Bridging Solar and Stellar Physics: Role of SDO in Understanding Stellar Active Regions and Atmospheric Heating

Abstract

The solar-stellar connection provides a unique framework for understanding magnetic activity and atmospheric heating across a broad spectrum of stars. Solar Dynamics Observatory (SDO) of NASA, equipped with the Helioseismic and Magnetic Imager, Atmospheric Imaging Assembly, and Extreme ultraviolet Variability Experiment, has enabled detailed Sun-as-a-star studies that bridge solar and stellar physics. Integrating spatially resolved solar observations into disk-integrated datasets, these studies provide insights into magnetic activity occurring in distant stars. This review highlights key results from recent analyses that employed all three SDO instruments to characterize active regions, quantify universal heating relationships, and reconstruct stellar X-ray and ultraviolet spectra. We discuss how these findings advance our understanding of stellar magnetic activity, provide predictive tools for exoplanetary environments, and outline future directions for applying solar-based frameworks to diverse stellar populations.

Figures (7)

• Figure 1: Representative full-disk images of the transiting sunspot (NOAA 12699) with corresponding light curves overlaid. Panels show HMI continuum and magnetogram, AIA 304, 171, and 131 Å channels, and Hinode/XRT. Images were taken at 00:00 UT on 2018 February 11. Figure was created based on 2020ApJ...902...36T.
• Figure 2: Schematic illustration of light curve variations during active region transits across the disk at different wavelengths. In each panel, the thick solid line shows the temporal variation of irradiance, whereas the reference quiescent level is indicated by the thin horizontal axis. The middle of the curves represents the time when the target active region is at the central meridian. Sunspots produce a central dip in visible and TSI curves; spotless plages remove this dip; and emerging fluxes introduce asymmetry about the central meridian in all wavelengths. Figure is reproduced from 2020ApJ...902...36T.
• Figure 3: Double-logarithmic scatter plots of solar irradiance versus total unsigned magnetic flux for five selected wavelengths, with power-law fits ($F \propto \Phi^{\alpha}$). Each panel shows the power-law index (slope) $\alpha$, correlation coefficient, and number of data points. Diamonds indicate the stellar data from the literature. Figure is reproduced from 2022ApJ...927..179T; however, the stellar magnetic flux for the stars was recalculated by correcting the error in the filling factor.
• Figure 4: Power-law index $\alpha$ derived from solar data plotted against the formation temperature of various spectral lines. Vertical bars show fitting errors; horizontal bars indicate temperature ranges for three X-ray observations. The $\alpha=1$ level is indicated by a sky-blue line. He I 10830 Å, which shows an anti-phased cycle variation against the solar activity, is indicated by an open circle. Colored symbols denote those compared with the stellar data in Figure \ref{['fig:cc']}. Figure is reproduced from 2022ApJS..262...46T.
• Figure 5: Sample synthesized spectra for stars having a total unsigned magnetic flux of 10$^{24}$, 10$^{25}$, and 10$^{26}$ Mx, derived using Equation (\ref{['eq:lambda']}). The model spectrum at a solar minimum value of $1.2\times 10^{23}$ Mx is also plotted in red as a reference. Figure is reproduced from 2023ApJ...945..147N.