A strong geomagnetic storm on Monday and Tuesday -- April 5-6 -- was the biggest since 2006, at least in terms of high planetary A index. On those days, the mid-latitude A index was 28 and 22 and the planetary A index was 49 and 46. This was caused by a mighty solar wind stream. A search for similar numbers over the past few years yields nothing. In 2009, the highest planetary A index was 19 on...
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