Sun-as-a-star Multi-wavelength Observations: A Milestone for Characterization of Stellar Active Regions

It has been revealed that "superflares" can occur on solar-type stars. The magnetic energy  of the flares is likely to be stored in active-region atmospheres. Therefore, to explain the energy storage and occurrence of the flares, it is important to monitor the evolutions of the active regions, not only in visible light but also in ultraviolet (UV) and X-rays. To demonstrate this, we perform multi-wavelength irradiance monitoring of transiting solar active regions by using full-disk observation data. As a result of this sun-as-a-star spectral irradiance analysis, we confirm that the visible continuum that corresponds to the photosphere becomes darkened when the spot is at the central meridian, whereas most of the UV, EUV and X-rays, which are sensitive to chromospheric to coronal temperatures, are brightened, reflecting the bright magnetic features above the starspots. The time lags between the coronal and photospheric light curves have the potential to probe the extent of coronal magnetic fields above the starspots. These results indicate that, by measuring the stellar light curves in multiple wavelengths, we may obtain information on the structures and evolution of stellar active regions.