Abstract:

Direct and reliable information of the variable sun is limited to the last 400 years of telescope-based observations of the sunspots on the sun’s surface. Satellite-based measurements of the solar energy input into the Earth’s system are even more limited to the last 40 years. In order to reconstruct solar activity over longer time scales one has to rely on indirect so-called proxy data.

Cosmogenic radionuclides are the most reliable proxies for reconstructing solar activity variations thousands of years back into the past. Several characteristics of solar variations have been identified in these records. These range from longer-term solar cycles (e.g. the 207-yr cycle), the bundling of solar minimum periods and possible longer-term changes in solar activity. However, quantitative reconstructions of solar activity still contain uncertainties that are illustrated by disagreements between different radionuclide-based solar activity reconstructions. 

New high-resolution data sets have recently revealed that solar storms can also leave significant signals in cosmogenic radionuclide records. This was quite unexpected since the known solar storms of the past 70 years were not powerful enough to leave significant signals in the proxy data.

I will present the different steps involved in solar activity reconstructions based on radionuclides and the comparison to the recently revised sunspot records. Furthermore, I will review our present knowledge on past solar storms that exceed anything that has been directly observed during the recent decades.