Neutral Atmosphere Measurement Techniques

Many different techniques have been developed in order to study the

atmosphere and determine its physical and chemical properties. The methods

employed reflect the most important parameters needed for a better understanding of the atmosphere. These include studying the constituents o f the atmosphere and their relative abundances, the temperatures o f different regions and the winds operating on the constituents.

As methods of atmospheric observation have advanced, the emphasis has moved from identification of emission features and their excitation processes to quantification. With the combination o f ground-based and in-situ measurements using optical and radio methods it is now possible that the "weather" and "climate" o f the thermosphere/ionosphere system can be monitored. This depends to a large extent on the use o f accurate theoretical models of the parameters being measured to establish "normal" conditions and also detailed and rigourous understanding o f the physical and chemical processes involved in the emission and transfer of radiation (Solom on, 1991). The techniques developed in order to establish a database for these atmospheric parameters fall broadly into three categories: direct sensing, indirect sensing and remote sensing.

M A G MAC BOOM LANG' m G9M W ATS' NA C5' L A N G V E F I L A P I M A G

DYNAMICS E.\PLPItER INSTRUMENT F abry.Perot interferom eter (FPI)

W ind and ten^pcrature spectrometer (WATS)

R etarding potential analyzer <RPA)

Ion drift m eter (IBM )

N eu ral m ass spectrom eter (MACS)

L angm uir probe (LANG)

Low altitude plasm a instrum ent (LAPl)

V ector electric Held instrum ent (VEFI)

M agnetic field in strum ent (MAG B)

G alactic background monitor (GUM)

MAIN,.M£A£Umi£NT

m eridional neutral w ind (u^) and neutral tem perature (T„)

zonal (vj,) and vertical n eu tral wind (w „) and tem peraure CTq)

m eridional ion drift (up and ion tem perature (Tp

to n a l (vj) and vertical ion drift (w p

neutral con stituent num ber densitiss of Ng, 0, N, Ar, and He

electron (N ^ and Ion (N p number d en sity and electron tem perature (T^j

p recipitating plasm a fluxes and pitch a n g le distribution

electric field (E)

m agnetic field (B)

star sensor for precise orientation o f m easu rem en ts

2.2.1 Direct Sensing

Direct sensing involves using an instrument to take measurements from within the region o f the atmosphere under investigation. Instruments are usually carried by

rocket or satellite. Satellite platforms have provided great scope for atmospheric

measurements over long periods, with full global coverage. Polar orbiting satellites have been launched such as Nimbus and Tiros which made up to 14 orbits in a 24 hour period and provided full global coverage at least twice per day (Houghton, 1977).

The Dynamics Explorer 2 satellite, shown diagramatically in Figure 2.1, was launched in 1983 and carried several instruments for direct sensing o f the upper atmosphere. This mission produced a large database o f measurements o f atmospheric variables, enabling improvements to be made to the existing models o f the atmosphere. Instruments on board included a mass spectrometer, a Fabry-Perot interferometer and a Langmuir probe.

The mass spectrometer is an instrument which can be used to determine the masses o f ions or neutrals in the atmosphere. The decrease in air density at higher altitudes allows experiments to be carried out in conditions approximating a vacuum, and in the case o f the mass spectrometer this is important as the principle o f its operation is to measure the effect o f electric and magnetic fields on the free motion o f

ions. In this respect it is necessary to ionize neutral constituents by electron

bombardment prior to their entry into the apparatus.

The Langmuir probe is a device which is placed in a plasma and from the characteristic curve o f current versus voltage for the probe, conclusions can be drawn on the conditions within the plasma. In regard to atmospheric research it is possible to determine the electron temperature for regions of the ionosphere and the relative abundances o f the most important ions in these regions.

The Fabry-Perot interferometer is a high resolution spectroscopic device which is based on the principle o f optical interference. It is used in atmospheric studies primarily due to its large light gathering power combined with high resolution. The range o f instruments on board the satellite demonstrates the usefulness o f satellite- based direct sensing, as a dataset can be built up consisting o f measurements taken at the same altitude and position, from many different instruments.

2.2.2 Indirect Sensing

Indirect sensing has long been used as a method for atmospheric research, where measurements are taken, from outside the region o f interest, o f the characteristics o f a sensor placed within the region. Satellites in near-earth orbits (200- 1100 km) are influenced by drag, which produces a decay in their orbits due to the frictional forces experienced as they traverse the upper atmosphere. This drag may be

deducec by tracking the orbits o f satellites. The density of the atmosphere at altitudes o f hundeds of kilometers may be inferred using this technique, which has been in use since saellites were first launched in the late 1950's.

lockets are also used for indirect sensing, sometimes by dropping spheres whose ate of descent is measured, and similarly the density o f the atmosphere can be inferred by the effect o f drag. This method is most useful between 90 and 130 km, below tie altitude at which satellite orbits can be maintained. Luminescent chemicals are usee at altitudes above 80 km also, providing wind data through measurements o f the trail produced by the chemicals. This technique is usually restricted to nighttime

and iwiight though sodium trails have been used in daytime. At lower altitudes

balloons have been used to take similar measurements since the 1940's.

2.2.3 Ronote Sensing

The technique o f remote sensing is based on either, (i), using a transmitter to propigate electromagnetic radiation through the region of the atmosphere being investigited and detecting the scattered radiation, or (ii), studying the radiation emitted from within the region o f interest by the constituent species. The first method is called activi remote sensing: examples include the incoherent scatter technique, which will be discussed in Section 2.3.1, and light detection and ranging (LIDAR). The second method is called passive remote sensing as it uses naturally occurring processes in the atmojphere as the source o f its measurements. The most common type o f passive remcte sensing technique is to observe naturally occurring em issions from the consituent species within the atmosphere, at visible or infrared wavelengths.

LIDAR works on the principle of scattering light at optical wavelengths and recoding a plot of intensity versus time lag, between emission of a signal from a laser and is detection. This instrument can be viewed as an optical analogue o f a radar. A variey of scattering processes combine to produced the backscattered spectrum and a

numler of techniques have been developed to take advantage o f these. Inelastic

scattering processes may be resonant, Raman, resonant-Raman and fluorescence. Elastc scattering processes are Rayleigh and Mie.

In a Rayleigh Lidar the Rayleigh-backscattered spectrum from air m olecules is recoded and the vertical density and temperature inferred from the profile. This procss is useful in the middle atmosphere (from 30 km to around 90 km) but is limitid at lower altitudes by the presence of aerosols which enhance the contribution o f Mie mattering and at higher altitudes by the decrease in returned signal, which falls off as tb atmospheric density rapidly decreases with height (Chanin and Hauchecorne,

Passive remote sensing commonly involves using interferometers, dispersion spectrometers or photometers to study the naturally occurring atmospheric em issions. The use o f photometers allows measurement of photon intensity at selectable wavelengths using narrow bandpass filters (0.3 - 1 nm) to isolate a portion o f the

spectrum. Dispersion spectrometers use a different method whereby light from a

broadbind source such as the night sky is incident on a dispersion apparatus, such as a ruled grating, and is separated into its constituent wavelengths which may then be detected individually. Using this method allows the measurement o f light intensities over a larger wavelength range.

Interferometers, such as the Fabry-Perot and Michelson interferometer, divide incident radiation into a number o f coherent rays which are recombined to produce optical interference. This method provides very high spectral resolution and has been used to study atmospheric emissions from the earliest days o f research in the field (Babcock, 1923). This thesis involves the use of Fabry-Perot interferometers to make observations of atomic oxygen emissions by passive remote sensing and details o f the method and relevant implementation will be given in Chapter 3.