The Cool Sun: New Insights from Cyra, ALMA, and Bifrost

Due to its abundance in the outer photosphere, the infrared vibrational spectrum of carbon monoxide (CO) serves as a powerful diagnostic of the temperature in the intermediate layers of the solar atmosphere – including the temperature minimum and photosphere/chromosphere interface. Confoundingly, the center-to-limb variation and limb extension of the lines indicate that cool (< 4000 K), CO-rich gas may permeate the solar atmosphere up to a height of 1000 km above the solar surface, instead of the traditional chromospheric temperature rise at z ~ 500 km predicted from continuum and atomic diagnostics (such as the reversal in the wings of Ca II H&K).

In this work, I present new observations of a portion of the fundamental (∆v=1) ro-vibrational band of CO, taken by the cryogenic spectrograph at the Goode Solar Telescope (GST/Cyra) at the Big Bear Solar Observatory between October 2021 and May 2022. These observations show several transient “cold bubbles”, which exhibit lower brightness temperatures and enhanced CO formation compared to the quiet Sun, some of which appear to be linked to small-scale flux emergence in the solar photosphere. By comparing these observations to synthetic CO spectra computed from MHD simulations of the solar atmosphere, I investigate how such features form in the solar atmosphere, and to what extent they might contribute to the “high altitude” CO seen above the solar limb. Additionally, I present the first coordinated observations of solar CO with the millimeter continuum (obtained from the Atacama Large Millimeter/submillimeter Array, or ALMA), in order to investigate the relation of these events to the “Chromospheric ALMA Holes” formerly observed by ALMA.