Measurements of spectral irradiance during the solar eclipse of 21 August 2017: reassessment of the effect of solar limb darkening and of changes in total ozone

Abstract

Abstract. Measurements of spectral irradiance between 306 and 1020 nm were performed with a GUVis-3511 multi-channel filter radiometer at Smith Rock State Park, Oregon, during the total solar eclipse of 21 August 2017. The radiometer was equipped with a shadowband, allowing the separation of the global (sun and sky) and direct components of solar radiation. Data were used to study the wavelength-dependent changes in solar irradiance at Earth's surface. Results were compared with theoretical predictions using three different parameterizations of the solar limb darkening (LD) effect, which describes the change in the solar spectrum from the Sun's center to its limb. Results indicate that the LD parameterization that has been most widely used during the last 15 years underestimates the LD effect, in particular at UV wavelengths. The two alternative parameterizations are based on two independent sets of observations from the McMath–Pierce solar telescope. When these parameterizations are used, the observed and theoretical LD effects agree to within 4 % for wavelengths larger than 400 nm and occultation of the solar disk of up to 97.8 %. Maximum deviations for wavelengths between 315 and 340 nm are 7 %. These somewhat larger differences compared to the visible range may be explained with varying aerosol conditions during the period of observations. The aerosol optical depth (AOD) and its wavelength dependence was calculated from measurements of direct irradiance. When corrected for the LD effect, the AOD decreases over the period of the eclipse: from 0.41 to 0.34 at 319 nm and from 0.05 to 0.04 at 1018 nm. These results show that AODs can be accurately calculated during an eclipse if the LD effect is corrected. The total ozone column (TOC) was derived from measurements of global irradiance at 306 and 340 nm. Without correction for the LD effect, the retrieved TOC increases by 20 DU between the first and second contact of the eclipse. With LD correction, the TOC remains constant to within natural variability (±2.6 DU or ±0.9 % between first and second contact and ±1.0 DU or ±0.3 % between third and fourth contact). In contrast to results of observations from earlier solar eclipses, no fluctuations in TOC were observed that could be unambiguously attributed to gravity waves, which can be triggered by the supersonic speed of the Moon's shadow across the atmosphere. Furthermore, systematic changes in the ratio of direct and global irradiance that could be attributed to the solar eclipse were not observed, in agreement with results of three-dimensional (3-D) radiative transfer (RT) models. Our results advance the understanding of the effects of solar LD on the spectral irradiance at Earth's surface, the variations in ozone during an eclipse, and the partitioning of solar radiation in direct and diffuse components.

Accurate 3D radiative transfer simulation of spectral solar irradiance during the total solar eclipse of August 21, 2017

“…The parametrization for this "limb darkening" effect used by Koepke et al (2001) is based on theoretical considerations from Waldmeier (1955). For our study, we follow the recommendations by Bernhard and Petkov (2019) and implemented the more accurate parametrization developed by , based on two measurement campaigns , ).…”

“…In the following, we will apply the model to simulate solar irradiance for the total solar eclipse 2017 at a location in Smith Rock State Park near Terrebonne, Oregon. Irradiance measurements from this site are available from Bernhard and Petkov (2019). At first, we will rebuild the environmental and atmospheric conditions at the measurement site, starting from a basic setup which will serve as a proof of concept.…”

“…were taken from the midlatitude summer profiles by Anderson et al (1986). The thereby specified molecular ozone profile was scaled to a total ozone column of 298 DU, corresponding to the value derived in Bernhard and Petkov (2019). The other profiles are not modified and aerosol is not included.…”

“…For the total solar eclipse over North America on August 21, 2017, different groups performed irradiance measurements, e.g. Calamas et al (2018) and Bernhard and Petkov (2019). The latter measured solar irradiance at 14 different wavelengths between 306nm and 1020nm with a sensitivity high enough to yield significant results also during totality.…”

“…The importance of limb darkening, in particular when deriving ozone, is also supported by findings from Groebner et al (2017) for the solar eclipse on March 20, 2015. To get a general overview of the results from many studies on atmospheric changes caused by solar eclipses, the reader is referred to Aplin et al (2016) In this study, we perform 3D model calculations for the conditions during the observations at totality by Bernhard and Petkov (2019). The comparison of the modelling results to the observations is an validation of our 3D radiative transfer model from ultraviolet to near infrared.…”