Temporal variability of the telluric sodium layer

The temporal variability of the telluric sodium layer is investigated by analyzing 28 nights of data obtained with the Colorado State University LIDAR experiment. The mean height power spectrum of the sodium layer was found to be well fit by a power law over the observed range of frequencies, 10 microhertz to 4 millhertz. The best fitting power law was found to be 10^\beta \nu^\alpha, with \alpha = -1.79 +/- 0.02 and \beta = 1.12 +/- 0.40. Applications to wavefront sensing require knowledge of the behavior of the sodium layer at kHz frequencies. Direct measurements at these frequencies do not exist. Extrapolation from low-frequency behavior to high frequencies suggests that this variability may be a significant source of error for laser-guide-star adaptive optics on large-aperture telescopes.Comment: 3 pages, 3 figures, accepted for publication in Optics Letter

Focus errors from tracking sodium layer altitude variations with laser guide star adaptive optics for the Thirty Meter Telescope

Laser guide star (LGS) adaptive optics systems for extremely large telescopes must handle an important effect that is negligible for current generation telescopes. Wavefront errors, due to improperly focusing laser wavefront sensors (WFS) on the mesospheric sodium layer, are proportional to the square of the telescope diameter. The sodium layer, whose mean altitude is approximately 90 km, can move vertically at rates of up to a few metres per second; a few seconds lag in refocusing can substantially degrade delivered image quality (15 m of defocus can cause 120 nm residual wavefront error on a 30-m telescope.) As well, the range of temporal frequencies of sodium altitude focus, overlaps the temporal frequencies of focus caused by atmospheric turbulence. Only natural star wavefront sensors can disentangle this degeneracy. However, applying corrections with representative focus mechanisms having modest control bandwidths causes appreciable tracking errors. In principle, electronic offsets measured by natural guide star detectors could be rapidly applied to laser WFS measurements, but to provide useable sky coverage, integrating sufficient photons causes an unavoidable time delay, again resulting in potentially serious focus tracking errors. However, our analysis depends on extrapolating to temporal frequencies greater than 1 Hz from power spectra of sodium profile time series taken at 1-2 minute intervals. In principle, with a pulsed laser, (e.g. 3-μs pulses) and dynamic refocusing on a polar-coordinate CCD, this focus tracking error may be eliminated. This result is an additional benefit of dynamic refocusing beyond the commonly recognized amelioration of LGS WFS spot elongation

Solar Response and Long‐Term Trend of Midlatitude Mesopause Region Temperature Based on 28 Years (1990–2017) of Na Lidar Observations

We present midlatitude solar response and linear trend from Colorado State University/Utah State University Na lidar nocturnal temperature observations between 1990 and 2017. Along with the nightly mean temperatures (_Ngt), we also use the corresponding 2‐hr means centered at midnight (_2MN), resulting in vertical trend profiles similar in shapes as those previously published. The 28‐year trend from _Ngt (_2MN) data set starts from a small warming at 85 km, to cooling at 87 (88) km, reaching a maximum of 1.85 ± 0.53 (1.09 ± 0.74) at 92 (93) km and turns positive again at 102 (100) km. The 6‐month winter trend is much cooler than the 4‐month summer trend with comparable solar response varying around 5 ± 1 K/100 SFU throughout the profile (85–105 km) with higher summer values. We explore the observed summer/winter trend difference in terms of observed gravity wave heat flux heating rate at a nearby station and the long‐term trend of gravity wave variance at a midlatitude. Between 89 and 100 km, the lidar trends are within the error bars of the Leibniz Middle Atmosphere (LIMA) summer trends (1979–2013), which are nearly identical to the lidar‐Ngt trend. We address the need of long data set for reliable analysis on trend, the extent of trend uncertainty due to possible tidal bias, the effect of a Pinatubo/episodic function, and the impact of stratospheric ozone recovery

Climatology of mesopause region temperature, zonal wind, and meridional wind over Fort Collins,Colorado (41°N, 105°W), and comparison with model simulations

Between May 2002 and April 2006, many continuous observations of mesopause region temperature and horizontal wind, each lasting longer than 24 h (termed full-diurnal-cycle observations), were completed at the Colorado State University Na Lidar Facility in Fort Collins, Colorado (41°N, 105°W). The combined data set consists of 120 full-diurnal-cycle observations binned on a monthly basis, with a minimum of 7 cycles in April and a maximum of 18 cycles in August. Each monthly data set was analyzed to deduce mean values and tidal period perturbations. After removal of tidal signals, monthly mean values are used for the study of seasonal variations in mesopause region temperature, zonal and meridional winds. The results are in qualitative agreement with our current understanding of mean temperature and wind structures in the midlatitude mesopause region with an observed summer mesopause of 167 K at 84 km, summer peak eastward zonal wind of 48 m/s at 94 km, winter zonal wind reversal at ∼95 km, and peak summer (pole) to winter (pole) meridional flow of 17 m/s at 86 km. The observed mean state in temperature, zonal and meridional winds are compared with the predictions of three current general circulation models, i.e., the Whole Atmosphere Community Climate Model version 3 (WACCM3) with two different simulations of gravity wavefields, the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA), and the 2003 simulation of the Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIME-GCM). While general agreement is found between observation and model predictions, there exist discrepancies between model prediction and observation, as well as among predictions from different models. Specifically, the predicted summer mesopause altitude is lower by 3 km, 8 km, 3 km, and 1 km for WACCM3 the two WACCM runs, HAMMONIA, and TIME-GCM, respectively, and the corresponding temperatures are 169 K, 170 K, 158 K, and 161 K. The model predicted summer eastward zonal wind peaks to 71 m/s at 102 km, to 48 m/s at 84 km, to 75 m/s at 93 km, and to 29 m/s at 94 km, in the same order. The altitude of the winter zonal wind reversal and seasonal asymmetry of the pole-to-pole meridional flow are also compared, and the importance of full-diurnal-cycle observations for the determination of mean states is discussed

Kinetic Temperature and Carbon Dioxide from Broadband Infrared Limb Emission Measurements Taken from the TIMED/SABER Instrument

The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) experiment is one of four instruments on NASA's Thermosphere-Ionosphere-Energetics and Dynamics (TIMED) satellite. SABER measures broadband infrared limb emission and derives vertical profiles of kinetic temperature (Tk) from the lower stratosphere to approximately 120 km, and vertical profiles of carbon dioxide (CO2) volume mixing ratio (vmr) from approximately 70 km to 120 km. In this paper we report on SABER Tk/CO2 data in the mesosphere and lower thermosphere (MLT) region from the version 1.06 dataset. The continuous SABER measurements provide an excellent dataset to understand the evolution and mechanisms responsible for the global two-level structure of the mesopause altitude. SABER MLT Tk comparisons with ground-based sodium lidar and rocket falling sphere Tk measurements are generally in good agreement. However, SABER CO2 data differs significantly from TIME-GCM model simulations. Indirect CO2 validation through SABER-lidar MLT Tk comparisons and SABER-radiation transfer comparisons of nighttime 4.3 micron limb emission suggest the SABER-derived CO2 data is a better representation of the true atmospheric MLT CO2 abundance compared to model simulations of CO2 vmr

The Relevance of Hanle Effect on Na and Fe Lidars

A laser resonant scattering process involves two steps, excitation and emission. That emission occurs spontaneously is well accepted. That the atoms involved in the emission are excited coherently by a laser beam leading to a non-isotropic angular distribution of emission (an antenna pattern) is not well known. The difference between coherent and incoherent excitation leads to the Hanle effect. In this paper, I discuss the physics of Hanle effect, and its influences on the backward scattering intensity of Na, K, and Fe atomic transitions and the associated Na and Fe resonant fluorescence lidar systems

Mesopause Region Temperature and Na Number Density Between 1990 and 2017

The Colorado State University (CSU) Na lidar performed nocturnal mesopause region Na density and temperature observations between March 1990 and March 2010 at Fort Collins, CO (41.6N, 105W). The lidar was relocated to Utah State University (USU) campus and has continued its regular observation at Logan, Utah (42N, 112W) since September 2010