SOLAR SECTOR BOUNDRY CROSSING: Last night, skies over Finland turned shocking green. What happened? Earth crossed through a fold in the heliospheric current sheet--a vast wavy structure that separates regions of positive and negative magnetic polarity in interplanetary space. This is called a "solar sector boundary crossing"; such crossings can cause geomagnetic disturbances around the poles.

[link to www.spaceweather.com (secure)]

https://imgur.com/ZNX6DsU