Noctilucent Clouds – NLC’s

Noctilucent Clouds over the northern horizon at Voorschoten (Netherlands), June 28-29, 1997, about 1.5 hours before local midnight. 10-second exposure with f2.8/50 mm. Note the wavy rippled structure of the Noctilucent Clouds, and the contrast with the dark common clouds at much lower atmospheric altitude (Photograph © 1997 by M. Langbroek)

Noctilucent Clouds (NLC’s) are much higher in the atmosphere than normal clouds: they occur at altitudes of 85 to 90 kilometers (compare: normal cirrus clouds occur at an altitude of 10 kilometers or less). It are thin but vast fields of ice-covered particles of interplanetary origin drifting high in the atmosphere. These tiny icy particles are believed to be typically in the order of 90 nm in diameter but can attain diameters of 200 to 400 nm. The ice particles largely find their origin in vaporized meteoric material. Small meteoroids entering the earth’s atmosphere vaporize entirely. As a result of this vaporization, heavy molecules are formed, mostly heavy ions of Ferro-oxides (iron oxides – rust in colloquial speech), like Fe⁺, FeO⁺, FeOH⁺ and FeO₂⁺. These heavy ions become nucleating agents for water-ice particles, and thus build up an icy coating and form the fields of icy particles that make up Noctilucent Clouds. Rietmeijer recently proposed (Planet. Space Sci. 49 (2001), 71-77) that in time these particles might react and form more complex Hydrated Iron molecules such as Goethite. Eventually, these will trickle down to the lower atmosphere and in that process lose their icy coating .

The icy particles making up NLC’s that formed when the products of interplanetary dust re-entry combined with the earth’s atmosphere, catch the last rays of sunlight due to their high altitude and hence start to illuminate in brilliant ghostly silverish, bluish and orange hues. Because of their altitude, they do that while the sky is already largely dark and normal clouds at much lower altitudes have long turned into darkness. These illuminating nighttime clouds are rare - they don’t occur each night and sometimes not even each month or year. In early twilight they become visible at altitudes up to 25-30 degrees at the northern horizon. As night progresses and the sun sinks deeper below the northern horizon, the altitude decreases, until only a thin stripe up to 2-3 degrees altitude remains near local midnight. As the clouds drift away, they may be gone a few hours later. The NLC’s you see at your horizon are generally at large distance from you – some 500 to 700 kilometers more to the north than you are. The NLC’s in above photograph were situated above southern Norway, but visible from the Netherlands, some 700 kilometers (over 400 miles) more to the south.

NLC’s often show wavy structures - as ripples in a stream- due to heigh altitude winds. In the course of a few minutes, their shapes can change. They can be very bright, but also very faint. A clear view at your local northern horizon is necessary.

Not all micrometeoroids entering our atmosphere completely vaporize. Some survive and start to float around in the atmosphere as true meteoritic dustparticles. These dustparticles themselves are called IDP’s – Interplanetary Dust Particles. They basically show the same structural composition as meteorites – some are Chondritic in character, others composed of Nickel-Iron. In time, they will trickle down to the lower atmosphere and then wash out with the rain to become embedded in the Earth sediments. NASA sends ESR reconnaissance planes (converted U2 spy planes) to collect these dustparticles at altitudes of 25 kilometers. These aircraft however cannot reach as high as the Noctilucent Clouds appear. All what we know about NLC’s proper, is largely through earthbased remote observation, and some limited sounding rocket data.

Note: an update on NLC particles provided by Frans Rietmeijer (UNM) is gratefully acknowledged.

Image and text © 2000 by M. Langbroek