tokens: for further testing and investigation. However, large active regions often last a long time and erupt repeatedly, so there are good reasons to take this hypothesis seriously. For such an active region, the scenario that we have found in the 2012 July case could come up again without much difficulty: the earlier eruptions from the active region can precondition the preexisting ambient solar wind, and the later successive ones from the same active region, while moving through the preconditioned wind, may interact en route to 1 au. This situation could also happen for active regions that are eruptive and close to each other. Historical records of some extreme events seem to support our hypothesis. Around the 1859 Carrington event, which also occurred in a weak solar cycle, long-duration brilliant aurorae were observed from August 28 through September 3. Green Boardsen ( 2006 ) suggest that the active region erupted successively and that two closely spaced CMEs interacted in interplanetary space, leading to the extreme nature of the Carrington event. Similarly, long-duration low-latitude auroral displays were seen for almost nine nights during 1770 September. Hayakawa et al. ( 2017 ) indicate that multiple consecutive eruptions from the same active region, which may have resulted in the “perfect storm” situation as we propose in Paper I , must have occurred for such a long duration of auroral observations. A more recent case, the ultrafast CME in 1972 August, reached the Earth in only 14.6 hr, which is the shortest transit time ever observed. Knipp et al. ( 2018 ) suggest that two earlier eruptions from the same active region likely cleared the path for the subsequent ultrafast event, which is exactly our “preconditioning” idea discussed in Paper I and here in the current work. We speculate that some of the 2003 Halloween storms may also be “perfect storms,” including the preconditioning effect and/or CME–CME interactions, because several large active regions continuously erupted for a long time (e.g., Skoug et al. 2004 ; Gopalswamy et al. 2005 ). We think that the hypothesis is valuable in that it points out the reality of extreme events from today's Sun. They generally are not simple and can involve many reinforcing factors. While the paper appears to stress the importance of interplanetary evolution of CMEs, the “perfect storm” concept also includes solar aspects, such as the persistent eruptive nature of the active region and the simultaneous presence of several big active regions on the disk. A particular instance is that widespread magnetic connectivity between concurrent active regions leads to sympathetic eruptions (e.g., Schrijver Title 2011 ; Jin et al. 2016 ; Wang et al. 2016 ). A nearly “global storm” could be made under this solar-side “perfect storm” scenario. This may have occurred during the 2003 Halloween period, which had a few large active regions, including one that emerged quickly and became connected to the
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