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• W1510062862 abstract "Rayleigh-Scatter lidar systems effectively use remote sensing techniques to continuously measure atmospheric regions, such as the mesosphere (45-100km) where in situ measurements are rarely possible. The Rayleigh lidar located at the Atmospheric Lidar Observatory (ALO) on the Utah State campus is currently undergoing upgrades to make it the most sensitive of its kind. Here, the important components of these upgrades and how they will effect the study of a particular atmospheric phenomena, atmospheric gravity waves, will be discussed. We will also summarize what has been done to the system during this year to bring us to the threshold of initial operations. I. INTRODUCTION The Rayleigh-Scatter lidar system at the Atmospheric Lidar Observatory (ALO) at Utah State University is currently going through a series of upgrades to make it the most sensitive instrument of its kind. The previous version of the Rayleigh lidar operated for more than a decade on the USU campus and was successfully used to measure relative densities and absolute temperatures in the mesospheric region of the atmosphere (45-90 km). From these measurements, 11 years worth of temperature data were collected that allowed researchers to study structures and trends in the temperatures over a long time frame. Also, atmospheric phenomena such as atmospheric gravity waves (AGWs), planetary waves, and noctilucent clouds were studied. After these upgrades to the system, it will be the most sensitive Rayleigh-scatter lidar in the world. Increasing the system’s laser power and collecting area achieve this. The altitude range will also have been greatly expanded by increasing the number of PMT detection channels. With the improved system, atmospheric phenomena, such as AGWs, which can travel throughout all altitudes of the Earth’s atmosphere and can have small temporal and spatial variations, can be better studied. There is still much that is unknown about AGW creation at low altitudes and the dynamics of AGW propagation horizontally and vertically throughout the atmosphere. It will be an excellent instrument for making AGW observations to determine their behavior and relate it to theory. II. BACKGROUND The term Rayleigh scatter refers to a particular type of scattering interaction between light and particles. In this case, the size of the particles is much smaller that the wavelength of the light. Another important aspect of Rayleigh scatter is that the wavelengths of the incident and scattered light are the same, or in other words the interaction between the light and scattering particle is elastic. For example, Rayleigh lidar systems use incident laser pulses to induce Rayleigh scatter off of N2 and O2 molecules in the atmosphere and return light of the same wavelength, in our case 532 nm. This is much longer than the size of N2 and O2 molecules, which are both on the order of 0.1 nm in size. Rayleigh scatter dominates over other types of scattering above approximately 25 km (35 km after a major volcanic eruption such as Mt Pinatubo). The interpretation of the return signal is most straightforward below 100 km, where the major constituents are turbulently mixed. In lower regions, different types of particles are added to the atmospheric mixture. These particles, such as aerosols (dust), are much bigger than N2 and O2 molecules. Incident light interacting with these particles results in a different type of scattering called Mie scatter. Mie scatter is characterized by scatterers that have sizes either on the order of or much larger than the wavelength of incident light. Their scattering cross sections are much bigger than those for Rayleigh scatter, with the result that the signal from Mie scattering is much bigger at altitudes where Mie dominates over Rayleigh scattering. A third type of scattering interaction differs from the previous two significantly. Raman scatter involves an inelastic interaction between the incident light and scattering particle; in other words, the incident and scattered wavelengths of light are no longer the same. The wavelength of the scattered light depends on the type of scatterer and the wavelength of the incident light. For example, the Rayleigh lidar will transmit pulses at 532 nm. When it scatters from N2, Raman scatter will lead to photons at 607 nm. The cross section for Raman scatter is the order of 10 smaller than for Rayleigh scatter. As a Results from an Extremely Sensitive Rayleigh-­‐Scatter Lidar Leda Sox, Vincent B. Wickwar Utah State University 2012 Rocky Mountain NASA Space Grant Consortium 2 result the signal is small enough that it would only be detectable with the upgraded system below about 40 km. Since lidar systems measure, specifically, the time of flight of a light pulse in order to determine the altitude or distance from the source to a scatterer, pulsed lasers are an essential component to their functionality. To detect the backscattered light, lidars first use large-mirror telescopes to capture the returned photons and then direct them into photon detectors that convert the physical, returned photons into a digital signal. The ALO Rayleigh lidar uses photomultiplier tubes (PMTs) for its detectors. After being directed onto the PMTs by the receiver optics, the photoelectrons from the photocathode cascade through the PMTs creating electronic pulses that are counted as a function of time (distance) by a multichannel-scaler (MCS). They are then downloaded to a PC, where they can be recorded and analyzed. Each lidar is comprised of slight variations on this basic setup. The previous version of the ALO Rayleigh lidar, in particular, used as its transmitter one Spectra Physics Nd:YAG laser frequency doubled to emit a 532 nm pulsed beam that had a average power rating of 18-W or 24-W depending on the stage in its history. Its receiver was made up of one 44-cm collecting mirror and one PMT channel. The previous system is depicted schematically in Figure 2. The large power output allowed measurements to be made high into the mesosphere to about 90 km while the dynamic range of the single detection channel limited the lower altitude to 45 km. The upgrades aim to increase the power output, receiving area and number of detection channels in order to increase the overall altitude range and sensitivity of the instrument." @default.
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• W1510062862 date "2012-01-01" @default.
• W1510062862 modified "2023-09-27" @default.
• W1510062862 title "Results from an Extremely Sensitive Rayleigh-Scatter Lidar" @default.
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