Solar UV-fluctuations underestimated

With approximately 60 percent, the ultraviolet radiation from the Sun contributes more significantly to the total irradiance variations of our star than previously thought.

The fluctuations in the Sun’s irradiance over the course of several years and decades are an important parameter in climate models. Researchers led by the Max Planck Institute for Solar System Research (MPS) in Germany have now shown that so far in such models  the influence of the of the ultraviolet (UV-) radiation has been underestimated. The UV radiation is considered the part of the solar radiation that is most relevant for climate studies. Instead of almost 30 percent, as previously thought, the UV radiation contributes around 60 percent to the overall variability of solar irradiance. Unlike other approaches, the MPS-researchers determine the solar irradiance on the basis of magnetic processes on the Sun itself: they evaluate the ever-changing number and brightness of bright and dark areas on the solar surface.

How bright does the Sun shine? And how bright did it shine ten, hundred and thousand years ago? These questions are crucial for our understanding of how our star effects Earth’s climate – and notoriously difficult to answer. Indeed, it has been known for more than a hundred years, that the Sun is a variable star subject to an approximately eleven-year-cycle. However, direct measurements of solar irradiance have been possible only since the beginning of the space age. The decisive quantity is the radiation incident on the Earth's atmosphere. From there it can even  influence processes on the surface. Since 1978, satellites provide such data. However, since each satellite is designed a little bit differently and the sensitivity of the instruments decreases with time, the data are sometimes difficult to compare.

This makes it all the more important to calculate the Sun’s irradiance in numerical models and thus perhaps to be able to reconstruct data from the past. Efforts of this kind have been going on for many years. Usually, parameters that have proven to change in parallel with the solar irradiance are used as a starting point. The model developed by the researchers from the MPS is the first one to consider more strongly physical aspects on the Sun itself. "We derive the irradiance of the Sun from physical processes that take place on its visible surface," explains project leader Natalie Krivova from the MPS.

For this, a closer look at the Sun is necessary as made possible for example by NASA’s spacecraft Solar Dynamics Observatory. "The key to the Sun’s behavior can almost always be found in its very dynamic magnetic fields”, says Sami K. Solanki, director auf the department “Sun and Heliosphere” at the MPS and coauthor of the new publication. The magnetic field can prevent hot plasma from deep within the star from rising to the surface thus creating sunspots. These are dark areas on the solar surface, which are particularly numerous in times of high solar activity. Near the sunspots, so-called faculae can be found, areas of increased brightness. "All in all, despite the many sunspots the Sun shines more brightly during its peak of activity “, says Krivova.

The researchers analyze images of the solar surface and measurements of its magnetic fields and use this as a starting point for their model. "Data of this kind has existed for several decades," says Krivova. "The number of sunspots, however, has been measured and recorded for centuries." The approach of the Göttingen researchers thus also allows for a glimpse into the past of our star.

"Our model is very successful in reconstructing the most reliable satellite data”, Kok Leng Yeo from the MPS, first author of the study, describes. The results differ from those of other models especially when it comes to the influence of the UV radiation. Instead of the previously assumed almost 30 percent, this part of the solar spectrum contributed approximately 60 percent to the overall variations of the Sun’s irradiance during the period between 1992 and 2004. According to the new study, divergent results of other models can be attributed mainly to unreliable data.

Overall, the intensity fluctuations of solar radiation are small. In long-term average they amount to only the fraction of a percent of the total irradiance. The ultraviolet radiation, however, shows greater fluctuations and is also regarded as particularly climate-effective. Since the Earth's atmosphere absorbs this radiation to a large extent, it influences critical chemical reactions in the upper layers of the atmosphere. Indirectly, these processes can also affect the temperature at the Earth's surface.

The extent, to which the higher UV fluctuations affect the result of climate models and forecasts, is unclear. "From our perspective, it is important to use the most accurate values ??for the influence of the Sun," says Krivova. In the past, calculations had shown that the Sun indeed influences the climate, but is not largely responsible for the sharp rise in global temperature seen in recent decades. "We do not expect this trend to change fundamentally," says Krivova.