Modeling spectral and total solar irradiance variability from any vantage point in the Solar System
by Isabela de Oliveira Martins
Date of Examination:2024-09-25
Date of issue:2024-12-17
Advisor:Prof. Dr. Laurent Gizon
Referee:Prof. Dr. Laurent Gizon
Referee:Prof. Dr. Stefan Dreizler
Referee:Prof. Dr. Ariane Frey
Referee:Dr. Natalie A. Krivova
Referee:Dr. Joanna Drazkowska
Referee:PD Dr. Olga Shishkina
Persistent Address:
http://dx.doi.org/10.53846/goediss-10832
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Description:Thesis Dissertation
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Abstract
English
The Sun is the primary external energy source for all planets in the Solar System. Its radiative flux, known as solar irradiance, plays a crucial role in planetary atmosphere modeling. Accurate spectral solar irradiance (SSI) measurements are essential for understanding the temperature, composition, and dynamics of planetary atmospheres, as well as their climate and weather patterns. Climate models depend on accurate SSI data to simulate solar forcing effects. Measurements of SSI at Earth are available from spacecraft missions, but similar data for other planets are limited. Mars is the only planet with SSI measurements, obtained by the Mars Atmosphere and Volatile EvolutioN spacecraft. The lack of SSI measurements for other planets hinders the study of solar influence on planetary atmospheres and climate. To address this gap, my dissertation proposes to use information about magnetic activity on both the near and far sides of the Sun to estimate solar irradiance variability from any vantage point in the Solar System. Near side measurements are obtained through direct observations, while far side information is derived from helioseismic holography. Far side information can also be obtained by using a surface flux transport model. This comprehensive approach enables sufficiently accurate irradiance variability estimations for planetary studies using the Spectral And Total Irradiance REconstruction (SATIRE) model, which attributes irradiance variations over periods longer than a day to the emergence and evolution of solar surface magnetic features -- i.e., sunspots and faculae. Irradiance is then calculated as the sum of contributions from the quiet Sun and these magnetic features. Others have also addressed this gap by interpolating Earth-based irradiance measurements and scaling them according to the Sun-planet distance in order to estimate solar irradiance variability at other planets. However, this interpolation approach overlooks the inherent solar irradiance variability over various timescales and the evolution of solar surface magnetic fields. The estimation of solar irradiance variability relies on area coverage calculations of magnetic features. In Chapter 2, synthetic full surface magnetograms generated by a Surface Flux Transport Model are converted into sunspots and faculae areas, and the wavelength-integrated spectral irradiance (total solar irradiance, or TSI) and the S-index (a proxy for ultraviolet irradiance variability) are calculated. The results demonstrate that simple phase-shifted Earth-based irradiance measurements are unreliable for other planets due to the dynamic nature of sunspots and faculae. The study finds agreement in S-index variability between the traditional interpolation method and the new approach, since this variability is dominated by faculae, which have longer lifetimes and cover a larger portion of the solar surface. However, significant discrepancies are observed in TSI variability estimates, particularly because sunspots, which are short-lived magnetic features that dominate the TSI variability, are not adequately captured by the interpolation approach. This finding highlights the limitations of an interpolation method that disregards far side solar activity. Chapter 3 builds on the previous chapter by addressing the challenge of estimating solar irradiance variability with limited data from the far side of the Sun. High-resolution magnetograms and continuum intensity images used for Earth-based irradiance reconstruction are unavailable for the far side, and therefore, magnetic field data alone must suffice. Here, I use Far side Active Region Magnetograms (FARM) -- which are derived from far side seismic images -- to estimate the area coverage of sunspots and faculae on the far side of the Sun. The results show that magnetic field data can serve as a reliable proxy for the area coverage of sunspots and faculae, enabling the estimation of solar irradiance variability at different positions in the ecliptic plane. The estimates are not as accurate as those obtained from the SATIRE model, but they provide a more reliable alternative to the interpolation method when near side data are unavailable.
Keywords: Solar Irradiance; Solar Irradiance Variability; Sun; Spectral Solar Irradiance; Total Solar Irradiance; Solar Surface Magnetic Fields; Sunspots; Faculae; Solar Irradiance Reconstruction; S-Index; Helioseismology; Helioseismic Holography
