WEAK IMPACT, BRIGHT LIGHTS: A weak CME impact on July 9th gave sky watchers more than they bargained for. Although the blow was feeble, both ends of the Earth lit up with auroras that persisted for nearly two days. Minoru Yoneto photographed the display from Queenstown, New Zealand:
The CME was effective in sparking high-latitude auroras because its wake contained a south-pointing magnetic field--a.k.a. "negative Bz". This opened a crack in Earth's magnetosphere, allowing solar wind to pour in and fuel the Northern and Southern Lights.
The moral to this story: Even weak CMEs can trigger bright auroras if the CME is dragging behind it a south-pointing magnetic field. The next CME is due on July 13th. Stay tuned. www.spaceweather.com