[Paper Review] Analysis and modeling of solar irradiance variations

K. L. Yeo|arXiv (Cornell University)|Dec 12, 2014

Solar and Space Plasma Dynamics201 references3 citations

TL;DR

This paper presents a high-precision reconstruction of total and spectral solar irradiance (TSI and SSI) from 1974 to 2013 using magnetograms from KPVT, SoHO/MDI, and SDO/HMI, combined with the SATIRE-S model. By calibrating magnetic features' contributions to irradiance and correcting for instrumental effects like stray light, the model achieves consistency with multiple satellite observations, resolving long-standing discrepancies in absolute radiometry and spectral dependence during solar cycles.

ABSTRACT

A prominent manifestation of the solar dynamo is the 11-year activity cycle, evident in indicators of solar activity, including solar irradiance. Although a relationship between solar activity and the brightness of the Sun had long been suspected, it was only directly observed after regular satellite measurements became available with the launch of Nimbus-7 in 1978. The measurement of solar irradiance from space is accompanied by the development of models aimed at describing the apparent variability by the intensity excess/deficit effected by magnetic structures in the photosphere. The more sophisticated models, termed semi-empirical, rely on the intensity spectra of photospheric magnetic structures generated with radiative transfer codes from semi-empirical model atmospheres. An established example of such models is SATIRE-S (Spectral And Total Irradiance REconstruction for the Satellite era). One key limitation of current semi-empirical models is the fact that the radiant properties of network and faculae are not adequately represented due to the use of plane-parallel model atmospheres (as opposed to three-dimensional model atmospheres). This thesis is the compilation of four publications, detailing the results of investigations aimed at setting the groundwork necessary for the eventual introduction of three-dimensional atmospheres into SATIRE-S and a review of the current state of the measurement and modelling of solar irradiance. Also presented is an update of the SATIRE-S model. We generated a daily reconstruction of total and spectral solar irradiance, covering 1974 to the present, that is more reliable and, in most cases, extended than similar reconstructions from contemporary models.

Motivation & Objective

• To reconstruct total and spectral solar irradiance (TSI and SSI) over the satellite era (1974–2013) with high temporal and spectral resolution.
• To resolve persistent discrepancies between satellite measurements and models in absolute radiometry, secular variation, and spectral dependence of solar cycle variability.
• To improve the accuracy of solar irradiance reconstructions by incorporating corrected magnetograms and accounting for instrumental effects such as stray light and point spread function (PSF) distortions.
• To validate the model against multiple independent observations, including SORCE, TIM, and ISS measurements, ensuring consistency across platforms.
• To provide a reliable, publicly available time series of TSI and SSI for use in climate modeling and solar-terrestrial physics research.

Proposed method

• Utilizes the SATIRE-S model, which simulates solar irradiance based on photospheric magnetic field distributions derived from full-disk magnetograms.
• Applies a multi-instrument harmonization technique to align KPVT, SoHO/MDI, and SDO/HMI magnetograms, ensuring consistent input data across missions.
• Corrects for instrumental effects such as the point spread function (PSF) and stray light in SDO/HMI data using lunar eclipse and limb observations to improve image fidelity.
• Employs a free parameter in the SATIRE-S model to optimize agreement with observed TSI and SSI, calibrated against satellite measurements.
• Performs error analysis to quantify uncertainties in the reconstructed TSI and SSI, including data gaps and instrumental drifts.
• Validates the model against independent observations from SORCE, TIM, and ISS-based instruments, particularly during the 2012 solar minimum.

Experimental results

Research questions

• RQ1 How accurately can total and spectral solar irradiance be reconstructed from photospheric magnetograms across multiple solar cycles?
• RQ2 To what extent do instrumental effects such as PSF and stray light distort the apparent properties of solar surface features and affect irradiance modeling?
• RQ3 What is the relative contribution of faculae and sunspots to the total solar irradiance variation over the 11-year cycle?
• RQ4 How do discrepancies in absolute radiometry and spectral dependence between satellite measurements and models arise, and can they be reconciled?
• RQ5 Can a unified model based on magnetograms from different instruments (KPVT, MDI, HMI) produce a consistent and reliable TSI and SSI time series over 40 years?

Key findings

• The SATIRE-S model successfully reconstructs TSI and SSI from 1974 to 2013 with high consistency across multiple satellite observations, including SORCE and TIM.
• The model shows that faculae contribute positively to TSI, while sunspots contribute negatively, with the net effect resulting in a 0.1% variation over the solar cycle.
• Stray light correction in SDO/HMI data significantly improves the accuracy of granulation and magnetic feature contrast measurements, reducing systematic biases.
• The reconstructed SSI shows good agreement with observations from the ISS and other instruments, particularly in the UV and visible ranges.
• The model resolves discrepancies in secular variation and spectral dependence, particularly during the 2008–2010 solar minimum, where previous models underestimated irradiance levels.
• The final TSI and SSI time series are publicly available at www.mps.mpg.de/projects/sun-climate/data.html for use in climate and space weather modeling.