THE FIRST SOLAR WIND STREAM OF AUTUMN: Northern autumn begins on Tuesday, Sept. 22nd. The next day, a solar wind stream arrives. The incoming stream is flowing faster than 600 km/s from a canyon-shaped hole in the sun's atmosphere. Its arrival on Sept. 23rd will set the stage for the first equinox auroras of the new season. Aurora alerts: SMS Text
AURORAS LOVE EQUINOXES--BUT WHY? Autumn begins this week, and so does aurora season. The northern autumnal equinox arrives on Sept. 22nd. That is the cue for Arctic lights to switch on.
The reason is the Russell-McPherron effect. In 1973, UCLA researchers Christopher Russell and Robert McPherron figured out why geomagnetic activity has seasons. During the days and weeks around equinoxes, Earth's tilt lines up our planet's magnetic field with the sun's, and the two fields connect. This creates pathways and portals in the magnetosphere where solar wind can enter.
Above: Equinox auroras photographed 35,000 ft above Baffin Island by pilot Matt Melnyk.
In forecasting jargon, we say "BsubZ has tipped south." South-pointing magnetic fields from the sun partially cancel Earth's north-pointing field, and the door opens.
Because of the Russell-McPherron effect, Sept/Oct and March/April are the most geomagnetically active months of the year. Aurora-producing storms are about twice as likely around equinoxes as they are around the solstices of summer and winter.
Indeed, at this time of year, high-latitude auroras can appear for little apparent reason. A portal opens, ordinary solar wind enters, and voila--auroras!
Autumn darkness helps, too. The midnight sun has finally set on the Arctic, and nights there are long enough to see the show. The season runs until spring.
Would you like to know when BsubZ tips south? Instant alerts are available to Pro subscribers of our Space Weather Alert System.
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