Solar flares are known to trigger magnetic storms in Geospace. The induced radiation and particles may result in harmful consequences on satellites and high- altitude airplanes' operation and cause errors with satellite systems such as GPS. It is widely accepted that solar flares represent a sudden release of magnetic energy stored in the corona above the sunspots. Flares occur not only on the Sun. Recently, massive flares (called "superflares") of solar-like stars were detected, and such superflares are estimated to release energies far exceeding the solar ones. Astronomers consider the link between the superflares and the starspots, in which magnetic fields are strongly concentrated. At some level, our Sun and distant Sun-like stars share common magnetic properties like spots and flares. Therefore, we may deepen our understanding of such common physical mechanisms through detailed investigations of the Sun and observations of stars. Magnetic activity of the Sun and stars influences the planets orbiting around them. In particular, strong ultraviolet radiation from a central star ionizes and strips away the planets' atmospheres. Thus, the measurement of ultraviolet radiation from distant stars is critical in investigating the habitability (capability to support life) of the planets.

To understand the stellar magnetic activity, we need to study both the stellar visible surface and their overlying atmospheres. Understanding the release and transport of magnetic energy and the effect of sunspot evolution on the chromosphere and corona will lead to a better comprehension of the mechanisms of flares. With current technology, we can conduct spatially resolved observations only on the closest star, the Sun. One approach is to study the Sun as a distant star and aggregate the light from the Sun as if it were emitted from a remote, point-like source. As a result, we can inform stellar observational data by utilizing solar data, leading to a better understanding of the stellar magnetic activity.

The international research team led by Shin Toriumi (ISAS/JAXA) investigated how disk-integrated sunlight in different wavelengths varies with time when a sunspot group transits the solar disk. Namely, the scientists treated the Sun as a point source to imitate distant stars and derived information that can be applied to stellar observations. They analyzed the solar data in different wavelengths, ranging from visible light to X-rays obtained by four spacecraft, including the solar-observing satellite "Hinode" and NASA's "Solar Dynamics Observatory (SDO)." "Hinode" was launched in 2006 and "SDO" in 2010, and both continue to monitor the Sun by taking images of the entire solar disk at regular intervals. Out of these large, decade-long data sets, the team choose periods for which a single, isolated group of sunspots marched across the disk, plotted the resulting light curves of different wavelengths, and investigated the causes of variations.

For the first time, the team looked into how the transit of individual sunspots and magnetic fields changed the light curves at different wavelengths. As a result, they found strong correlations between the ultraviolet radiation (which are sensitive to the chromosphere) and the photospheric magnetic flux.

Figure 1 shows the light curves in different wavelengths for the case of a sunspot group transiting the solar disk. These curves are overlaid on the corresponding solar images, chosen to be at the point in time when the sunspots are located near the central meridian. First, in the visible continuum that corresponds to the photosphere (top left), the sunspots are darker than the surroundings, resulting in a dip in the light curve as the spot group crosses the solar disk. The magnetic flux measured in the photosphere (top middle) attains its peak value when the spot group is around the disk center. The ultraviolet light curves that originate from the matter at chromospheric (top right) and coronal temperatures (bottom middle) are also peaked when the spot group is near the central meridian. The investigation finds that ultraviolet light curves (bottom left), sensitive to transition- region temperatures, are sometimes darkened. The transition region is the thin atmospheric layer located between the chromosphere and the corona. Whereas the coronal and chromospheric intensities are enhanced, the intermediate layer shows, interestingly, the opposite behavior.

Why does this occur? The team calculated the relative amount of plasma having different temperatures (a property called "emission measure"). It was found that the density of plasma at temperatures between 600,000 Kelvin (1.1 million degrees Fahrenheit) and 800,000 Kelvin (1.4 million degrees Fahrenheit), corresponding to the transition region, was reduced in the region surrounding the sunspots (about 40% of the visible solar hemisphere) (see Figure 2, left). Compared to the spotless quiescent case,

Moreover, the research team revealed that the ultraviolet light curves that probe the chromosphere (Figure 1, top right) show good correlations with the photospheric magnetic flux (Figure 1, top middle). Additionally, time lags exist between the evolution of photospheric and coronal light curves, the lengths of which reflect the spatial extents of coronal loops. These results indicate that we may obtain information on starspots and magnetic fields on far distant stars by measuring light curves in multiple wavelengths, even though spatially resolved observations of these targets are not feasible.

DOI: 10.3847/1538-4357/abadf9