Name/Affiliation: Klaus-Peter Schröder (Universidad de Guanajuato)
Solar cycle 24 UV radiation is lowest since decades
Using spectra taken by the robotic telescope "TIGRE" and its mid-resolution (R=20.000) HEROS double-channel echelle spectrograph, we present our measurements of the solar Ca II H+K chromospheric emission, using moonlight. For a direct comparison with historic observations since the early 1960ies, we derive Mount Wilson S-indices from the calibration given by O.C. Wilson and colaborators. Our measurements probe the double-peaked, weak activity cycle 24 at representative phases: in its inter-peak plateau and during its 2nd peak (last winter). Our respective S-values suggest that this maximum has the lowest chromospheric emission since at least 60 years - following the longest and deepest minimum since a century. These observations suggest a similar long-term (on a scale of decades) low of the far-UV radiation, which should be considered by the next generation of climate model. The current, very interesting activity behaviour calls for a concerted effort on long-term solar monitoring.