Joint Diagnostics of Circumsolar Sky Brightness Using Coronagraphic Measurements and Aerosol Optical Inversions at Mauna Loa

Abstract

Atmospheric aerosols strongly influence daytime sky quality for solar coronal imaging, yet few studies directly link aerosol properties and sky-brightness measurements within ~2° of the Sun. Here we compare externally occulted coronagraphic measurements of near-Sun radiance with aerosol-constrained inferences derived from direct-Sun and sky photometry. Our analysis focuses on Mauna Loa Observatory, a well-characterized high-altitude site for atmospheric and solar observations. We present coronagraphic measurements of near-Sun radiance at 1.54 +/- 0.77° from solar disk center acquired between 2006 and 2007 by an ATST Sky Brightness Monitor (SBM). These data are directly compared with circumsolar radiances inferred at 1.54° using AERONET almucantar measurements and aerosol optical retrievals. We find quantitative agreement between these two approaches, enabling extension to multi-decadal analyses of circumsolar radiance and its relationship to aerosol properties and related proxies (e.g., the Angstrom exponent) using AERONET data from 2000-2025. Near-Sun radiances are expressed relative to the solar disk-center radiance, facilitating direct comparison with related studies. Finally, we synthesize physically based true-color images of the circumsolar sky under representative aerosol conditions as an observational aid, in part to illustrate that visually enhanced solar aureoles do not necessarily imply poor infrared coronal observing conditions. This methodology provides an extended framework for assessing daytime coronal sky quality at existing and future observing sites.

Figures (14)

• Figure 1: Diurnal evolution of circumsolar conditions on three Mauna Loa days representing Rayleigh-like (2006-12-20), fine-mode dominated (2007-04-13), and enhanced coarse-mode forward-scattering (2007-01-15) regimes. Top row: SBM near-Sun ($\Theta \sim\!1.54^{\circ}$) radiance normalized by solar disk-center radiance and corrected for airmass. Middle row: spectral AERONET aerosol optical depth; square symbols mark values inferred via the almucantar inversions. Thin error bars represent the variability of measurement triplets. Bottom row: NOAA/GML direct-normal irradiance (black) and diffuse-to-direct ratio (blue).
• Figure 2: AERONET aerosol property retrievals for the three representative cases shown in Figure \ref{['fig:compare3days']}. (a) Aerosol volume size distributions at the almucantar scan times. (b) Corresponding normalized aerosol phase functions, highlighting the enhanced forward scattering in the coarse-mode case that drives the strong near-Sun radiance observed by the SBM at $\sim\!1.54^{\circ}$ ($6\,R_{\odot}$).
• Figure 3: Comparison of measured AERONET almucantar sky radiances (circles) and analytical model radiances from Equation \ref{['eq:sky_lam']} (dashed curves) for three representative aerosol regimes at Mauna Loa: a Rayleigh-like day (2006-12-20), a fine-mode--dominated day (2007-04-13), and a coarse-mode forward-scattering day (2007-01-15). Panels (a)–(c) show radiances over the full range of scattering angles at four wavelengths, with the Rayleigh contribution indicated separately (dash-dotted lines). Panels (d)–(f) show an expanded view of the near-Sun region with a linear scaled y-axis in relative radiance units. Here, the independent co-temporal Sky Brightness Monitor (SBM) measurements are overplotted (diamonds).
• Figure 4: Wavelength dependence of near-Sun radiances at $\Theta \approx 1.54^{\circ}$ ($\approx 6\,R_{\odot}$) for the same three representative days shown in Figure \ref{['fig:almu_rad']}. AERONET-inferred radiances (circles) and SBM measurements (triangles) are normalized by the solar disk-center radiance and corrected for airmass. Dashed lines indicate best-fit power-law slopes, parameterized by $\gamma$.
• Figure 5: Two-dimensional histograms comparing AERONET-inferred circumsolar radiance at $\Theta \approx 1.54^{\circ}$, derived from sky-radiance inversions of almucantar measurements, with contemporaneous SBM near-Sun radiance measurements for four wavelength pairs. Radiances are expressed in units of $10^{-6}$ of the solar disk-center radiance per unit airmass. Each panel corresponds to one AERONET channel paired with the nearest SBM wavelength. Logarithmic binning emphasizes the dynamic range of the measurements. The solid line indicates the one-to-one relation whereas the dashed red lines show a $\pm$ 25% deviations from unity. One-dimensional histograms of the SBM measurements are shown in the background.