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A new satellite has captured its first views of enigmatic glowing clouds whose proliferation may be linked to climate change.
NASA launched the Aeronomy of Ice in the Mesosphere (AIM) satellite on 25 April on a mission to investigate "night-shining" or noctilucent clouds. These clouds float 80 kilometres above the ground and are made of tiny ice crystals. Because they are so high up, the Sun still reaches them and makes them glow after sunset and before sunrise, when the ground is in darkness.
The clouds have been observed since the 1800s, but in recent years they have brightened and grown more numerous. They were first observed above Earth's polar regions, but have now spread to latitudes as low as 40°. Some scientists suspect their proliferation is related to increasing greenhouse gases, which can actually cause the upper reaches of the atmosphere to cool.
Now, the AIM satellite has inaugurated a new era in noctilucent cloud research, obtaining its first views of these ghostly structures.
The satellite spotted its first noctilucent clouds on 25 May. Since then, the clouds have become more numerous and have spread to lower latitudes (see map at right of the clouds' distribution on 11 June).
Low temperatures
AIM has observed the pattern of noctilucent clouds around the north pole rotating over time. Russell suspects this may be due to areas of especially low temperature moving around rather than the clouds themselves moving. Temperatures where the clouds form hover at about -140° C.
Some researchers suspect the increase in the clouds seen in recent years could be due to human activity. Carbon dioxide absorbs energy in the upper atmosphere as well as radiation leaving the Earth's surface and then reradiates it very efficiently.
About half of that energy returns to the Earth's surface, causing warming. But because the carbon dioxide helps radiate energy away from the upper atmosphere, there is a net cooling there.
Human activities
"Temperature certainly is one plausible explanation for why we're seeing more of the clouds and at lower latitudes and brighter," AIM chief scientist James Russell III of Hampton University in Hampton, Virginia, US, told New Scientist.
He also points out that the amount of water vapour in the upper atmosphere has increased in the past 50 years, perhaps as a result of increased methane in the atmosphere from farming and other human activities. Methane can react with oxygen in the atmosphere to create water molecules.
Noctilucent clouds can be seen from the ground with the naked eye, appearing after twilight when the sky first gets dark and also just before the sky brightens prior to sunrise. They glow a silvery blue colour. "They're very beautiful," Russell says. "You can't miss them – they're iridescent and bright and they have a lot of structure."
Seasonal effect
The northern hemisphere's noctilucent cloud season ends towards the end of August. When northern summer is over, the satellite will study the appearance of the clouds in the southern hemisphere, where they are prevalent between mid-November and mid-March.
The satellite's Cloud Imaging and Particle Size instrument produces panoramic images of the noctilucent cloud distribution. The Solar Occultation for Ice Experiment (SOFIE) provides measures the chemical makeup of the clouds and their altitude, and the Cosmic Dust Experiment measures space dust entering Earth's atmosphere.
Mission scientists are trying to determine whether this dust provides the "seeds" for ice particle formation for noctilucent clouds or if this role is instead played by dust particles blown upward from Earth.
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