Ground-based Observations of the Solar Sources of Space Weather (Invited Review)

Monitoring of the Sun and its activity is a task of growing importance in the frame of space weather research and awareness. Major space weather disturbances at Earth have their origin in energetic outbursts from the Sun: solar flares, coronal mass ejections and associated solar energetic particles. In this review we discuss the importance and complementarity of ground-based and space-based observations for space weather studies. The main focus is drawn on ground-based observations in the visible range of the spectrum, in particular in the diagnostically manifold H$\alpha$ spectral line, which enables us to detect and study solar flares, filaments, filament eruptions, and Moreton waves. Existing H$\alpha$ networks such as the GONG and the Global High-Resolution H$\alpha$ Network are discussed. As an example of solar observations from space weather research to operations, we present the system of real-time detection of H$\alpha$ flares and filaments established at Kanzelh\"ohe Observatory (KSO; Austria) in the frame of the ESA Space Situational Awareness programme. During the evaluation period 7/2013 - 11/2015, KSO provided 3020 hours of real-time H$\alpha$ observations at the SWE portal. In total, 824 H$\alpha$ flares were detected and classified by the real-time detection system, including 174 events of H$\alpha$ importance class 1 and larger. For the total sample of events, 95\% of the automatically determined flare peak times lie within $\pm$5 min of the values given in the official optical flares reports (by NOAA and KSO), and 76\% of the start times. The heliographic positions determined are better than $\pm$5$^\circ$. The probability of detection of flares of importance 1 or larger is 95\%, with a false alarm rate of 16\%. These numbers confirm the high potential of automatic flare detection and alerting from ground-based observatories.