HELENA - It's the Autumnal Equinox, the beginning of autumn in the northern hemisphere.
An equinox happens twice a year, in March and September, when the Earth's axis and orbit align. The sun is positioned directly over the equator, giving both the northern and southern hemispheres an almost equal amount of daylight and darkness.
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If this is the beginning of your favorite season, you're not alone. The aurora borealis, aka the northern lights, loves a good equinox. Around the autumnal and vernal equinoxes, there is an increase in auroral activity.
For most of the year, the Earth's and sun's magnetic fields are misaligned, leaving our planet's magnetic field less open to the aurora, causing effects of the solar wind. However, at the equinox, the two magnetic fields line up but point in opposite directions, allowing for greater capture of the charged particles that cause the northern lights.
In 1973, two geophysicists named Christoper Russell and Robert McPherron presented a theory involving an increase in aurora activity at the equinoxes. With the Earth's poles almost perpendicular to that of the sun, this maximizes the Earth's poles ability to capture charged particles coming from the sun.
Most displays of the northern lights require large solar flares and coronal mass ejections to trigger the light show at night.
But the Russell McPherron effect can allow the formation of the aurora without these solar storms based on just the geometry of the Earth to the sun around the time of the equinox. So keep an eye on the sky at night because this time of year, the leaves are not the only thing that becomes colorful.
