The Roar of the Aurora

July 10, 2008
by Seth Shostak, Senior Astronomer, SETI Institute

It's the mother of all earthly radio transmissions, a broadcast that's been on the air for billions of years. However, and despite the long run, it's one radio program that you'll probably give a pass: it sounds like Fast-Finger Freddie twisting the shortwave dial at a few hundred RPM.

This cacophony of radio static from Earth is known as Auroral Kilometric Radiation (so-called because the wavelength of the emission is typically kilometers long). AKR is generated when fast-moving particles boil off the Sun, gush into space, and then get manhandled by Earth's magnetic field. The same circumstance accounts for the aurora borealis - those ghostly celestial displays that quietly amuse bored Canadians and insomnious polar bears.

But AKR, which has been in the news lately, has caught the notice of many space fans. They see it as just the sort of signal that could tip off aliens about Earth's existence, a kind of radio fingerprint of our world. And if that's possible, then perhaps we might use our radio telescopes to detect the AKR billowing off ET's planet.

Adding to the allure, AKR is no pipsqueak signal: the power involved is measured in millions of watts.

So, could aliens be tuning into this, the most powerful radio transmitter on Earth?

In principle they could, but in practice this would be tougher than a three-dollar steak. One problem for any extraterrestrial listeners is confusion with other solar system emitters. Both the Sun and Jupiter - each of which have magnetic fields many thousands of times stronger than Earth's - belch more powerful natural signals into space than we do. (Mercury, Saturn, Uranus and Neptune are also members of this cosmic chorus.) The aliens would have to wield highly directional antenna arrays to pick out Earth from this noisy crowd.

Of course, the aliens may have such sophisticated instruments. But we don't. Our best low-frequency radio telescopes couldn't find an alien Earth's AKR emissions. Even so, radio astronomers are still hoping to detect the static from so-called "hot Jupiters" around nearby stars. Being so close to their suns, these blistering behemoths would be showered by charged particles. The particles would twist and shout once they got embroiled in the planet's magnetic field, producing signals that are many thousands of times more powerful than the AKR from our own planet. But hot Jupiters are a special case.

So you might wonder, if finding multi-megawatt natural transmissions from Earth-like worlds is difficult, why do we bother to look for ET's radio broadcasts with our SETI experiments? The answer is two-fold: first, deliberate transmissions could be far more tightly beamed (thereby wasting less energy on a signal that largely dissipates into empty space). Second, an intentional radio ping might be narrow-band.

The last point is important. Nature, lacking a degree in electrical engineering, produces radio signals that are wastefully wide-band. Think of lightning: the color is close to white (in other words, the light's spread over all wavelengths), and if you have your AM radio on during a storm, you'll note that the associated radio crackle is also wide-band. You can hear it no matter where you tune. While there are exceptions to this general behavior (both interstellar masers and the 21 cm emission from neutral hydrogen come to mind), it's usually the case that nature's radio transmissions - from quasars to pulsars to AKR - are spread all over the dial.

On the other hand, ET, benefitting from higher education, could build a transmitter capable of corralling a lot of energy into a very narrow band. This would produce a signal that's far easier to detect than nature's broadcast splatter. If you do the numbers, you'll see that - at light-years distance - AKR, despite all the megawatts, is enormously harder to find than even a modest radar installation.

So yes, it's interesting that an Earth-like planet will have its own, thoroughly natural radio signature. Indeed, as our radio telescopes improve, this is one more phenomenon that legions of grad students will no doubt study. But it is the deliberate, carefully-engineered signal - a signal able to stand head and shoulders above this noisy background - that could reveal something far more precious than the rowdy interplay of magnetic fields and charged particles: the presence of intelligent life.