With the technological advances, the methods for exploring atmospheric properties have advanced considerably during the past 20 years. In what follows, the most common modem instruments are described.
i) FABRY-PEROT INTERFEROMETERS
In the upper atmosphere, photochemical reactions are accompanied by the emission o f radiation in the infra-red, visible and ultra-violet parts o f the electromagnetic spectrum. The emissions are referred to as airglow and are always present at all latitudes. Outside the auroral (high-latitude-) regions airglow is virtually unstructured, whereas magnetospheric disturbances can cause highly structured airglow profiles in auroral regions. The regions o f airglow emission move with the neutral air and thus carry information about the neutral winds. Observing the apparent wavelength of a prominent airglow line makes it possible to determine its Doppler-shift and thereby the line-of- sight neutral air velocity. Fabry-Perot Interferometers (FPI’s) measure these airglow emissions for specific (known) emission frequencies and thus give as primary parameters the neutral gas temperature (through the emissivity or spectral width) and the line-of-sight neutral wind (through the Doppler-shift). From these, the vector velocities can be derived as secondary parameters. Since airglow o f one constituent peaks at one specific height, the measured winds and temperature are values for that altitude only. However, different constituents have their airglow emission peaks at different heights. By using a selection o f filters one FPI can therefore measure temperatures and winds for different altitudes.
ii) RADARS
Atmospheric gas is a medium of differently sized particles and irregularities. As radiation propagates through the gas it is frequently scattered in all directions. If scattering particles are spaced at half a wavelength along the ray path o f the radiation, the scattered and weak signals will add up to a strong and measurable signal. This volume scattering is used to measure atmospheric properties with radars. Essentially, a radar instrument emits well-defined radiation signals into the atmosphere and measures the backscattered signal. This signal is then analyzed in terms of
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intensity, frequency and echo shape. The intensity is controlled by the backscatter mechanism, the frequency will through its Doppler shift give information about the line-of-sight velocity o f the scattering medium and the signal shape can provide information such as the temperature. The basic technique o f radars can be applied in many different ways which differ mainly in the properties o f the signal sent out. A distinction is therefor made between four different types o f radars, the coherent- and incoherent radars, AfST/MLr radars and lidars. O f these, the coherent- and incoherent scatter radars measure in the ionosphere, whereas the other two measure neutral gas parameters below 100 km altitude. While lidars measure in the visible spectrum, the other radars all use electromagnetic radiation in the radio frequency spectrum.
Coherent scatter radars measure the physical structures o f the ionized gas which change slowly enough for successively arriving scattered pulses to be almost identical. This means that the initially weak scattered pulses add up to a strong signal, thus improving the sensitivity o f measurements. Measurements are carried out with all radars by sending out a signal pulse and receiving the backscatter signal with the antenna. When most o f the backscatter signals have arrived (with travel times depending on the distance or altitude at which they are scattered) a new signal is sent out. The scattering particles are electrons and the returned signal provides information about the altitude of measurement (through the travel time of the signal), line-of-sight plasma velocity (th ro n g its Doppler-shift) and plasm a density (through its backscatter intensity).
Coherent scatter signals rely on irregularities in the plasma since these cause sudden changes o f the plasma’s refractive index, thus improving its reflective properties. These irregularities occur mostly along the magnetic field lines and correspond to waves perpendicular to the magnetic field lines. Therefore, coherent scatter radars measure perpendicular to the magnetic field lines. The height range in which they operate is limited since plasma irregularities appear mostly in the E
region at low latitudes (caused by the equatorial electrojet) and in the E- and F-region at auroral latitudes. The geometry does not allow a vertical angle between the ground based instrument and the field lines in the F-region, but when operating the radar in the HF band (at 8-20 MHZ) the waves are refracted in the ionosphere until they meet the irregularities at right angle. By operating several instruments from different locations which look at the same sky section, more comprehensive information about the region can be obtained. This principle has been used for the Super Dam network o f radars which are globally distributed.
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The received signal strength is much weaker since the scattering electrons move thermally and the amplification o f the waves cannot take place as for coherent scatter signals. Incoherent scatter radars therefore require high power transmitters, a large antenna equipped with the most sensitive receivers and sophisticated data processing equipment. Their main disadvantages are therefore the large costs associated with such a facility. The first incoherent scatter radar was constructed in the early 1960s, and currently less than a dozen radar facilities operate worldwide. They are mostly national or international facilities in order to cover the high costs. Still, though, incoherent scatter radars are the most powerful ground based technique for measuring ionospheric parameters. Incoherent scatter occurs in all directions and not only perpendicular to the magnetic field lines. The basic differences between an incoherent- and a coherent scatter measurement are therefore the strength of the signal pulse sent out and the direction o f viewing (which is flexible for incoherent scatter radars). In principle, therefore a facility can be used for both coherent and incoherent measurements. However, the signal processing techniques used are different as well as the parameters measured and the altitudes investigated. Since incoherent scatter does not depend on specific events in the atmosphere (while the coherent scatter relies on turbulence) measurements can be carried out for the D-, E- and F regions at all latitudes. The parameters primarily derived from the signal analysis are the electron- and ion temperatures, their densities and line-of-sight velocities. By making a series o f assumptions, neutral air parameters such as winds and temperatures can be derived as well. Examples for networks o f incoherent scatter radars are the globally distributed instruments for the Coupling, Energetics and Dynamics o f Atmospheric Regions (CEDAR) program and the high-latitude facilities for the EISCAT consortium.
M S T (Mesosphere Stratosphere Troposphere) and M L T (Mesosphere Lower Thermosphere) radars work in a similar manner to the incoherent scatter radars but measure in the neutral atmosphere instead o f the ionosphere. Radio waves are scattered by neutral particles in the homosphere (below 100 km) where turbulence occurs. The backscatter returns are strongest from altitudes around 10-12 km and 60-75 km, but measurements are carried out also between these regions. A preferred operation frequency lies at 50 MHZ, implying that the backscatter occurs from irregularities with a spatial period o f around 3 m. These turbulence features are often associated with gravity wave phenomena and MST/MLT radars are thus often used for studies o f gravity waves and also tides. The primary parameter measured is the neutral line-of-sight velocity
which again is derived from the Doppler shift o f the returned echo. Since the upper mesosphere region is difficult to observe by other techniques, MST/MLT radars offer a unique means for
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measuring that region. A large number o f instruments are operated by the CEDAR community.
Lidar instruments rely on similar principles to incoherent scatter radars, but the radiation used is in the visible spectrum. The signals undergo Raleigh scattering on particles much smaller than the wavelength. The brightness of the backscattered signal gives the line-of-sight neutral velocity and
neutral density from which, by using the hydrostatic equation, the temperature and pressure can be derived, provided the composition is known. Alternatively, the temperature can also be derived from the Doppler broadening of the returned signal. While early instruments relied on searchlights, the recent ones use lasers and thus achieve a height range o f up to 100 km. Lidars have since the early 1970s been used for tidal and gravity wave studies. The lack of an absolute calibration of atmospheric transmission implies that the density profiles obtained need to be fitted either to theoretical models or experimental data. Lidar instruments are relatively cheap and often operate continuously.
I. 5. NUMERICAL MODELS