Unlike today's blank sun, the solar disk 20 years ago was peppered with sunspots, including a monster named "AR9393." The biggest sunspot of Solar Cycle 23, On March 29, 2001, AR9393 hurled a pair of CMEs directly toward Earth. The first one struck during the early hours of March 31, 2001. The leading edge of the shock front was dense (~150 protons/cc) and strongly magnetized -- traits that give rise to powerful geomagnetic disturbances. Within hours, an extreme geomagnetic storm was underway, registering the maximum value of G5 on NOAA storm scales.
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