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Thursday, July 24, 2008

Substorms solved - or are they?

The THEMIS team has announced that the primary mystery of substorms has been resolved:

"We have discovered what makes the Northern Lights dance," declares UCLA physicist Vassilis Angelopoulos, principal investigator of the THEMIS mission. The findings appear online in the July 24 issue of Science Express and in print August 14 in the journal Science.

What is a substorm?

Well, broadly speaking they are massive bursts of energy from the night-side part of the Earth's magnetic field; here the field is stretched back into space, almost like a comet tail. Energy from the solar wind gets carried into this magnetotail in the form of increased magnetic field strength and energetic charged particles until at some point the energy is released, a good portion of it carried Earthward.

Some of those particles dive along the Earth's magnetic field and produce the bright and dynamic auroral displays (e.g. see the pretty picture) that can be seen off to the north. Some of them hang around a big longer in the inner magnetosphere and drift around to the dayside. Some even get accelerated and add to the population of relativistic electrons in the radiation belts; these are the really dangerous ones.

The primary question about substorms (though only one of many) is: 'how is the energy release initiated?'

There are two principle competing theories* and the big difference between them is the order in which physical processes occur in the magnetotail. It has been known for a long time that magnetic reconnection** plays a significant role (sorry to my astronomy colleagues as I have been reliably informed that you get bored hearing about this). What was not clear was whether this initiated the substorm process (or more accurately the substorm expansion phase) or whether it occurred after some other process had already started the ball rolling.

Over the years the different physics that occurs within the substorm have pretty much all been observed (well, as far as we know...), but knowing in which order they occur has always been tricky. To find that out we would need probes along the magnetotail recording simultaneously as the substorm occured, plus a whole network of ground based observatories so that we can interpret the satellite data in proper context.***

That's what THEMIS is: five satellites spread out along the tail with a huge and impressive network of ground instrumentation across North America (and a couple in Europe for good measure - one of them operated by a UK scientist). Now, it seems, this ground and space based mission has come good.

The discovery came on what began as a quiet day, Feb 26, 2008. Arctic skies were dark and Earth's magnetic field was still. High above the planet, the five THEMIS satellites had just arranged themselves in a line down the middle of Earth’s magnetotail—a million kilometer long tail of magnetism pulled into space by the action of the solar wind.

That's when the explosion occurred. A little more than midway up the THEMIS line, magnetic fields erupted, "releasing about 1015 Joules of energy," says Angelopoulos. "For comparison, that's about as much energy as a magnitude 5 earthquake."
And the satellites and ground-based cameras saw it from start to finish:

"For the first time, THEMIS has shown us the whole process in action—from magnetic reconnection to aurora borealis," says Sibeck. "We are finally solving the puzzle of substorms."

So it looks as if we can tick a box: magnetic reconnection is the first step in the process. Or at least that is my interpretation of this press release. This could be the answer to a question that has been around for over 30 years.

However, I would bet that this is far from the definitive answer.

I saw talks in the special Substorms in the Age of THEMIS session that we held at NAM that suggested that the story is actually a whole lot more complicated. One in particular hinted very strongly that two different mechanisms occurred in two substorms that quickly followed one another. I'm keen to see if THEMIS also sees another substorm; one that doesn't follow the same pattern as the February 2008 example. There is time yet.

Plus the substorm debate is far from over; there are many other unanswered questions surrounding the substorm: What triggers the reconnection and why does it happen when it does? What determines the location of the aurora? Why do periodic substorms have such large particle injection regions? etc.

For a look at a some recent work that addresses a different aspect of the substorm issue grab a copy of August's Astronomy and Geophysics, where one of the Rishbeth prize winners was looking at many substorms at Earth. The other prize winner was looking at a product of auroral substorms -kilometric radiation- but on Saturn, rather than Earth. This could be a very useful tool for identifying extra-solar planets that have a magnetic field, something that might well be a prerequisite for complex life.

*A primer on potential candidates

**Reconnection is the process by which two different magnetic fields (pointing in opposite direction) merge to produce a new configuration. In the Earth's magnetotail this means that the field close to the equatorial plane gets pinched together causing a change in shape that propels some of the charged particles Earthward. Essentially the field goes from being stretched out in a long tail, to looking more like a dipole field. This animation demonstrates the basic concept nicely.

***I hereby resist the urge to point out to STFC the stupidity of withdrawing from ground-based STP facilities. Except for this footnote...

1 comment:

Andrew W said...

The science paper is here:

It's focussed on one isolated substorm (though they do say they have a couple of other examples) and on a quick read through sounds pretty convincing. I'm still willing to bet that by the end of the mission they'll have a good few examples of substorms fitting both theories though...