 The recovery phase may last as short as 8 hours or as long as 7 days.The size of a geomagnetic storm is classified as moderate (−50 nT > minimum of Dst > −100 nT), intense (−100 nT > minimum Dst > −250 nT) or super-storm (minimum of Dst < −250 nT).In 1931, Sydney Chapman and Vincenzo C. A. Ferraro wrote an article, "A New Theory of Magnetic Storms", that sought to explain the phenomenon.