interesting to note that the coefficients determined by multi-linear regression for the short and long-term differ by about a factor 1.5, with the latter being higher. This may indicate that part of the long-term may not really be due to the effects described by the proxy, but due to some other mechanism, also related to activity. • But, as this index does not provide a long-term change of the minima we need to introduce a trend for each cycle separately to ‘explain’ the changes of TSI from minimum to minimum. SPD-AAS/AGU Meeting, Fort Lauderdale, May, 27-30 2008

  8. Where may the trend of the TSI minima come from? • Not only the MgII index does not change substantially between minima, but also F10.7, the CaK index and more obviously the sunspot number and PSI. • The open field BR of the Sun seems to show changes between minima which are very similar to the ones observed in TSI. Also the magnitude of these changes relative to the amplitude of the cycle are of the same order. • The question why only the minima seem to be important for TSI is not definitively answered. There is a hint: the short-term variance of BR is very high, so high that only 27-day averages can be reasonably plotted. So removing the short term would possibly leave a very small cycle amplitude which may even be comparable to 10% of the cycle variation which according to Foukal and Bernasconi (2008) may not be explained by the surface structures. • Another question is why is the isotropic BR related to TSIwhich is only irradiance in the ecliptic. The open field is isotropic and it might be representative for luminosity, indicating that the connection is through a photospheric temperature change related to activity. This would explain, that only TSI is affected and not the spectral irradiance which depends on magnetic changes in the solar atmosphere and not on temperature change. • Anyway we may do the correlatation. SPD-AAS/AGU Meeting, Fort Lauderdale, May, 27-30 2008

  9. What does all this mean for long-term changes of TSI • This sensitivity is determined from measured data, so it is an observational result. • It can directly be compared with the values determined independently by Wang et al. (2003) from flux transport calculations for the open field and a relation between the photospheric fields and TSI. The results is 0.58 Wm-2/nT. This is 1.3 times higher than our value. What does this mean? • The cosmic rays and thus the production of cosmogenic isotopes, such as 10 Be, is modulated by the interplanetary field, which is proportional to the open field. • Again, from observations this relation can be determined. Weighted mean relation: BR = -(0.57 ± 0.50) + (0.45 ± 0.03) BIMF • With this relation and the observed 10Be data during the Maunder Minimum we get for the open field a value of BR 0.3 nT. We conclude, that the minima of TSI during ths period were at about 1364.6 Wm-2 or about 0.9 Wm-2 below the value during the minimum between cycles 21 and 22. This corresponds to a somewhat smaller value than reported by e.g. Wang et al., 2003. SPD-AAS/AGU Meeting, Fort Lauderdale, May, 27-30 2008

  10. Conclusions • During the last 30 years the cycle averaged TSI shows a decline after 1980 of about 50 mWm-2 /decade (see Lockwood & Fröhlich, 2007, 2008). Comparing the minima values this is most pronounced in cycle 23. This most recent decrease cannot be explained by changes deduced from e.g. the MgII index or magneto grams, but needs a trend, the origin of which may be found in a temperature change with solar activity, represented by the strength of the solar open field. • Comparison with the observed BR ,allows to determine a sensitivity of TSI of about 0.45 Wm-2/nT. The long-term changes of the minima can be reconstructed back to 1890ties from magnetic measurements on earth and from 10Be production rate back to the Maunder Minimum and further. • An important result is that an activity related temperature change – probably also related to the overall magnetic fields - may be responsible for the long-term TSI change, which does not influence the spectral irradiance. Thus, SSI has no long-term trend, as shown by the proxies. It is only modulated by the strength of the cycle, as TSI. SPD-AAS/AGU Meeting, Fort Lauderdale, May, 27-30 2008

  11. This the end……….Thanks SPD-AAS/AGU Meeting, Fort Lauderdale, May, 27-30 2008

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