Sept. 14, 2018: The northern autumnal equinox is only a week away. That means one thing: Cracks are opening in Earth’s magnetic field. Researchers have long known that during weeks around equinoxes fissures form in Earth’s magnetosphere. Solar wind can pour through the gaps to fuel bright displays of Northern Lights. Here’s an example from Yellowknife, Canada:
“On Sept. 5-6, we could see auroras in the sky all night long, with a bright outburst of pink shortly after midnight,” says photographer Yuichi Takasaka.
During the display, a weak stream of solar wind was blowing around Earth. At this time of year, that’s all it takes. Even a gentle gust can breach our planet’s magnetic defenses.
This is called the the “Russell-McPherron effect,” named after the researchers who first explained it. The cracks are opened by the solar wind itself. South-pointing magnetic fields inside the solar wind oppose Earth’s north-pointing magnetic field. North and South partially cancel one another, opening a crack. This cancellation can happen at any time of year, but it happens with greatest effect around the equinoxes. Indeed, a 75-year study shows that September is one of the most geomagnetically active months of the year–a direct result of “equinox cracks.”
NASA and European spacecraft have been detecting these cracks for years. Small ones are about the size of California, and many are wider than the entire planet. There’s no danger to people on Earth. Our planet’s atmosphere intercepts the rush of incoming particles with no harm done and a beautiful afterglow.
Stay tuned for more Arctic lights as autumn approaches.