Sept. 27, 2023: (Spaceweather.com) When Vincent van Gogh painted “The Starry Night” in 1889, little did he know he was working at the forefront of 21st century astrophysics. A paper recently published in Nature Communications reveals that the same kind of waves pictured in the famous painting can cause geomagnetic storms on Earth.

Above: Vincent van Gogh’s ‘Starry Night’, which he painted in 1889: more

Physicists call them “Kelvin Helmholtz waves.” They ripple into existence when streams of gas flow past each other at different velocities. Van Gogh saw them in high clouds outside the window of his asylum in Saint-Rémy, France. They also form in space where the solar wind flows around Earth’s magnetic field.

“We have found Kelvin-Helmholtz waves rippling down the flanks of Earth’s magnetosphere,” says Shiva Kavosi of Embry–Riddle Aeronautical University, lead author of the Nature paper. “NASA spacecraft are surfing the waves, and directly measuring their properties.”

This was first suspected in the 1950s by theoreticians who made mathematical models of solar wind hitting Earth’s magnetic field. However, until recently it was just an idea; there was no proof the waves existed. When Kavosi’s team looked at data collected by NASA’s THEMIS and MMS spacecraft since 2007, they saw clear evidence of Kelvin Helmholtz instabilities.

“The waves are huge,” says Kavosi. “They are 2 to 6 Earth radii in wavelength and as much as 4 Earth radii in amplitude.”

This computer model shows van Gogh waves moving down the flank of Earth’s magnetosphere. Credit: Shiva Kasovi. [full-sized animation]

Imagine a wave taller than Earth curling over and breaking. That’s exactly what happens. Kelvin-Helmholtz waves naturally break onto Earth’s magnetic field, propelling energetic particles deep into the magnetosphere. This revs up Earth’s radiation belts, triggering geomagnetic storms and auroras.

A key finding of Kavosi’s paper is that the waves prefer equinoxes. They appear 3 times more frequently around the start of spring and fall than summer and winter. Researchers have long known that geomagnetic activity is highest around equinoxes. Kelvin-Helmholtz wave activity could be one reason why.

Our planet’s seasonal dependence of geomagnetic activity has always been a bit of a puzzle. After all, the sun doesn’t know when it’s autumn on Earth. One idea holds that, around the time of the equinoxes, Earth’s magnetic field links to the sun’s because of the tilt of Earth’s magnetic poles. This is called the Russell-McPherron effect after the researchers who first described it in 1973. Kavosi’s research shows that Kelvin-Helmholtz waves might be important, too.

Northern autumn has just begun, which means Kelvin Helmholtz waves are rippling around our planet, stirring up “Starry Night” auroras. Happy autumn!

Sept. 14, 2023: (Spaceweather.com) Mark your calendar. On Oct. 14th, the Moon will pass in front of the sun, producing an annular solar eclipse visible from North and South America. In this map, the yellow band shows where the eclipse will be best:

This should not be confused with a total eclipse, where the Moon completely covers the sun. During an annular eclipse, the Moon is a little too small for complete coverage–a result of the Moon’s elliptical orbit. When the Moon passes in front of the sun, an annulus remains visible, creating a “ring of fire” in the sky. It is still very cool. Indeed, you will literally feel cool as the diffuse shadow of the Moon passes overhead, dimming the landscape and lowering the air temperature by a few degrees.

The “path of annularity” is about 100 miles wide. Along its centerline, the sun will be ring-shaped for more than 4 minutes, with as much as 95% coverage of the solar disk. Be careful! Even a 5% sliver of the sun can be blindingly bright. Watch the event using ISO-approved eclipse glasses.

Outside the path, observers will see the sun turn into a crescent, fat or thin depending on how far they are from the centerline. This is called a “partial eclipse,” also best seen using eclipse glasses. Almost all of North and South America is in the partial eclipse zone.

Above: An annular eclipse over Southern California in Jan. 1992. Photo credit: Dennis L. Mammana

There’s a lot to experience during an eclipse. Listen for changes to birdsong and insect sounds. Wildlife is known to respond to the arrival of the Moon’s shadow.   Also, look under leafy trees. Crescent-shaped sunbeams lancing through the foliage can dapple the ground with tiny images of the eclipsed sun. Amateur astronomers with solar-filtered binoculars can watch brilliant beads of sunlight glittering through through lunar mountains especially around the Moon’s north and south poles.

To learn more about the eclipse, we recommend GreatAmericanEclipse.com. Their Field Guide to the 2023 and 2024 Solar Eclipses is a must-have for anyone planning an eclipse adventure, and they also sell safe eclipse glasses and solar binoculars.

Sept. 13, 2023: (Spaceweather.com) An unexpected CME hit Earth’s magnetic field on Sept. 12th (1237 UT) and sparked a G2-class geomagnetic storm. Magnetically, the CME remained connected to our planet for more than 13 hours, allowing plasma from the CME’s wake to enter Earth’s magnetosphere. This fueled a display of auroras photographed as far south as Missouri (+40.2N) and Nebraska (+40.9N).

Matthew Merrell knew the storm was underway, and he waited anxiously for the sun to set over his home in central Minnesota. As darkness fell, he saw the auroras:

“It was a great start to the night with pillars visible as soon as the sky was dark enough,” says Merrell. “By 10:00 pm it was just a gentle glow with very little motion.”

Merrell witnessed only the subsiding tail-end of the storm, a G1-class event. At its G2 apex, the storm produced “BRIGHT” and “stunning” activity over Scotland, “massive rays” above Ireland, and an all-sky explosion of color over Iceland. Magnetometer needles in Britain swung wildly with more than 15 hours of dramatic undulations.

Forecasters did not see this coming. One surprised NOAA analyst called it a “stealthy CME.” In retrospect, it was probably one of many relatively bright CMEs that left the sun on Sept. 8th, shown here in a 24 hour time-lapse from the Solar and Heliospheric Observatory (SOHO):

When the sun spits out so many storm clouds in such a short period of time, it can be hard to disentangle them and figure out which one(s) might be heading toward Earth. Notably the movie includes a distinct halo CME. At the time it was thought to be a farside event, but maybe it is the one that hit Earth yesterday.

Did you miss the storm? Subscribers to our Space Weather Alert Service received instant text messages when the CME arrived and, later, when the geomagnetic storm began. Give it a try! Aurora alerts: SMS Text