There was an outbreak of auroral displays on Friday the 20th April – across the Canadian border and lighting up Illinois and Indiana (and many other States). The display was caused by an inteplanetary shock wave according to http://spaceweather.com (April 23rd) – but there is a mystery. The shock waves are supersonic disturbances in the solar wind which cause discontinuities in the speed, temperature, and density of the plasma. During periods of high solar activity these are routinely delivered to Earth by corononal mass ejections (CMEs) – explosions on the Sun. However, there was no big explosion on the Sun last week and the Sun is at minimum activity in its current cycle – so what caused the shock wave event? No one knows. This is in spite of the fact there are multiple satellites keeping an eye on the Sun twenty four hours of the day, week in and week out. How did it remain undetected by space cameras or during its journey from the Sun to the earth, noticed in its final stages. This unannounced interloping ejection of the solar wind caused a moderate geomagnetic storm which came with strangely coloured and widespread auroral displays.
