Don’t despair over fall dropping summer leaves, autumn is aurora season in the northern hemisphere. Auroras dance in the wake of geomagnetic storms – the product of blustery solar winds vibrating Earth’s magnetic field. Consider our magnetic field (magnetosphere) a protective barrier of protons and electrons. When uppity solar winds smack Earth’s magnetic bubble dislodged particles shower the atmosphere with auroras iconic glow. Spring and fall have twice the geomagnetic storms as summer and winter, the product of interplanetary tilt near the equinoxes.
Interplanetary tilt – think of our sun as an enormous magnet, all planets in our solar system orbit within the Sun’s magnetosphere. At magnetopause, the point at which the farthest reach of Earth’s magnetosphere battles solar forces, our magnetic field points north. No harm no foul unless the Sun’s magnetic field tilts south, a defining characteristic occurring near the equinoxes. When north and south facing magnetic fields link up, Earth’s magnetic field is partially cancelled at point of contact, with wild abandon displaced particles dance the northern lights.
Science calls the north/south facing magnetic portal Bz. Negative south pointing Bz’s allow solar wind energy to penetrate Earth’s magnetic fortress, positive north pointing Bz’s slam the door shut. Bz’s ebb and flow in response to Earth’s wobbly axis. Every spring and fall Earth’s axis careens obligingly towards gates of the Bz portal – aurora season has arrived.
Hang on for a lesson in solar dynamics – Earth is experiencing a solar sector boundary crossing. Let me explain….
The sun produces wind (currently 410.9 Km/second) that blasts across the cosmos. Just like Earth, our Sun has a magnetic field – known as the interplanetary magnetic field (IMF). Whipped into a spiral rotation, wind driven IMF rotates in one direction. It divides into spiral sections pointing to and away from the sun along the ecliptic plane ( a direct line between Earth and the Sun). The edge of this swirling mass has a surface separating polarities of planetary and solar magnetism called the heliosphere current sheet.
Earth’s magnetic field points north at the magnetopause (the point of contact between our magnetosphere and the IMF). If the IMF happens to point south at contact (scientific term, southward Bz) the two fields link causing partial cancellation of Earth’s magnetic field – in other words, opening a temporary door for solar energy to enter our atmosphere. Welcome solar sector boundary crossing – a phenomenon born of high solar wind and coronal mass ejections (CME’s – aka solar flares).
It takes 3 or 4 days for magnetism to sort itself out – in the meantime, and barring the occasional high frequency radio disruption, wonky GPS and cell phones, peppered with sudden power grid failure events – we’re treated to kick ass auroras.