As a name, Hessdalen may be more familiar to UFO watchers than scientists. The valley in Norway is prone to "strange, hovering, flashing balls of lights" best attributed, as some believe, to alien origins. Now scientists say they're on the verge of an explanation: The valley is a giant natural battery.
But first, let's set the scene, as Caroline Williams does wonderfully in her piece about the Hessdalen phenomenon for New Scientist. On a cold, dark Norwegian night about a dozen times a year, you might see this:
Sometimes the lights are as big as cars and can float around for up to 2 hours. Other times they zip down the valley before suddenly fading away. Then there are the blue and white flashes that come and go in the blink of an eye, and daytime sightings that look like metallic objects in the sky.
The Hessdalen phenomenon has been around for at least a century, but it reached a peak in the 1980s when the lights suddenly started appearing 10 or 20 times a week. UFO enthusiasts took this as a sign that the valley was a portal to other worlds. Google Hessdalen today, and you'll still find plenty of UFO conspiracy sites.
But that strange bursts of light in the 1980s attracted physicists, too, interest piqued by the idea of some unexplained natural phenomenon. In the decades since, they have determined the glow likely comes from air turned into plasma.
The unique geology of the valley could be responsible for this plasma. The valley is formed by rocks on one side rich in copper and the other rich in iron and zinc—not unlike the cathode and anode of a battery. Sulphuric acid, leached from the abandoned sulphur mine at the bottom of the valley, could then turn the river into the weak acid of an electrolyte. But where does the charge to energise plasma come from?
It may be extraterrestrial after all. The Hessdalen phenomenon seems especially common after a display of Northern Lights, reports Williams, when solar wind ionises the earth's atmosphere. If you've got a subscription to New Scientist, do read her full feature on the Hessdalen phenomenon.
The science is certainly not yet definitive, but the details so far are intriguing. For me, the idea of a valley-battery created by natural geology and charged by solar winds coming from 90 million miles away feels like a far more astonishing explanation than aliens. [New Scientist]
Top video: A purported observation of the Hessdalen phenomenon in August 2013.