Storms on the Sun

Abstract

Solar flares and coronal mass ejections are the most important storm phenomena in the atmosphere of the Sun. The observation that a geomagnetic storm commenced only some 17 hours after the flare observed by Carrington and Hodgson in 1859, and many subsequent events suggesting a similar flare–storm relationship, led to the hypothesis that the flares were the drivers of the nonrecurrent magnetic storms at the Earth. The evidence for a causal connection from flares to storms was, however, not particularly good. Large flares can be observed without ensuing magnetic storms and storms, also nonrecurrent ones, often occur without any notable preceding flare activity on the Sun. But if it is not a flare, what would be the driver? The answer came with the first CME-observations using a space-borne coronagraph [Tousey, 1973]. The misconception of addressing the flares as primary storm drivers prevailed in some parts of the solar–terrestrial physics community for a long time, even after subsequent spacecraft observations of CMEs and their in situ characteristics in the solar wind had convincingly shown that CMEs are the main drivers of nonrecurrent magnetic storms. (For a discussion of this “solar flare myth”, see Gosling [1993].) Finally, the excellent SOHO coronagraph images of CMEs during solar cycle 23 brought the real storm drivers to the attention of the entire community concerned with severe space weather.