Instruments:csl

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Colorado State University Lidar

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Acknowledgments

The Colorado State University sodium lidar is operated by the Colorado State University with support from the National Science Foundation. It is designated as a Ground Based Instrument (GBI) for the TIMED satellite mission, and so has additional support from CEDAR/TIMED NSF and NASA (National Aeronautics and Space Administration) funding.

Data Description

The Colorado State University (CSU) narrow-band sodium (Na) lidar is located at Fort Collins, Colorado at (40.59N, 105.14W), 1570 m above sea level. It was developed with CEDAR support and has been in operation since 1990. The sodium layer is located between about 70 and 120 km, with a peak near 92 km. At the end of 2001 at 92 km, the apex magnetic lat,lon coordinates were (49.61, -40.17), where the magnetic inclination and declination angles were 67.58 deg and 10.33 deg. The magnetic local time at 0 UT is 1612 MLT.

The centroid wavelength of the sodium D2 transition is at 589.158 nm, which corresponds to a centroid frequency of f0=5.0886E+14 Hz. The peak of the sodium D2a transition is located at fa=-0.6514 GHz relative to the centroid frequency.

The 2-frequency system is the simpler, backup system, where the cavity of the laser is tuned between fa and a second higher frequency, fc=0.1878 GHz above the centroid frequency f0. The sodium density is determined from the the signal intensity at fa, Sfa, normalized to Rayleigh scattering at a lower altitude, e.g., 30km, while the temperature can be determined from the ratio of the signal intensity, Sfc, received at fc to that at fa, Sfc/Sfa, for the 2-frequency operation.

The 3-frequency system uses an acoustic optic modulator (AOM) which shifts the frequency from fa where the continuous wave (cw) laser is locked to a lower frequency fl=-1.2814 GHz, and a higher frequency fh=-0.0214 GHz, relative to f0. With 3-frequency operation, the temperature and the line of sight (los) velocity are determined collectively from two intensity ratios: the ratio 0.5(Sfh+Sfl)/Sfa, which is mainly sensitive to temperature, and a second ratio of (Sfh-Sfl)/Sfa mainly sensitive to the los velocity.

The CSU lidar has two telescpes, so they can both be used and pointed in different directions to get concurrent observations of Na, temperature, and los winds that are reported in different kindats. The telescopes are often pointed 60 degrees in elevation to geographic north (azimuth=0 deg) and geographic east (azimuth=90 deg), in kindats=17011-2, where
Horizontal winds = los winds/cos(60) = 2*los winds
assuming zero vertical winds are stored in kindats 18011-2.

Up to the end of 2000, most observations were made during the night, and most were taken with 2-frequency operation using 1200 lidar shots per min. By the end of 2000, a total of 501 nights of temperature and sodium density data were taken and analyzed. The hourly, nightly and monthly mean profiles from 51 nights in 1993 are stored in kindats 17001, 17002 and 17003.

The nocturnal mean temperatures at 87+/-1.85 km for each night with 4 hours of data or longer were calculated from nightly mean photofiles directly, and from the mean of hourly averaged temperatures from 419 nights between 1990 and 1999. They were vertically smoothed by a running Hanning window with FWHM (Full Width at Half Maximum) of 3.7 km. The signal to noise ratio was better for the nightly mean photofiles obtained by averaging the photon files over the entire night before the temperatures were calculated. Therefore, this nightly mean temperature was defined to be the temperature.

The nocturnal mean temperatures at 87 km are provided for colleagues who wish to compare the measured OH airglow rotational temperatures. She and Lowe (1998) suggest that under normal conditions, the centroid of the OH layer is at 87 km and that the OH temperature can then be used as proxy for temperatures at 87 km. However, this proxy should be used with care, because under unusual conditions when the OH profile is highly disturbed, sometimes with the appearance of double peaks, the OH temperature may differ from the lidar temperature at 87 km in excess of 20 K.

Daytime observations are made using a Faraday filter which consists of a sodium atomic vapor cell in an imposed magnetic field between a pair of crossed polarizers. The broadband background sunlight is removed by the crossed polarizers. At the proper setting of vapor density in the Na cell and the imposed magnetic field, the received sodium light, linearly polarized after the first polarizer, has its polarization rotated by 90 deg and is retained. This technique has been used for both the 2-frequency system and the 3-frequency system to find daytime horizontal winds. Daytime lidar data in the CEDAR Database started in April 2002.

At present, the width of the range gates (code 126) is 150 m for the lidar observations. The lidar fires 3000 shots per minute (50/s), evenly divided among the frequencies used. Two minute observations are the shortest periods retained by the system, and these are usually summed over 1 UT hour (code 61) and smoothed in altitude using a Hanning window [Press et al., 1986] to give a vertical Full Width at Half Maximum (FWHM, code 4021) resolution of ~2 km and ~4 km, respectively for nighttime and daytime data. FWHM = # samples in range ave * width of range gate * cos(90-elevation) / 2, or 2.01 km for 31 samples, at 60 degree elevation angle, with 150 m gates. The vertical resolution and laser shots per minute varied in the past and may vary again in the future. In some cases higher resolution data may be available from the data providers, where the UT is the 2 min ending time.

For TIMED/CEDAR, the 3-frequency operation is intended, and a mean sodium density, neutral temperature, and neutral los wind are given for each telescope associated with a particular azimuth at 1.0 km intervals between 75 and 115 km at regular UT intervals (kindat 17011) and every night or day (kindat 17012), where the night or day average is computed from the interval averages weighted by the number of profiles used in each interval (code 415). Also included are measurement uncertainties calculated from the standard deviation of the photon noise present in the stated temporal and spatial resolution. The horizontal wind components (zonal or meridional) from the number of telescopes used are stored in kindats 18011 and 18012 for the UT interval and average values along with copies of the temperatures in those directions.

Three days of data will be available each month starting January 2002 for TIMED/CEDAR use. Other periods are available from the data providers and are listed at [csldates.html /instr/csldates.html]. The January through March 2002 data were revised in August 2002.

The physics behind and the technology of the 2-frequency operation can be found in She et al. (1992). Those who are interested in the technique should contact the data providers.

References 

Key references for the technology of and the science derived from the Colorado State University (CSU) narrow-band sodium (Na) lidar are:
Press, W. H., Flannery, B. P., Teukolsky and W. T. Vetterling, "Numerical Recipes, the art of scientific computing", p. 425, The Cambridge University Press, 1986.
She, C. Y., H. Latifi, J. R. Yu, R. J. Alvarez II, R. E. Bills and C.S. Gardner, Two-Frequency Lidar Technique for Mesospheric Na Temperature Measurements, Geophys. Res. Lett., 17, 929-932, 1990.
She, C. Y., J. R. Yu, H. Latifi and R. E. Bills, High-Spectral-Resolution Fluorescence Lidar for Mesopheric Sodium Temperature Measurements, Appl. Opt., 31, 2095-2106, 1992.
Chen, H., C. Y. She and Eric Korevaar, Na Vapor Dispersive Faraday Filter, Opt. Lett. 18, 1019-1021, 1993.
She, C. Y., and J. R. Yu, Doppler-Free Saturation Fluorescence Spectroscopy of Na Atoms for Atmospheric Applications, Appl. Opt., 34, 1063-1075, 1995.
Yu, J. R. and C. Y. She, Climatology of a mid-latitude mesopause region observed by a lidar at Ft. Collins, CO (40.6N, 105W), J. Geophys. Res., 100, 7441-7452, 1995.
She, C. Y. and U. von Zahn, The concept of two-level mesopause: Support through new lidar observation, J. Geophys. Res., 103, 5855-5863, 1998.
She, C. Y., S. W. Thiel and D. A. Krueger, Observed episodic warming at 86 and 100 km between 1990 and 1997: Effects of Mount Pinatubo eruption, Geophys. Res. Lett., 25, 497-500, 1998.
Williams, B. P., C. Y. She, and R. G. Roble, Seasonal climatology of the nighttime tidal perturbation of temperature in the midlatitude mesopause region, Geophys. Res. Lett., 25, 3301 - 3304, 1998.
She, C. Y., and R. P. Lowe, Seasonal temperature variations in the mesopause region at mid-latitude: comparison of lidar and hydroxyl rotational temperatures using WINDII/UARD OH height profiles, J. Atmo. Solar-Terr. Physics, 60, 1573-1583, 1998.
Krueger, D. A., and C. Y. She, Observed "long-term" temperature change in a midlatitude mesopause region in response to external perturbation, Earth, Planets and Space, 51, 809-814, 1999.
Chen, S. S., Z. L. Hu, M. A. White, D. A. Krueger and C. Y. She, Lidar observations of seasonal variation of diurnal mean temperature in the mesopause region over Fort Collins, CO (41oN, 105oW), J. Geophys. Res., 105, 12,371-12,379, 2000.


1993 Summary Nightly Hourly Plots for Colorado State Lidar

At 0.5 km intervals between 70 and 119 km, a mean temperature and sodium density are plotted every hour. There are 4 areas in the summary plots for each of 2 nights of data. The top plots show the neutral temperature as a function of height and time, while the bottom plots show the log of the sodium density.

Summary Nighttime Ave Tn at 87 km for Colorado State Lidar

Summary 3-Frequency Night and Day Plots for Colorado State Lidar

At 1.0 km intervals between 75 and 115 km, a mean sodium density, temperature and mean horizontal wind are plotted every hour. There are 4 areas in the summary plots for each night/day of data. The top plots show the log of the sodium density and the neutral temperature as a function of height and time from a combination of the two telescopes if present, while the bottom plots show the neutral horizontal winds from one or both of the telescopes. Local midnight is at 0701 UT. The plots go from 0 UT to 27 UT, so there can be data up to 3 UT on the day after the date listed in the plot.

-Revised 01 Jun 2006 by Barbara Emery

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