Abstract
Abstract. We calculate the variation of spectral solar irradiance in the umbral shadow of the total solar eclipse of 21 August 2017 and compare it to observations. Starting from the Sun's and Moon's positions, we derive a realistic profile of the lunar shadow at the top of the atmosphere, including the effect of solar limb darkening. Subsequently, the Monte Carlo model MYSTIC (Monte Carlo code for the phYSically correct Tracing of photons In Cloudy atmospheres) is used to simulate the transfer of solar radiation through the Earth's atmosphere. Among the effects taken into account are the atmospheric state (pressure, temperature), concentrations of major gas constituents and the curvature of the Earth, as well as the reflectance and elevation of the surrounding area. We apply the model to the total solar eclipse on 21 August 2017 at a position located in Oregon, USA, where irradiance observations were performed for wavelengths between 306 and 1020 nm. The influence of the surface reflectance, the ozone profile, the mountains surrounding the observer and aerosol is investigated. An increased sensitivity during totality is found for the reflectance, aerosol and topography, compared to non-eclipse conditions. During the eclipse, the irradiance at the surface not only depends on the total ozone column (TOC) but also on the vertical ozone distribution, which in general complicates derivations of the TOC from spectral surface irradiance. The findings are related to an analysis of the prevailing photon path and its difference compared to non-eclipse conditions. Using the most realistic estimate for each parameter, the model is compared to the irradiance observations. During totality, the relative difference between model and observations is less than 10 % in the spectral range from 400 to 1020 nm. Slightly larger deviations occur in the ultraviolet range below 400 and at 665 nm.
Highlights
When modeling real world processes, we generally want to achieve one of two main goals: if measurements have not yet been performed, we can use the simulation to make predictions about the expected outcome and give recommendations for the experimental setup
We apply the model to the total solar eclipse on 21 August 2017 at a position located in Oregon, USA, where irradiance observations were performed for wavelengths between 306 and 1020 nm
Outside the totality at 17:12:00 UTC, a decrease in the total ozone column leads to an increase in irradiance like one would expect for a reduction of an absorbing gas
Summary
When modeling real world processes, we generally want to achieve one of two main goals: if measurements have not yet been performed, we can use the simulation to make predictions about the expected outcome and give recommendations for the experimental setup. If measured data exist, we can validate and enhance our conceptual understanding of the phenomenon under investigation by comparison with the model results. In this case, one can use the measurements to verify the model itself, in order to increase the credibility of model predictions. Zerefos et al (2000) performed measurements in the UV domain during the total solar eclipse in 1999 over Europe. They discussed the implications of solar limb darkening and the ratio of diffuse to direct irradiance during a solar eclipse on ozone measurements
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