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 August 30, and in 2008 it was 37 on October 11. This is indicative of how quiet space weather has become in the past few years. In 2007, the planetary A index reached 30 on April 1 -- we have to go back to 2006 to find any geomagnetic activity as strong, when the planetary A index reached 63 and 104 on December 14-15.
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