Vernal equinox auroras

March 20th began with the vernal equinox and ended with this outburst of Spring-green over Tromsø, Norway:

Spring is aurora season. For reasons not fully understood by scientists, the weeks around the vernal equinox are prone to Northern Lights. Auroras are caused by solar activity, but the Sun doesn’t know what season it is on Earth. Intensive outbursts are called auroral substorms and they have long puzzled space physicists. Often sighted in springtime, substorms erupt with little warning and sometimes shocking intensity. (NASA Science)

Here are some nice images. (More at SpaceWeather.com):

Featured image: Aurora display over Sommarøy / Tromsø, Norway (Credit: Frank Olsen)