(a) The monthly sunspot number (same as Fig. 2a). (b) The seasonal variations of the GPS phase scintillation occurrence rate around noon (10–14 MLT) and (c) around midnight (22–02 MLT) as a function of MLAT and month of year. The solar elevation angles of 0° and 30° at 110 km altitude above Ny-Ålesund are shown in white curves. Note that the color scales are different in panels b and c. The green solid and dashed vertical lines present the spring and autumn equinoxes, respectively.