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Light Detection And Ranging
Light Detection And Ranging (LIDAR) is a remote sensing technology that measures distance by illuminating a target with a laser and analyzing the backscattered light. In recent years lidar became popular as a technology to make high-resolution maps for geology, seismology, and forestry, among other areas. This innovative technology you can even find it in cell phone applications.
One of the first major applications of lidar was the mapping of the moon during the Apollo 15 mission in 1971. Soon after, lidar for meteorological applications was constructed to measure various components and parameters of the Earth’s atmosphere. Lidar technology has since expanded vastly in capability and systems that are used to perform a range of measurements, including profiling clouds, measuring winds, studying aerosols, and quantifying various atmospheric constituents and parameter from the troposphere to the lower thermosphere.
Principal of Operation
Lidar is an active remote sensing instrument that sends out a pulsed laser beam and receives the light scattered back from the atmosphere. At which distance the scattering process has occurred is calculated from the time elapsed between emission and detection of the light. A variety of information about the scattering medium can be derived from the intensity, spectral composition, and polarization of the scattered light, depending on the emitted light's properties. The scattering and absorption of the laser beam are proportional to the scatterers' density and absorbers in the atmosphere. The various types of lidar instruments are classified according to the atmospheric parameter measured or the scattering process or atmospheric constituent used.
Arecibo Lidar Facility
The work at the Arecibo Lidar Facility is mainly focused on the so-called mesosphere and lower thermosphere (MLT) region at around 85 – 115 km of altitude but also includes the stratosphere down to 30 km and ion observations as high as 160 km altitude. The MLT region is experimentally challenging to access as it is too high for aircrafts or atmospheric balloons and too low to fly satellites because of the atmospheric friction. This region of our atmosphere is where meteors ablate (burn up) because of their speed entering the earth atmosphere and the increasing density of the atmosphere. The meteor trail is leaving behind a permanent layer of metal atoms, molecules, and dust. Resonance lidar can detect the quantity of a given atomic species from the remains of meteors. Atmospheric constituents at these altitudes can provide useful information about temperature, composition, and chemistry. The atomic metal layers can also be used as a tracer to study the dynamics of the upper atmosphere and to understand processes from short-term variabilities to long-term seasonal variations for climatological purposes.
Potassium Resonance Lidar
The potassium Doppler-Resonance lidar at Arecibo Observatory was primarily developed for the purpose of measuring the temperature in the mesosphere and lower thermosphere (MLT) region by probing the spectral shape and width of the resonance line of mesospheric potassium atoms. With the so-called three frequency technique, it is possible to derive the atmospheric temperature, wind, and potassium density from these measurements. The transmitter unit includes an alexandrite power laser, a seeding laser that is locked to the Doppler-free spectrum of the potassium D1 spectral line, and acousto-optic modulators to tune the emitted wavelengths to three specific points at the spectral shape of the resonance line. The receiver unit consists of an 800 mm diameter telescope mirror that collects the backscattered light from the atmosphere and focuses it on a fiber cable. On the detection bench, the light is then filtered and converted to electronic signals for data recording and analyzing.
Long-term seasonal studies for the climatology of the nighttime mesopause temperature and potassium density. This ongoing study is expected to reach 30 years of observations as ‘Climatology’ is defined by the World Meteorological Organization (WMO) or at least cover two solar cycles.
A short time series of 33 months of temperature observations has been published here:
A longer time series of the mesospheric potassium layer is published here:
These time series are already giving valuable information; however, it needs to be continued to understand, for example, the influence of the solar cycle on temperature and density.
Tidal and gravity waves are a major driver of the atmosphere. Short-term (during one or several consecutive nights) observations of the temperature (and wind) structure of the mesopause give valuable information on the gravity wave spectrum, wave-breaking, and tidal motions. A general overview of the gravity waves activities is given in this publication . However, the tidal activities and the coupling between the different atmospheric layers is missing. In future efforts, it is planned to combine the Rayleigh lidar and Resonance lidar measurements to gather a temperature profile from around 30 to 105km. With this, the coupling and wave activities in the stratosphere and mesosphere can be investigated. This can also help to understand the forcing from the MLT system into the ionosphere from below. A rare example of simultaneous observation of the neutral and ionized atmosphere can be found here:
The formation of neutral atomic layer enhancements, sometimes called "sporadic" or "sudden" layers, are still debatable. Resonance measurements can retrieve temperature profiles of these layers and look for wind shears or other dynamics that may lead to their formation. Working in combination with the multi-metals lidar and Sodium lidar (see separate tap please insert link), we can also gain inside to the chemical reactions taking place within the layers.
Set up of the Potassium Resonance Lidar