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Skywatching

Auroras on other worlds

Two of the most striking astronomical images taken in recent years show the aurora.

One was taken from Earth orbit and shows a ring of aurora like a crown around the North Magnetic Pole.

The other shows a similar ring around one of the magnetic poles of the planet Jupiter.

We know auroras happen on Saturn, Uranus and Neptune. No doubt there are other planets scattered around the universe that experience the same spectacles. The recipe for auroral displays includes three main ingredients:

  • Planet with an atmosphere
  • Strong magnetic field
  • Nearby star.

To have a magnetic field, a planet needs a hot core with flows of hot, electrically conductive material, like molten iron and nickel.

When planets form, they pick up a bit of magnetism from the cosmic material that made them. These interact with the flows of hot core material, forming a natural dynamo.

Huge electrical currents are generated which in turn produce strong magnetic fields that extend beyond the planet, far out into space.

Mars is a smaller world than ours. Its core cooled faster and the flows stopped. The result is that the Red Planet has just little patches of remnant magnetic fields and consequently no auroras.

Mercury doesn’t have an atmosphere. Venus has only a very weak magnetic field.

The Sun appears in the sky as an extremely bright disc. The disc is a layer called the photosphere, because that is where the light and heat are radiated into space. Its temperature is about 6,000 degrees Celsius.

Above the photosphere is a layer called the chromosphere, and above that the corona, which extends far into space, beyond the Earth.

The corona is very hot, about a million degrees. This is odd, because something between the 6,000-degree photosphere and the cold of space should have a temperature lower than 6,000 degrees, certainly not a million degrees.

One theory we have for this is that the huge turbulence at the photosphere sends sound waves upward and, as they move into more and more rarefied material, they get bigger until they dissipate as heat.

The high temperature makes the solar atmosphere unstable, so that it expands outward as a high-speed wind.

This wind, of million-degree plasma and magnetic fields, flows at hundreds to thousands of kilometres a second, ranging from a "breeze" to a "gale,” with an occasional "storm.”

In pre-space-age books the Earth's magnetic field was depicted as a sort of doughnut shape. Then we learned about the solar wind, which blows it into a teardrop, tapering to a point facing away from the Sun.

Gales and storms in the solar wind create instabilities in the Earth's magnetic field that accelerate electrons. These follow the magnetic field down into the Polar Regions, where they collide with atoms of nitrogen and oxygen in the air, making them glow.

Pulses and instabilities in the electron streams create the changing curtains, rays and flickering blobs of the aurora.

The occurrence of aurora on planets out as far as Neptune indicates the solar wind is still blowing strongly even that far out in the Solar System.

Now, thanks to the Voyager spacecraft, we know the solar wind extends out well beyond the planets, until it finally collides with the interstellar plasma, marking the edge of the Solar System.

On our world, the aurora has attracted artists and scientists, and has accumulated a wealth of myths from many cultures living at high latitudes.

Since inhabited planets need a star, a magnetic field to prevent the stellar wind scraping away the atmosphere, and an atmosphere to breathe, it is likely there is an accumulation of auroral myths on those distant worlds too.

Will we ever find out?

  • Jupiter and Saturn lie low in the southeast before dawn.
  • Mars is now slowly sinking into the sunset glow.
  • The Moon will reach Last Quarter on the 2nd and be New on 10th.

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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

Ken.T[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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