The Roar of the Aurora

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

Thiscacophony 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 circumstanceaccounts for the aurora borealis —those ghostly celestial displays that quietly amuse bored Canadians andinsomnious polar bears.

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

Adding tothe allure, AKR is no pipsqueak signal: The power involved is measured inmillions of watts.

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

Inprinciple they could, but in practice this would be tougher than a three-dollarsteak. One problem for any extraterrestrial listeners is confusion with othersolar system emitters. Both the sun and Jupiter — each of which have magneticfields many thousands of times stronger than Earth's — belch more powerfulnatural signals into space than we do. (Mercury, Saturn, Uranus and Neptune arealso members of this cosmic chorus.) The aliens would have to wield highlydirectional antenna arrays to pick out Earth from this noisy crowd.

Of course,the aliens may have such sophisticated instruments. But we don't. Our bestlow-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. Theparticles would twist and shout once they got embroiled in the planet'smagnetic field, producing signals that are many thousands of times morepowerful than the AKR from our own planet. But hot Jupiters are a special case.

So youmight wonder, if finding multi-megawatt natural transmissions from Earth-likeworlds is difficult, why do we bother to look for ET's radio broadcasts withour SETI experiments? The answer is two-fold: First, deliberate transmissionscould be far more tightly beamed (thereby wasting less energy on a signal thatlargely dissipates into empty space). Second, an intentional radio ping mightbe narrow-band.

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

On theother hand, ET, benefitting from higher education, could build a transmittercapable of corralling a lot of energy into a very narrow band. Thiswould produce a signal that's far easier to detect than nature's broadcastsplatter. 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 modestradar installation.

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

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Seth Shostak
Senior Astronomer, SETI Institute

Seth Shostak is an astronomer at the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, California, who places a high priority on communicating science to the public. In addition to his many academic papers, Seth has published hundreds of popular science articles, and not just for; he makes regular contributions to NBC News MACH, for example. Seth has also co-authored a college textbook on astrobiology and written three popular science books on SETI, including "Confessions of an Alien Hunter" (National Geographic, 2009). In addition, Seth ahosts the SETI Institute's weekly radio show, "Big Picture Science."