LIDAR GLEME: Global Lidar Exploration of the MEsosphere

LIDAR | A Mesospheric Lidar Mission

The mesosphere remains to this date one of the least explored and understood parts of the Earth atmosphere: Situated at altitudes ~50 to 90 km, it is too high for balloon experiments and too low for orbital vehicles, due to significant atmospheric drag. Thus mesospheric measurements are primarily performed either by rocket experiments or by lidars and radars from the ground. Long-used for ground-based observations of the atmosphere, lidars are now being carried by a new generation of Earth observing satellites (e.g., CALIPSO and the upcoming ADM-AEOLUS missions) which are studying primarily the lower layers of the atmosphere. It is the overall objective of this study to investigate the prospect and advantages of using a spaceborne lidar system dedicated to the study and monitoring of the mesosphere.


Abstract
We present the first description of the Global Lidar Exploration of the MEsosphere (GLEME) mission concept designed to study mesospheric dynamics and chemistry. Determination of small scale waves in the mesosphere is the primary motivation and science focus of this mission and has driven the preliminary mission design.

The lidar is designed to obtain high spatial and temporal resolution temperature and horizontal winds in the mesosphere, allowing determination of gravity wave characteristics, heat and momentum wave flux, and the effects on the background atmosphere. We have developed a novel measurement scheme that allows the determination of the vector winds at high resolution.

Finally we discuss next steps in instrument and mission definition and future flight opportunities.
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Gleme Science Objectives

This mission concept is motivated by the outstanding mesospheric science questions that only a high resolution and accuracy instrument such as a lidar can provide. These include issues in mesospheric chemistry and composition, transport phenomena, noctilucent clouds (NLCs), metallic layers and meteoric dust, and most importantly small scale waves.

Preliminary Mission Design

Lidar Mission A Small
Lidar Mission A Small
Lidar Mission A Small

Our preliminary choice of orbit balances power and coverage both over topography and over the polar regions.

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Viewing Geometry

Lidar Geometry A Small
We develop a novel measurement scheme uses two orthogonal look directions offset ~30° from nadir and the satellite motion to provide common volume measurements with two orthogonal lines of sights for the decomposition of vector horizontal winds. This obtains high temporal and spatial resolution horizontal winds and temperatures.
Lidar Geometry A Small

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Doppler Shifts

Lidar Doppler Small
Significant complications for a spaceborne resonance fluorescence lidar are the Doppler shifts associated with the spacecraft frame-of-reference relative to the Earth’s. which may be split into two effects: Doppler Shift due to spacecraft motion and Doppler Shift due to Earth’s rotation. The combined effect of the shifts adds significant requirements to the laser transmitter subsystem, as well as to the receiver. The effects are presented here. The effective Na line frequencies are varying functions of latitude and are displaced by ~9GHz between the fore and aft looking telescopes.

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Measurement concept

Lidar Concept Small
In order to resolve both temperature and wind measurements, the two frequency technique is not sufficient. Typically a three frequency technique displaced evenly about the D2a peak at offsets of ±630 MHz is used.

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Preliminary Error Analysis

Lidar Analysis A Small
Lidar Analysis B Small
Lidar Analysis C Small
We calculated the errors that we would expect for retrievals of temperature and LOS wind in the core mesosphere region and determined that Na is the optimal choice for this lidar mission.

Our preliminary error analysis shows temperature, wind, and sodium density at high enough resolution to obtain a significant part of the gravity wave spectrum.

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Summary

We have developed a preliminary design for a spaceborne, LEO mesosphere lidar mission that will study gravity waves and their effects on the mean state and variability. We demonstrate that sodium is the optimal choice for this lidar to obtain high enough temporal and spatial resolution. Significant trade-offs may be required in terms of integration time (spatial resolution) and the instrument design.
Preliminary GLEME lidar parameters
Wavelengths λ0 = 589.16 nm (vacuum)
λ1,4 = λ0 ± 5 GHz
λ2,3 = λ0 ± 5 GHz ± 630 MHz
λ5,6 = λ0 ± 5 GHz ± 630 MHz
Pulse energy 50 mJ
Pulse Repetition Rate 33 Hz (3 frequencies)
Average optical power 5 Watts
Average electrical power consumption 500 Watts

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