Earth’s Aurora Radio Emissions -A Signal to “Exo-Worlds”?

Lightning_kpno_trim  Chirps and whistles of our planet's auroral kilometric radiation (AKR) might be the first thing an extraterrestrial civilization is likely to hear from Earth. In reality, they are the sounds that accompany the aurora. The European Space Agency's Cluster mission is showing scientists how to understand this emission and, in the future, search for alien worlds by listening for their sounds.

AKR is generated high above the Earth, by the same shaft of solar particles that then causes an aurora to light the sky beneath. For decades, astronomers had assumed that these radio waves traveled out into space in an ever-widening cone, rather like light emitted from a torch. Based on Cluster, astronomers now know this is not true.

By analyzing 12,000 separate bursts of AKR, a team of astronomers have determined that the AKR is beamed into space in a narrow plane, similar to placing a mask over the torch with just a small slit in the middle for light to escape.

"Whenever you have aurora, you get AKR," says Mutel. That includes aurora on other planets, too. Visiting spacecraft have seen aurora and detected AKR on Jupiter (image above) and Saturn, the two largest gas giants in our Solar System.

Not only will this new understanding of how the AKR is beamed into space help astronomers understand the magnetic environment of those gas worlds, it will also help them search for similar planets around other stars.

Although looking for AKR from extra solar planets will require much larger radio telescopes than are currently available, these instruments are on the drawing boards. Once these planets have been identified, the AKR can be monitored for how it winks on and off, allowing astronomers to calculate how long the planet takes to rotate.

"We can now determine exactly where the emission is coming from," says Robert Mutel, University of Iowa, who conducted the three-year study with colleagues. For each of the AKR bursts they analyzed, the astronomers pinpointed its point of origin to regions in Earth's magnetic field just a few tens of kilometers in size. These were located a few thousand kilometers above where the light of the aurora is formed.

"This result was only possible because of the Cluster mission's four spacecraft," says Mutel. Consisting of four nearly identical spacecraft flying in formation, Cluster allowed the scientists to precisely time when the AKR washed over each of the satellites. Using this information, the scientists triangulated the points of origin, in a similar way to the way GPS navigation works.

AKR was discovered by satellites in the early 1970s. It is blocked from reaching the ground by the ionosphere, the upper reaches of Earth's atmosphere. This is just as well because otherwise it would overwhelm the transmissions from all our radio stations. It is 10,000 times more intense than even the strongest military radar signal.

It also provides new routes of investigation into the magnetic fields of other stars, many of which have magnetic fields thousands of times stronger than the Sun. They too, will produce radiation similar to AKR and these can be monitored.

Posted by Casey Kazan. ESA Cassini Image  shows radio emissions from Saturn.

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