Technical Description of Figure:
The ~daily OMI solar index (line-core to line-wing ratios) measurements of 3 upper-Balmer series hydrogen lines between 400-500 nm are plotted [black] for a 1-year period. The total solar irradiance (TSI) [red] predominantly tracks the development and passage of big sun-spot groups on short time scales (months), and agrees well with the OMI Balmer line index. The TSI data were inverted, scaled and de-trended in order to help demonstrate this agreement. A solar index derived from the CH 430 nm spectral blend (tracking changes in the populations of solar faculae) is also shown [blue]. The Balmer line indices in the TROPOMI data set (not shown) behave identically to the OMI Balmer index.
Scientific significance, societal relevance, and relationships to future missions:
The flourishing field of exo-planet studies relies on robust detection techniques, which requires precise characterization of intrinsic variability patterns of the hosting stars to discriminate planetary signals. The stellar variability patterns are traced using various techniques (line-indices inclusive) and show a bewilderingly complex variety. Even for the best-studied star, our Sun, there is on-going debate on major drivers of the observed changes.
A seemingly simple question to ask is: Do all major spectral transitions (covering a wide range of the extended solar atmosphere) vary in unison on the solar-rotation (~month) time scale? Most UV-visible spectral lines predictably follow the rise and fall of numbers of the (hot) facula, as well as the changes in areas of active solar networks. However, we find an interesting exception while studying the 'textbook' hydrogen Balmer lines (Hβ,Hγ,δ). These lines have been used in numerous astrophysical applications for over a century. We employ the line-index approach (i.e. core-to-wing ratio) in order to minimize instrumental effects, which enables very high individual-measurement precision: ~80 ppm (part-per-million) for the OMI data shown here, and ~30 ppm in the TROPOMI measurements. This precision is essential for our conclusion that the studied Balmer lines defy the general trends and show high sensitivity to the rise and gradual decay of big sun-spot groups, instead of the anticipated governance by hotter solar areas. Our result may help in deciphering the complex stellar-activity patterns in search for exo-planets.
The ~daily OMI and TROPOMI solar irradiances are publicly available at : https://daac.gsfc.nasa.gov/
References: Marchenko, S., Criscuoli, S., DeLand, M. T., Choudhary, D. P., Kopp, G., Solar Activity and Responses Observed in Balmer Lines, Astronomy and Astrophysics, 646, 81-86, 2021.