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Analysis of historical solar observations and long-term changes in solar irradiance

dc.contributor.advisorKrivova, Natalie A. Dr.
dc.contributor.authorChatzistergos, Theodosios
dc.titleAnalysis of historical solar observations and long-term changes in solar irradiancede
dc.contributor.refereeSolanki, Sami K. Prof. Dr.
dc.subject.gokPhysik (PPN621336750)de
dc.description.abstractengThe Sun is the main external driver of Earth's climate. Various mechanisms of the solar influence on climate have been proposed. The debate is ongoing, but variation in the radiative flux of the Sun is among the main candidates. Direct measurements of the solar irradiance exist for merely 40 years, which is a rather short period to derive conclusions about any possible long term changes in solar irradiance and their possible influence on climate. The main driver of the irradiance variations on time-scales of days to decades, and possibly longer, is believed to be the solar surface magnetism. Models have been developed that reconstruct the irradiance by using appropriate proxies of the magnetic activity of the Sun. Irradiance models require input data representing both dark and bright magnetic features emerging at the solar surface. The most widely ever used proxies are the sunspot areas (available since 1874), the sunspot number (available since 1700), and the sunspot group number (available since 1610). However, these records do not provide direct information on bright features. Their evolution has to be inferred from the sunspot data via certain assumptions whose justification is still very unclear. Therefore, there is a strong need for a more direct facular proxy. %in order to resolve the ambiguities of the models. Ca II K full-disc spectroheliograms are uniquely suited for that purpose. Observations in the Ca II K line started as early as in 1892 at various sites, providing a good temporal coverage of the whole 20th century. However, these data suffer from a variety of problems hindering their immediate applicability. The historical Ca II K observations are stored in photographic plates or films, which have a non-linear response to the incident radiation. Information on this relation has not been recorded for the majority of the historical observations. Furthermore, a plethora of artefacts have been introduced on these images during their various life stages. These artefacts need to be accurately accounted for in order to provide meaningful results from such data. We have developed a method to recover the relation for the response of the plates to the incident radiation by using information that is stored on the solar disc of the image. This method is based on the assumption that the darker parts of the quiet Sun regions remain unchanged in time. We have also developed a method of calculating the quiet Sun background of the images, which includes the centre-to-limb variation and takes into account various large-scale artefacts. We have shown that the accuracy of this method is greater than that of previously proposed techniques. We have also reassessed the relation between the magnetic field strength and the Ca II K contrast, by using a larger number of Ca II images than was done in earlier such studies. We find that this relation can be well described with a power law function, and the best fit parameters are unaffected by the activity level or the position on the disc. Hence this relation potentially allows a reconstruction of pseudo-magnetograms from the available Ca II K observations covering almost the whole 20th century, that can be used by irradiance models. The sunspot data (i.e. records of sunspot number), despite their extensive use, are not free of problems either. The process of cross-calibrating different records by individual observers has recently become a matter of debate. This debate brought to light that the majority of the methods used so far fail to take into consideration the non-linearity that arises due to different observing capabilities of the observers. We addressed the issue of the shape of the relation for the inter-calibration between different group sunspot number series. We have shown, with the aid of synthetic observations derived from the royal Greenwich observatory sunspot area records, that it is strongly non-linear, contrary to what is commonly assumed. We have developed a method to recalibrate the sunspot group number series with a non-linear non-parametric method based on daily statistics between the observers. Using Monte Carlo simulations we have accounted for the error propagation, which has not been done by any previous reconstruction. Our reconstruction of the group sunspot number favours moderate activity levels for the 18th and 19th century and supports the existence of the modern grand maximum of solar
dc.contributor.coRefereeKollatschny, Wolfram Prof. Dr.
dc.contributor.thirdRefereeUnruh, Yvonne Dr.
dc.subject.engsolar irradiancede
dc.subject.engCa II K spectroheliogramsde
dc.subject.engphotometric calibrationde
dc.subject.engsunspot number seriesde
dc.affiliation.instituteFakultät für Physikde

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