Two decades ago, developments in solar modeling revised numbers on the sun’s elemental abundances. Though this helped resolve some anomalies, it also resulted in a serious discrepancy between theory and observation regarding the location of the boundary between the convective and radiative zones — called the “CZ boundary.”

This discrepancy could be resolved by an increase in radiative opacities. Transmission opacity experiments identified a higher than predicted opacity for iron, but those findings remained at odds with plasma opacity theory.

Hoarty et al. conducted an experiment using an alternative technique to infer iron opacity at the temperatures and densities seen in the solar interior. At the National Ignition Facility (NIF), they conducted a radiation burn-through experiment combined with radiation-hydrodynamics calculations, a Bayesian statistical analysis technique, and machine learning.

Their results showed that under the conditions of the CZ boundary, the iron opacity was consistent with plasma opacity theory.

“In two NIF shots we have been able to answer the question of whether a high iron opacity can largely explain the CZ boundary problem in solar physics,” said author David Hoarty. “The answer is no. Solar physicists will have to discount iron opacity as an explanation.”

The findings will help guide theoretical solar physics and demonstrate an innovative experimental technique.

“Though transmission opacity experiments remain the best technique for opacity measurements in general, this work has shown that there is a place in opacity measurement for radiation burn-through techniques, combined with modern statistical analysis and machine learning,” Hoarty said.

Source: “Radiation burn-through measurements to infer opacity at conditions close to the solar radiative zone-convective zone boundary,” by David Hoarty, John Morton, Jonathan C Rougier, Michael Rubery, Yekaterina P Opachich, Damon Swatton, Scott Richardson, Robert F Heeter, K McLean, Steven J. Rose, Theodore Sonne Perry, and Bruce A. Remington, Physics of Plasmas (2023). The article can be accessed at