The St. Patrick's Day Storm

What happened

The strongest geomagnetic storm of Solar Cycle 24, reaching G4 (severe) levels on March 17, 2015. Two CMEs merged en route to Earth to produce a longer-than-forecast Kp 8 storm that drove aurora as far south as New Mexico and Oklahoma, disrupted GPS-based precision navigation, and became one of the most heavily studied storms of the modern satellite era.