Here is an article from spaceweather.com:
SURPRISE GEOMAGNETIC STORM: On March 18th, a crack opened in Earth’s magnetic field. Solar wind poured in, fueling a moderately strong (G2-class) geomagnetic storm. Ruslan Merzlyakov sends this picture from Mårup (Hjørring), Denmark:
Yesterday’s unexpected storm of Northern Lights was visible with the naked eye,” says Merzlyakov. “For the first time in months, Lady Aurora visited Denmark!”
NOAA forecasters had said there was a slight chance of minor G1-class storms on March 18th. The actual storm was much more intense, spreading auroras to lower latitudes than expected. In Europe the display spilled all the way down to Germany.
MAGNETIC CRACKS AND STORMS: For the past five days, Earth’s magnetic field has been in a state of unrest as an unusually-wide stream of solar wind blows around our planet. Literally, the geomagnetic field has been shaking back and forth. This plot from Stuart Green’s backyard magnetometer in Preston, UK, shows the unrest, highlighted by a G2-class geomagnetic storm on March 18th:
Blue squiggles in Green’s chart represent changes in his local magnetic field caused by the buffeting of solar wind high overhead. “Magnetic activity has been relatively high since March 14th with several distinct periods of storminess,” he says. “Magnetometers all around the world are registering this.”
The speed and pressure of the solar wind are key factors in stirring up magnetic storms–but not the only factor. Even more important is the formation of cracks in Earth’s magnetic field, which allow solar wind to penetrate. How do we know when cracks are forming? Green has prepared another plot to answer this question:
“I lined up NOAA solar wind data with my own magnetometer chart,” he says. “Note the red curve in the solar wind data. Much of the magnetic unrest I’ve been recording correlates with negative Bz, when the magnetic field of the approaching solar wind tips south.”
Indeed, that is exactly how cracks form. The magnetic field in the solar wind points south, partial cancelling Earth’s north-pointing magnetic field. Solar wind pours in through the resulting weak point. These cracks tend to form most often during weeks around equinoxes–a phenomenon known as the “Russell-McPherron effect.”
Note to auroraphiles: Keep an eye on Bz right here on Spaceweather.com. When it tips south (becomes negative), that is the best time to watch for Northern Lights.
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