Why Northern Lights displays are stronger around spring, autumn equinoxes
Aurora-chasing season has arrived with the spring equinox, a time when space weather experts say we are more likely to see vivid displays of the dancing technicolor lights known as the aurora borealis or Northern Lights.
According to NOAA's Space Weather Prediction Center (SWPC), the best times of year to see auroras are around the spring and fall equinoxes.
Earth's astronomical seasons are based on the Earth's position with respect to the Sun, as our planet orbits our star. At the spring equinox in March and autumn equinox in September, the equator is aligned with the Sun.
"Due to subtleties in the way the solar wind interacts with Earth's magnetosphere, there is a tendency towards larger geomagnetic storms, and thus better auroras, to occur near the equinoxes," according to the SWPC.
This increased Northern Lights activity around the equinoxes is known as the "Russell-McPherron effect," according to the University of Alaska Fairbanks (UAF) Geophysical Institute. UAF experts say the chances of seeing aurora in interior Alaska in March and September are roughly twice as high as they are during the summer and winter solstice.
Even though there is a strong tendency to see more intense aurora around these times, this phenomenon is not fully understood.
Shawn Dahl, service coordinator at NOAA's SWPC, said the reaction also depends on the type of space weather feature causing a geomagnetic storm.
Coronal hole high-speed streams (CH HSS) or coronal mass ejections (CME) are solar events that can produce solar storms and Northern Lights on Earth. The SWPC issues space weather forecasts when these events appear to be headed toward Earth, and the spring equinox plays a role in those forecasts.
"We see this routinely and we add it to our forecast thinking in the (SWPC) office," Dahl said. "And the overarching thing for the spring equinox is that you need the feature to be of negative polarity for that more pronounced effect to happen."
Meanwhile, Dahl said a positive polarity is needed during the fall equinox.
"The thinking of this is all the way connected to the southern pole, which is now positive. Polarity just enhances it," Dahl said.
This week, the SWPC issued a G2 (Moderate) Geomagnetic Storm Watch on its five-point scale for a CH HSS headed toward Earth, but it was a positive polarity feature. Dahl said the forecast might have been upgraded if it had been negative polarity.
The large coronal hole contains less dense, cooler plasma traveling at high speeds, streaming along the Sun's magnetic field lines toward Earth. When these charged particles interact with oxygen and nitrogen in Earth's atmosphere, they briefly energize the molecule, giving off light as they de-energize, producing colorful Northern Lights.Original article source: Why Northern Lights displays are stronger around spring, autumn equinoxes
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