|Publication type:||Doctoral thesis|
|Title:||Reconstruction of solar irradiance variations in cycles 21-23 based on surface magnetic fields|
|Advisors / Reviewers:||Stenflo, Jan Olof|
Solanki, Sami K.
|Publisher / Ed. Institution:||Cuvillier|
|Publisher / Ed. Institution:||Göttingen|
|Abstract:||The main aim of this thesis is to reconstruct solar irradiance variations over the current and the previous two solar activity cycles. The model is based on the assumption that all irradiance changes on time scales from days to the 11-year solar cycle scale are entirely caused by the evolving distribution of the magnetic field on the solar surface. The most successful recent reconstructions are based on the observations (magnetograms and continuum intensity images) performed by the Solar Oscillations Investigation/Michelson Doppler Interferometer (SOI/MDI) onboard the ESA/NASA satellite Solar and Heliospheric Observatory (SoHO) which are available from 1996 until now. However, it is important to extend the reconstruction over a longer time period (i.e. for the pre-SoHO period). This is possible with ground based observations of the National Solar Observatory/Kitt Peak Vacuum Telescope (NSO/KPVT), which date back to the year 1974. Two data sets are available. One of these is the set of the older magnetograph (NSO-512) before 1992. The other, newer data set is the record of the spectromagnetograph (NSO-SPM) from 1992 to 2003. In a first step we compare and intercalibrate the magnetograms as well as continuum images recorded by the NSO-SPM at Kitt Peak and the MDI on board SoHO in order to test how well the ground-based NSO-SPM data can be used to reconstruct total solar irradiance (TSI) variations. We demonstrate that NSO-SPM data can be employed to reconstruct TSI variations with almost the same accuracy as earlier shown for MDI data. In a second step we calculate the reconstructions of TSI also based on the NSO-SPM data, but for the whole time period of these data, i.e. from 1992 to 2003 (covering parts of cycles 22 and 23). The comparison with observational data gives a good correspondence for the whole period, with no bias between the two solar activity cycles on time scales longer that the solar rotation period, and suggests that the source of the irradiance variations is the same for cycles 22 and 23, namely the evolution of the magnetic flux at the solar surface. Next we include the older NSO-512 data set, which overlaps with the NSO-SPM data for a few months, back to the year 1974, i.e. from the minimum of cycle 21 to the declining phase of cycle 23. The reconstructed irradiance is compared with three composites of total solar irradiance measurements. A good correspondence is found with the total solar irradiance composite from PMOD/WRC, with no bias between the three cycles on time scales longer than the solar rotation period. This suggests that the same driver of the irradiance variations, namely the evolution of the magnetic flux at the solar surface, is acting in cycles 21, 22 and 23. The agreement with the other composites (ACRIM and IRMB) is less marked. In particular, a secular increase in the irradiance exhibited by these composites is not present in the reconstructions. This result implies that either no secular trend in the irradiance is present over cycles 21-23 or that any such trend is unrelated to surface magnetism. It weakens the claims in the literature for the presence of such a trend and has some implications for the influence of solar irradiance variations on climate in recent decades. Furthermore, based on a statistical analysis of the reconstructed total solar irradiance in cycles 21-23, we identify the contributions of different magnetic features on the solar surface to the variations of the total solar irradiance and evaluate the ratio of umbral to sunspot area. Finally, we study the latitude distribution of solar activity as presented by sunspot positions and areas using the five lowest statistical moments of the latitudinal distribution of all complete sunspot cycles since 1874 and compare these moments with each other. Remarkable correlations are found between some of the moments. The same analysis when applied to different dynamo models reveals significant differences between the models and demonstrates that such moments are a powerful diagnostic to distinguish between rival dynamo models. Such an analysis also provides the possibility for improved estimates of the butterfly diagram for earlier times when only sunspot numbers were available and hence to improve irradiance reconstructions for the period since the Maunder minimum.|
|Further description:||Ph.D. Thesis, Diss ETH No. 16199, Zürich|
|License (according to publishing contract):||Licence according to publishing contract|
|Departement:||School of Engineering|
|Appears in collections:||Publikationen School of Engineering|
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