Extreme Space Weather Events in Solar Cycle 24

Solar cycle 24 is a weak cycle that has generally lacked very energetic events as experienced at Earth. For example, there were only two ground level enhancement (GLE) events that were of modest strengths in comparison with 16 GLEs in solar cycle 23. However, STEREO, widely separated from the Sun-Earth line, witnessed very fast coronal mass ejections (CMEs) of backside origin, on July 23, 2012, and 2017 with the fluxes of energetic particles and various solar wind parameters increasing to unusual values. They could have been labeled as extreme events with potential to severely impact the Earth if they had occurred a week or so earlier. We examine these behind-the-limb events and other energetic events in September 2017, and compare them with previous events that had been put in to extreme categories. We discuss how difficult it would be to forecast these solar cycle 24 events before they occurred, on the basis of our current knowledge. We suggest the importance of the environment into which the eruptions were launched such as very low-density regions caused by previous CMEs and highly-compressed stream interaction regions, in addition to the peculiarities of the eruptions themselves.