Sci­en­tists at­tribute the bright nights of old to UV light’s ef­fect

Northwest Arkansas Democrat-Gazette - - STYLE - JOANNA KLEIN

It was night, but peo­ple could see, al­most as if it was day. There were no street­lamps, no flood­lights, no can­dles, sun or moon. But they could read doc­u­ments, make out peb­bles on the ground and spot de­tails of land­scapes hun­dreds of yards away. Dis­tant moun­tains were il­lu­mi­nated. Some called it the noc­tur­nal sun.

Re­ports of ob­ser­va­tions like these, dat­ing to an­cient Rome, have long per­plexed sci­en­tists and on­look­ers. Sci­en­tists in Canada may have an an­swer.

In a study pub­lished in Geo­phys­i­cal Re­search Let­ters, Gordon Shep­herd and his col­league Young-Min Cho, at­mo­spheric sci­en­tists at York Univer­sity, ex­plain how waves in Earth’s at­mos­phere may have made these an­cient bright nights pos­si­ble.

A bright night starts with a dull light called air­glow, which is found more than 60 miles above the earth’s sur­face. Nor­mally, Earth’s at­mos­phere is made up mainly of ni­tro­gen and oxy­gen, in their molec­u­lar forms. That just means that rather than a single atom of oxy­gen, for ex­am­ple, two of them are stuck to­gether. But up in those heights, ul­travi­o­let light from the sun sep­a­rates the atoms in these mol­e­cules.

At night, when the sun is gone, they come back to­gether, re­leas­ing en­ergy as they re­unite. This en­ergy is visible as light, and the pres­ence of oxy­gen, which re­searchers fo­cused on in the study, can make it ap­pear green.

Sci­en­tific in­stru­ments are sen­si­tive enough to de­tect it, but not hu­man eyes — un­til a bright night, when an un­ex­pected align­ment of waves in our up­per at­mos­phere am­pli­fies that once in­vis­i­ble air­glow, mak­ing it much brighter.

These waves, called zonal waves, are in­flu­enced by se­vere weather on Earth’s sur­face and travel around the up­per at­mos­phere. Shep­herd and Cho ze­roed in on four kinds of zonal waves us­ing im­ages taken from a satel­lite de­ployed in the 1990s to mea­sure air­glow and other fea­tures of the at­mos­phere.

Usu­ally, the waves peak in dif­fer­ent places along their jour­neys around Earth. But “ev­ery once in awhile, the waves end up in the same spot,” Shep­herd said. “Just imag­ine waves in the ocean pil­ing up to­gether. That makes a big­ger wave.”

And when they su­per­im­pose like that, the in­ten­sity of the air­glow in­creases so much that it’s pos­si­ble for the naked eye to see it, and may ex­plain those noc­tur­nal suns of the past.

Once su­per­im­posed, the waves will stay that way for a while be­cause they move so slowly, said Shep­herd, so bright nights will last two to four nights. And, ac­cord­ing to his anal­y­sis of the satel­lite im­ages, one bright night can shine over ar­eas as big as Europe.

In his­tor­i­cal re­ports, peo­ple did not re­ally men­tion what was go­ing on in the sky, Shep­herd said. “They were just aware that sud­denly they could see things in their en­vi­ron­ment.”

With so much light pol­lut­ing our nights now, it is nearly im­pos­si­ble to make out a bright night when it oc­curs in most places, let alone find a pho­to­graph of one. When many of the ob­ser­va­tions took place, cam­eras weren’t yet in­vented. And a pho­to­graph from Earth of the bright sky at night wouldn’t be that im­pres­sive, Shep­herd said. It’s some­thing to ex­pe­ri­ence first­hand.

And to do that re­quires pa­tience, luck and a very spe­cial place. The weather pat­terns that pro­duce the waves lead­ing to bright nights tend to fa­vor mid­dle lat­i­tudes. “If you pick one lo­ca­tion, like New York City, it would only hap­pen about once a year,” Shep­herd said. And even then, you’d need a clear night and no light pol­lu­tion.

“We have an­i­mal species that are dis­ap­pear­ing. We have glaciers that are dis­ap­pear­ing. And bright nights too are dis­ap­pear­ing, be­cause there are so many city lights ev­ery­where,” Shep­herd said. “There are go­ing to be fewer and fewer places where peo­ple can see them, and if they did, they’d have to wait a long time.”

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