Interplanetary magnetic field cracks open an auroral portal

Pale green auroral patches and rayed arcs illuminate the bottom third of the northern sky earlier this morning. Photo: Bob King

Another bleary-eyed morning with a happy heart. So it is after a late-night encounter with the northern lights. Last night’s display wasn’t in the forecast but just one of those things that happens time to time, when the interplanetary magnetic field dips southward.

Auroras bloom in Earth’s skies for at least several reasons: solar flares, coronal mass ejections, coronal holes AND fluctuations in the sun’s magnetic field.

The spiral shaped IMF, invisible to the eye, permeates the solar system with the sun’s magnetic field. Credit: NASA with my own additions

Streams of high-speed electrons and protons, known as the solar wind, boil off the sun, spreading its magnetic field across the solar system.

Called the interplanetary magnetic field or IMF, it continually rushes past Earth and the rest of the planets like waves flowing around rocks in a pond.

With one difference. Since the sun rotates, the IMF and solar wind are twisted into a spiral. Picture the IMF as water spraying from a rotating lawn sprinkler.

Earth has a magnetic field too. While it deflects most of what the sun deals out,  it’s not without its vulnerabilities or “soft spots”. When the gusty, ever-fluctuating IMF tips south in the Earth’s vicinity, it cancels our magnetic field at the point of contact, opening a crack in the planet’s armor. Solar wind particles seize the opportunity and stream directly into the upper atmosphere to spark auroras.  That’s what happened last night. Thank you IMF.

Look at this beast! Sunspot group 1520, seen this morning July 9, has a complex magnetic field, making it a potential hotbed of solar flares in the coming weeks. It’s also big enough to see with the naked eye using a safe solar filter. For timely aurora alerts, follow my tweets at AstroBob_bk. Credit: NASA

There’s a small chance for auroras again tonight from a coronal mass ejection blasted our way by last week’s large sunspot group 1515, now departing over the sun’s western edge. A worthy replacement has already sprung up along the sun’s eastern edge, the behemoth group numbered 1520. I easily spotted it this morning with nothing more than a pair of solar eclipse glasses. The sketch below shows how it looked facing east around 10:30.

Sketch of naked eye sunspot group 1520

Last night I grabbed another look at Nova Sagittarii No. 4, the topic of yesterday’s blog. I hope you did too. The star was faintly visible at magnitude 8.2 in 10×50 binoculars from a dark site. For the moment, it’s fading after its initial outburst – typical nova behavior – but keep an eye out for surprises. Novae sometimes re-brighten on their way back to obscurity.

While you’re up late squinting after auroras, don’t miss the last quarter moon rising around midnight in Pisces. It’s on its way to a wonderful conjunction with Jupiter and Venus this Sunday.

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About astrobob

My name is Bob King and I work at the Duluth News Tribune in Duluth, Minn. as a photographer and photo editor. I'm also an amateur astronomer and have been keen on the sky since age 11. My modest credentials include membership in the American Association of Variable Star Observers (AAVSO) where I'm a regular contributor, International Meteorite Collectors Assn. and Arrowhead Astronomical Society. I also teach community education astronomy classes at our local planetarium.

2 thoughts on “Interplanetary magnetic field cracks open an auroral portal

  1. of COURSE the aurora was out last night! Just my luck. I finally made the drive up to Duluth (Silver Bay actually) and camped out on the beach Saturday night. I had hoped to see some action, but the moon was so bright all night that I could barely sleep in the tent! No such luck. But I did get a great shot of early morning sunrise with Venus and Jupiter. Next time!

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