New Observations Of Chromospheric CO

The detailed temperature structure of the solar photosphere and chromosphere still holds some mysteries. A key example is the challenge of trying to reconcile observations of spectral lines from the CO molecule into the 1-D models of the solar atmosphere. The line core temperatures of this molecular band suggest average temperatures around 200 K lower than expected, while observations at the limb indicate the presence of CO at what would be considered chromospheric heights. Observations at higher resolution and with additional context – provided by multi-wavelength datasets – might help resolve some of these puzzles.

In this work, I present observations of a solar pore observed in the CO 4.7 µm band with the McMath Pierce Solar Telescope. These observations were coordinated with simultaneous IBIS/DST observations in the Fe I 709.0 nm, Na I 589.6 nm, and Ca II 854.2 nm lines. These well understood lines, whose regions of sensitivity span the base of the photosphere to the middle chromosphere, provide a reference against which observations of the CO spectra may be compared.

Comparisons of the stronger, higher-forming CO lines with the atomic spectra suggest that different components of the CO spectrum are sensitive to regions of the solar atmosphere spanning the temperature minimum (z = 500 km above the surface of the photosphere) to chromospheric heights. Velocity phase spectra between the Fe and CO lines suggests the CO line core velocity is sensitive to the upper photosphere, while comparisons with the calcium line scans suggests that the CO line core intensity has significant contributions from both photospheric and chromospheric heights. Additionally, a time series of the CO line core intensity reveals several intermittent, CO-rich features, which appear primarily around the boundary of the pore. These “cold bubbles” occur on spatio-temporal scales which are distinct from the 5 minute oscillations and solar granulation, but are instead consistent with the chemical reaction timescale of CO at z = 1000 km.

A thorough understanding of the height of formation and behavior of these lines is particularly valuable in anticipation of observations of the 4.7 µm CO band at much better resolution from DKIST’s Cryo-NIRSP instrument.