Date and time (in UT-hour) of MOD and SEV level AA based on Kp-index. Each block represents a 3-h period. Orange (solid) and red (crosshatched) shading indicate MOD and SEV level activity. Yellow (diagonal striped) shading indicates a 3-h gap in MOD or SEV activity observed during an event. Each line represents a different event. Grey shading indicates events observed during solar maximum years, similar to Figure 2a.