This week's science question is - Using satellite sensors that operate in the visible wavelengths, is there a way to observe the polar regions during the winter months?
Recent climate studies in the Arctic indicate that sea ice is melting at rates faster than previously thought. According to one study, the rate at which sea ice is currently disappearing (about 9% per year), could mean that most of the perennial sea ice will be a thing of the past by the end of the 21st century. The same study also finds that temperatures in the Arctic are increasing at the rate of 1.2 degrees Celsius (2.2 Fahrenheit) per decade. In fact, observations have shown that this past year's perennial ice cover is the least extensive ever observed during the satellite era (more than 30 years). Things haven't gotten any easier over the years for poor old Santa. Evidently he somehow knew that the ice around the North Pole might melt and long ago decided to take his sled and reindeers aloft.
One of the characteristics of polar regions is that they're light during the summer and dark during the winter. This is so because our planet's axis is tilted 23 1/2 degrees from vertical (90 degrees). The Earth's Northern Hemisphere is now tilted away from the Sun, while our fortunate brethren in the Southern Hemisphere are leaning toward the Sun. When the Sun lays over the Tropic of Capricorn (23 1/2 degrees south of the Equator) at noon, as it does now, the entire Southern Hemisphere is bathed in light, and the Antarctic region receives light all day long. For the Northern Hemisphere, however, daylight is at a minimum at this time of year, and north of 66 1/2 degrees latitude (Arctic region), there's no sunlight at all. The point is, you can't see the Sun or anything else (discounting refraction) if it's more than 90 degrees from your local zenith.
In the Arctic now, it's not only dark, but it's bitter cold - not what most people would consider to be a great combination. It many ways, people who live in the Arctic during the winter are cut off from the world. Even many satellites sensors can't penetrate the polar darkness. For example, most of those satellites that sense in the visible wavelengths, relying on reflected sunlight, are rendered pretty much useless. Our eyes sense light from about .4 to .7 microns - this is considered the visible part of the spectrum. It's funny, for some reason Santa and his coursers could see just fine in the pitch dark, but throw a little fog into the equation, and Christmas would have been ruined without Rudolph's glowing nose.
One of the advantages of satellites operating in the infrared and microwave portion of the electromagnetic spectrum is that they're able to observe the polar surface and atmosphere with little regard to how the Earth is illuminated. However, it's possible to see the North Pole in the dead of winter with some visible satellite sensors. The key is that another source of illumination is required, other than direct sunlight. How about reflected sunlight from the Moon?
Each year, the Sun ranges between a declination of 23 1/2 degrees north and 23 1/2 degrees south as it circles the sky. The Moon goes through a similar up and down motion, but every month. Actually, the Moon ranges a bit further north and south than does the Sun (a little over 5 degrees further north and south). On a seasonal time frame, the full Moon assumes a place in the sky where the Sun will be 6 months hence. In other words, in mid winter, the full Moon is high in the sky at midnight, like the Sun in summer at noon. Conversely, the full Moon in summer is much lower in the sky. Thus, during winter, the moon illuminates the Earth at a much higher incidence angle than does the Sun. For instance, at 45 degrees north latitude, the midday Sun is approximately 22 degrees (2 fists held at arm's length) above the southern horizon on December 21. Whereas, the full moon at midnight on the date of the winter solstice is about 68 degrees above the southern horizon.
It just so happens that the Moon will be full tonight (Thursday the 19th of December). Sometimes the December full Moon nearest to the solstice is known as the Long Night Moon or the Moon Before Yule. This means that at midnight, it'll be pretty close to being directly over the Tropic of Cancer (23 1/2 degrees north latitude). Therefore, in places like northern Canada, northern Alaska and northern Siberia, the Moon it'll be up all day (24 hours), as is the Sun in summer. Thus, it could provide a weak light source for satellites that operate in the visible wavelengths.
Defense Meteorological Satellite Program (DMSP) satellites observe the Earth in visible light at night as well as during the day. During the day, when the illumination source is the Sun, a solid state detector is used, and at night, when the Moon's out, a special photomultiplier tube is used. You've probably seen the satellite photos of city lights at night. These photos are taken with DMSP sensors when the Moon is new or in the crescent phase. Despite the fact that light levels under full sunlight are between 5 and 6 orders of magnitude greater than under full moonlight, features having a high albedo, like snow and ice, are bright enough to reflect light even at low levels of brightness and are therefore, quite easy to see under moonlight skies. This is evident to anyone who has ever been outside at night when the ground is snow covered and the full Moon is high in the sky.
Most all other satellites operating in the visible wavelengths are incapable of nighttime imaging. For instance, Landsat satellites have seldom been used when the solar illumination is less than 10 degrees (the Sun is at least 10 degrees above the horizon). Even though Landsat, MODIS, and a number of other satellites offer more spectral information at a better resolution than DMSP satellites, they're much less sensitive to low light levels.
Of course, the Moon also illuminates the mid latitudes and tropics as well as polar areas, so moonlight can also be used to observe these regions too. Look at the DMSP satellite photos of the Indian subcontinent (from the two attachments). The darker picture was taken when the Moon was new, and consequently city lights are quite obvious, and the brighter picture was taken during a full Moon. On the latter, note that the city lights are now washed out but that some features, such as snow in the Himalayan Mountains, are readily identifiable. Even a gibbous Moon shines with enough reflected sunlight that snow, ice and clouds can be recognized on the DMSP nighttime photos.
If you're out looking for Old Saint Nick on Christmas Eve, you might want to check out Jupiter, Saturn and Venus. Saturn is up all night now, and is unusually bright. It's one of the brightest objects in the night sky. Jupiter is even brighter than Saturn. It rises well after sunset when Saturn's already high in the sky. Venus is brighter still - about 6 times brighter than Jupiter! Look for it before sunrise on Christmas morning - you can't miss it. Keep your eyes peeled, you may see a shooting star, or maybe that's Santa blazing across the night sky.
Happy Holidays, and all the best in the New Year!
Dr. Foster originated this series and did it as a solo project for the GSFC website for SEVEN YEARS! This year Dr. Foster has decided to share the enthusiasm he has for this project with other Goddard scientists and will be posing questions on a semi-regular basis.