Estimating Electron Densities in the Middle Solar Corona using White-light and Radio Observations

TL;DR

This study uses low-frequency OVRO-LWA radio imaging to estimate coronal electron densities in the middle corona (1.7–3.5 R_), providing an independent diagnostic that complements white-light pB inversions and a data-driven MAS MHD model. Radio densities are obtained via an iterative, hydrostatic-approximation approach that accounts for ray refraction and uses brightness temperature as a diagnostic, yielding densities that agree with white-light results within about a factor of two. A representative density model is derived: $n_e(r)=10^6\ \text{cm}^{-3}[1.27(r/R_)^{-2}+29.02(r/R_)^{-4}+71.18(r/R_)^{-6}]$, valid from 1.7 to 3.5 R_, and the results are cross-validated against MAS predictions and standard density prescriptions (Leblanc, Saito, Newkirk). The work demonstrates that OVRO-LWA can routinely provide robust, independent coronal density estimates in a region poorly covered by traditional imaging, filling a crucial observational gap and enabling long-term monitoring of the quiescent corona.

Abstract

The electron density of the solar corona is a fundamental parameter in many areas of solar physics. Traditionally, routine estimates of coronal density have relied exclusively on white-light observations. However, these density estimates, obtained by inverting the white-light data, require simplifying assumptions, which may affect the robustness of the measurements. Hence, to improve the reliability of coronal density measurements, it is highly desirable to explore other complementary methods. In this study, we estimate the coronal electron densities in the middle corona, between approximately $1.7-3.5R_\odot$, using low-frequency radio observations from the recently commissioned Long Wavelength Array at the Owens Valley Radio Observatory (OVRO-LWA). The results demonstrate consistency with those derived from white-light coronagraph data and predictions from theoretical models. We also derive a density model valid between 1.7--3.5 $r_\odot$ and is given by $ρ(r')=1.27r'^{-2}+29.02r'^{-4}+71.18r'^{-6}$, where $r'=r/R_\odot$, and $r$ is the heliocentric distance. OVRO-LWA is a solar-dedicated radio interferometer that provides science-ready images with low latency, making it well-suited for generating regular and independent estimates of coronal densities to complement existing white-light techniques.

Figures (5)

• Figure 1: White-light total brightness (left column panels) and polarized brightness (right column panels) images from LASCO/C2 coronagraph observations for all the dates under study. The total brightness images are composited with extreme ultraviolet images at $193$ Å from SDO/AIA.
• Figure 2: Example radio images at 71 MHz for different days. Colors indicate the brightness temperature. The lowest contour level is at $5\%$ of the peak and then increases in multiples of 2. The black dashed circle represents the optical disk of the Sun. The white dashed circle indicates the heliocentric distance beyond which we apply the density estimation technique presented here, for this frequency. The white ellipse drawn at the bottom-left corner of each panel shows the corresponding instrumental resolution.
• Figure 3: Density estimates obtained on different dates are shown in separate panels. The date corresponding to each panel is indicated in the respective title. The comparison highlights the density estimates from our dataset, based on radio and white-light observations, consistent with previous findings and model-predicted densities.
• Figure 4: Red points show the densities derived using OVRO-LWA data from April 30, 2024. The blue line shows the fitted model. The fitted coronal density model is provided in the figure as well.
• Figure 5: Left panel: Shows the densities obtained by inverting solar maps at 71 MHz from April 26, 2024. Middle and right panel: Shows the density variation along 1D cuts through the center of the Sun in the sky plane along the equator and poles, respectively.