Lidar laser doesn’t just put on a strange show over the Logan night sky; it’s a leader in atmospheric studies
By Lindsay Nemelka
The mysterious green beam can be seen shining above campus on clear nights; and even though lighthearted suspicions of UFO contact circulate, the beam is actually a super concentrated laser used to take atmospheric temperatures.
Utah State University’s Science Engineering Research building is home to the Atmospheric Lidar Observatory, and the world’s biggest Rayleigh laser. Head of the lidar project for the National Science Foundation is Vincent Wickwar, a professor of more than 20 years at USU. Working closely with him for almost 2 years is physics undergraduate student Marcus Bingham and graduate student Leda Sox.
WHAT IS LIDAR?
The “most visual experiment in the valley,” according to Wickwar, is a complex system of powerful lasers, a telescope, fiber optics, mirrors, light amplifiers and detectors. The Lidar (Light Detection and Ranging) laser relies on Rayleigh Scattering to measure temperatures. “No one has anything else this big,” said Wickwar. This is the largest (the most sensitive and the biggest telescope) Rayleigh Scatter lidar in the world.
RAYLEIGH SCATTERING: Light has different wavelengths that break apart and scatter once they hit various sizes of atmospheric particles and molecules (mostly N2 and O2). Certain particles are more adept to scattering light of a certain wavelength, and reflecting it at a particular angle.
CHECK THIS OUT: (Click here to learn how Rayleigh Scattering makes the sky appear blue)
Taking accurate atmospheric temperatures is a difficult task for most instruments. Lidar works by shining a laser into a certain atmospheric height; the wavelength at which the laser light bounces back allows Wickwar to determine particle densities, absolute temperatures and observe phenomena such as gravity waves and noctilucent clouds.
GRAVITY WAVES: Ever seen what looks like a field of plow marks in the clouds? These are formed when air waves move through weather fronts or over mountain tops. The air is forced to move vertically into more stable altitudes, however at a colder temperature the air will then want to sink. This falling and rising of air in vertical waves increases the amplitude and causes waves to break. This creates the turbulence and nonlinear effects in the middle atmosphere. If clouds are present, the waves will create rows of clouds with clear areas between the rows.
NOCTILUCENT CLOUDS: The highest clouds found in the atmosphere—noctilucent clouds are located in the mesosphere, and can only be seen during twilight when sunlight reflects off the ice crystals in the cloud when the lower sky is darkening. These phenomena appear when changes in the upper atmosphere occur, though it isn’t fully understood how this happens. Scientists believe that the increasing frequency sightings of these clouds are connected to climate change. “These are big particles, so the signal we get stands out from the rest,” said Wickwar.
All particles give off radiation as wavelengths. Long and short wavelengths are direct indicators of the temperature of an object. Hot particles give off short wavelengths, while cold particles emit longer wavelengths. Very accurate temperatures can be taken by detecting the wavelengths of atmospheric particles. The density of particles is also a major factor in determining temperature. Using Lidar data, Wickwar plots the density of molecules at a specific altitude. “If it is cold… the density falls off fast; if it is hot, if falls off slowly. So basically looking at the slope, we get the temperature,” said Wickwar.
HOW LIDAR WORKS:
The 18 watt laser generates pulses of infrared light at 1064nm. “Doesn’t sound big compared to your 25 watt light bulb, but it’s in one small, narrow wavelength region and in one direction. Put a piece of paper here and you’d burn right through it,” said Wickwar.
This infrared light is transferred to visible light by converting the beam to a wavelength of 532nm; the wavelength of the color green. The strong, excess infrared radiation that failed to convert to green light passes through a dichroic filter, while the green beam is reflected up a chimney to the deck. The roof of the large deck is motorized to slide out so the beam can travel into the atmosphere.
INFRARED: Light is emitted in different wavelengths. When a wavelength is longer than that of visible light it is called infrared light. Infrared includes thermal radiation that objects release, and is directly related to their temperature. Night vision goggles can detect infrared radiation that comes off of objects when visible light is low. Most of the sun’s energy is emitted as infrared light.
DICHROIC: When light is transferred through a dichroic filter, only certain wavelengths are able to pass through while the rest are reflected. For example, a blue dichroic filter will reflect only strong blue light, which reaches the eye and becomes visible. This is because all other light wavelengths are absorbed within the different refractive indexes built within the glass, while the blue wavelength is reinforced. (You may remember playing with these in school as a kid).
Though it appears as one continuous beam of light, each 8 nanosecond pulse generated moves at the speed of light; and 30 pulses are generated per second. The beam may seem vertical, but actually diverges at an angle of 0.5 miliradians. The mirror which diverts the beam is mounted on a giant rotator, so the beam is able to be directed at any point in the sky within 45 degrees of vertical.
COOL FACT: TV screens refresh images at the same rate of the Lidar laser—30 pulses per second, in order for the screen to appear like one continuous moving picture.
TRY THIS: When looking at the beam, move your head rapidly from side to side in order to see the distinct pulses. (My roommates and I tried to do this unsuccessfully with our TV)
The green beam is tailored to reach atmospheric heights from 25 km up to 150 km, way into the lower thermosphere; (the average plane flies at 10 km).
ATMOSPHERIC HEIGHTS: The Troposphere is the layer of atmosphere in which exists life on earth. It extends 20 km around the equator, and thins out to about 9 km at the poles. This is the layer in which most weather occurs. The Stratosphere reaches 50 km above the surface of the earth and holds 19% of the atmosphere’s gasses. Temperature increases the further you go up in this region. The Mesosphere is next, extending 90 km above earth’s surface and where air thins out and the temperature decreases. Meteors break apart at this level, which is considered the middle atmosphere. Above the Mesosphere is the Thermosphere—earth’s upper atmosphere. Extreme radiation from the sun is absorbed by molecules in this 690 km high layer. The Lonosphere is the outermost layer of the atmosphere where space meets and satellites orbit. It extends 10,000 km above the surface of the earth.
The light hits atmospheric particles (including meteorite particles) and scatters, sending the backscattered wavelengths, which are reflected by four mirrors 1 ¼ meter in diameter, down the chimney into a Newtonian telescope. Though the beam is intense enough to cause blindness, so little light is backscattered that a 44 cm diameter telescope is needed in order to capture it.
LIDAR LASERS AS WEAPONS? Though lasers are often seen on firearms, the laser itself is not the source of the weapon’s firepower. Lasers create high energy pulses but only for a brief period. However, even a ½ watt laser is enough to cause eye damage. Wickwar recalled an experiment he tried by bringing the point of the laser to the edge of the building and put a piece paper at its end. The beam particles had traveled and spread out enough that the energy density had decreased; so much that the paper took 30 minutes to brown. Wickwar said, “It’s not one of these futuristic weapons to shoot down a plane. We’re nowhere near that power.” Weaponized lasers are usually involved with gas dynamics.
The telescope focuses the wavelengths to a single point and reflects them through a chopper; which blocks dust particle signals from the very lowest altitudes (about 40 km) and prevents detector-damaging wave “noise” from skewing data.
NEWTONIAN TELESCOPE: The earliest known functional reflecting telescope was built in 1668 by Sir Isaac Newton. Light is reflected using a concave mirror to focus wavelengths to one point, and a flat mirror to reflect it. Newton built this telescope because he sought to prove his theory that white light is made up of a spectrum of colors.
The rest of the light passes through the chopper into a photomultiplier tube—which is so sensitive it can detect a single photon. (Much painstaking work is done to keep these optical surfaces clean, such as bringing in dust filters and piping clean nitrogen gas into the detection chamber).
The signal is averaged over 250 ns and stored on a computer where the data is converted to temperature and atmospheric density. Bumps in the chart are always exciting for Wickwar, and normally indicate atmospheric phenomena.
IN SHORT: The laser generates infrared light. The infrared wavelength is transformed to green visible light. The green light is reflected by a dichroic filter up a small chimney and up into the atmosphere. The light reflects off particles in the atmosphere and bounces back down to the Newtonian telescope. The telescope directs the wavelength into a single beam and bounces it into a detector where the different wavelengths are recorded.
Because other instruments are unable to take accurate density and temperature readings of the middle atmosphere, Wickwar says that’s why lidar instruments are so important. “This is sort of unique in what it can cover; there’s a big hole in the middle and that’s where the Rayleigh and lidar is unique as a ground-based instrument,” he said.
On clear nights, the laser probe is sent into the sky while a team of graduate and undergraduate students work below as sitters. During the 11 years of operation, the Lidar has recorded about 900 nights of data.
Wickwar’s team is currently working on adding another 24 watt laser, trying to combine the output of two lasers in order to widen the atmospheric detection range. “We’re in the upgrading stages,” said Marcus. Wickwar explained that “this is also one of the first efforts to combine the output from all the mirrors into one, and also one of the first to combine the input of two lasers instead of one.” Currently accurate readings can be taken from 40 to 110 km, but will soon be expanded to a height of over 200 km by adding more detectors and the second laser.
WHY SHOULD YOU CARE?
The reason why atmospheric temperature is important is because it directly links to global warming, said Wickwar. Tracking temperatures in the middle atmosphere may give scientists a good indication about earth’s climate change.
DID YOU KNOW: Atmospheric temperatures are exactly the opposite of what you’d expect. The coldest temperatures are reached during summer, and in the winter the temperature warms up. This is due to the high amount of excess radiation from warmer temperatures leaving earth’s atmosphere in the summer, cooling the upper region of gases.
With over 11 years of temperature data, Wickwar and his team plot changes in temperature over time. Wickwar said they are seeing an overall decrease in the amount of time it takes for temperatures to change—a decrease 10 times bigger than what has been predicted by scientists. Findings are also showing an increased amount of water vapor in the lower stratosphere—another indicator of global warming.
COOL CAMPUS, COOL RESEARCH
Marcus wasn’t able to pinpoint exactly what he does with the Lidar project because of the various odd jobs he performs, such as cleaning giant delicate mirrors, tweaking computer programs, and shopping for lenses; but he loves all the different skills he’s developing. “It’s been really fun because I’m pretty much a hands-on learner, and so being able to do this kind of stuff and use what skills I have has been a great experience.”
Wickwar said that USU is unique, not only because it holds the amazing Lidar, but because of all the research that happens here. He said that Marcus gets involved with different tasks because “that’s what happens with experimental science. –We get students significantly involved, they get real projects to do.”
Marcus and Wickwar are looking forward to more than just another night of aligning light beams; they hope to gather enough significant data about earth and its changing atmosphere to make a difference.