Neon Insights from Old Solar X-rays: a Plasma Temperature Dependence of the Coronal Neon Content

TL;DR

This study analyzes solar X-ray data to show that the coronal Ne/O abundance ratio increases with plasma temperature, suggesting the solar neon abundance should be revised upward, impacting solar models and stellar activity understanding.

Contribution

It demonstrates a plasma temperature dependence of the coronal Ne/O ratio and proposes a revision of the solar neon abundance based on stellar activity evidence.

Findings

Ne/O ratio varies by a factor of two or more

Ne/O increases with active region plasma temperature

Solar neon abundance should be revised upward by about a factor of two

Abstract

An analysis using modern atomic data of fluxes culled from the literature for O VIII and Ne IX lines observed in solar active regions by the P78 and Solar Maximum Mission satellites confirms that the coronal Ne/O abundance ratio varies by a factor of two or more, and finds an increase in Ne/O with increasing active region plasma temperature. The latter is reminiscent of evidence for increasing Ne/O with stellar activity in low-activity coronae that reaches a "neon saturation" in moderately active stars at approximately twice the historically accepted solar value of about 0.15 by number. We argue that neon saturation represents the underlying stellar photospheric compositions, and that low activity coronae, including that of the Sun, are generally depleted in neon. The implication would be that the solar Ne/O abundance ratio should be revised upward by a factor of about two to n(Ne)/n(O)~0.3. Diverse observations of neon in the local cosmos provide some support for such a revision. Neon would still be of some relevance for reconciling helioseismology with solar models computed using recently advocated chemical mixtures with lower metal content.