Glow And Glow Alike – Saturn And Earth Share Aurora Link

Artist illustration of the northern auroral oval on planet Earth. Credit: NASA

Auroras will do what they will do, forecast or no. Two days ago, NOAA space weather forecasters predicted a quiet night last night, but there’s no question the aurora was out. We received a couple reports plus the usual online indicators were hopping with activity. All that was shut out here by clouds and rain, but observers in the northern U.S. with clear skies and a bent for staring into them saw the aurora last night.

Tonight a minor flurry of auroras may happen once again, so be on the lookout. Anytime you see the northern lights, keep in mind you’re looking at one small section of a much larger structure called the auroral oval. These “crowns” of glowing gases are centered about 70 miles high over Earth’s north geomagnetic pole in the Canadian Arctic and over the south geomagnetic pole in Antarctica.

Satellite photos of the auroral oval taken every six minutes over a half hour. The oval is dynamic, changing in brightness and width in response to particle flows from the sun. Credit: NASA

The ovals are where electrons and protons in the solar wind are funneled by our planet’s magnetic field into the upper atmosphere. There they crash into and energize nitrogen and oxygen molecules; rays and colors are produced when the excited atoms emit tiny bursts of light as they return to their “rest” states. I like to visualize them as an ever-shifting interface between sun and Earth – a place where two worlds touch on a subatomic level.

The ovals do not rotate with the Earth but stay in place as the Earth rotates under them. They’re not perfectly circular halos either. The northern oval sags furthest south toward the equator around local midnight, making that the usual optimum time for aurora viewing.

On February 18, 2012 the oval expanded southward well into the U.S. making for a great display. Credit: NOAA

Still, much of the U.S. rarely experience auroras because the oval doesn’t reach that far south except during powerful solar storms.

Like stretching a rubber band, the auroral oval expands with strong influxes of particles from the sun during times of high solar actitvity. When the sun is “quiet”, the ovals skinnify and shrink back toward the poles.

Back in January I described Jupiter’s auroral ovals and how they’re caused not by interaction with the sun’s wind but by sulphur spewed from active volcanoes on the moon Io. Caught up in the planet’s rapid 10-hour rotation, electrified sulfur atoms follow Jupiter’s magnetic field until they crash into the planet’s upper atmosphere, setting it aglow. Big Jove’s own aurora-making machine doesn’t need much help from the sun.

Hubble Space telescope photo in UV light showing Saturn’s north and south ovals. Credit: NASA/ESA

Saturn’s different. It’s poles are capped by auroral crowns, but unlike Jupiter they’re created primarily by fluctuations in the zappy solar wind.

When the wind blows hard and points in a favorable direction, Saturn responds with expanding ovals. When weak, the ovals ratchet back poleward. Sound familiar? Saturn’s oval mimics that of planet Earth.

Enceladus sends beams of electrons down to Saturn’s upper atmosphere creating an auroral footprint. Credit: NASA/ Cassini Imaging Team

There are a couple key differences. Hydrogen gas dominates Saturn’s upper atmosphere. When excited by incoming electrons, it glows deep ruby red unlike the more common green auroras produced by oxygen molecules here on Earth. Some photos show it as blue – that’s because they’re taken in ultraviolet light, where the aurora stands out better against sunlight reflected by the planet’s cloud tops.

Listen to the hiss-like radio noise generated by electrons moving along magnetic field lines from Enceladus to a glowing patch of ultraviolet light on Saturn. The sound is typically inaudible to the human ear, but has been amplified electronically. 

There’s also a faint “footprint” of auroral light just beyond the oval’s edge courtesy of the moon Enceladus. Enceladus spews water vapor from cracks in its icy crust at the rate of 400 lbs. (200 kg) per second which becomes electrically-charged when it interacts with Saturn’s magnetic field. As Enceladus revolves around the planet, that cloud of particles – called a plasma – stimulates a rain of electrons to plummet down one of the planet’s magnetic field lines straight into the atmosphere to form a patch of aurora. Jupiter’s moon Io does the same thing.

Saturn now rises in the southeastern sky around 11 p.m. in the constellation Virgo. The next time the aurora’s aflame on our planet, you can picture something very similar happening nearly a billion miles away.

8 Responses

  1. Giorgio Rizzarelli

    Thank you for the article. A good companion to the one you wrote a few months ago about Jupiter’s aurora. Interesting that Saturn’s auroras are somehow in the middle between Earth’s and Jupiter’s (because of the footprint from Enceladus). Looking forward to future articles about the auroras on other planets. I read for example that Venus, since he lacks its magnetic field, can have auroras spread on all the planet.
    PS The radio whistle sounds quite like 1950s sci-fi soundtracks 🙂

    1. astrobob

      Hi Giorgio,
      Thanks! Long ago I thought all auroras were the same but it’s been most interesting to learn otherwise. I agree on the whistle. Do you have a favorite old ’50s sci fi movie? I still like The Day the Earth Stood Still and Forbidden Planet.

      1. Giorgio Rizzarelli

        Forbidden Planet me too (with its very unusual soundtrack). And if you allow me to include year 1960, George Pal’s The Time Machine.

        1. astrobob

          Oh, yes, The Time Machine – that was also a favorite. How could I forget? That one really fired up my imagination. For the comet photo I used a Canon 300mm f/2.8 lens from work that we share as a photo staff. No multiplier. It almost was too much for my small tracking mount and took a lot of careful, deliberate tweaking to work just right.

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