Condign volumes 1 to 3

Read from the scan by GLM-OCR; expect the odd misread word.

# UNCLASSIFIED

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17. Radar Reflections No information has been found on the possible radar reflectivity from charged dust aerosols, but some data is available on radar scatter from Dusty Plasmas'. There may well be some overlap in the terminology and applicability. The latter topic is covered at Working Paper No.5, on radar reflectivity.

## SUMMARY

18. Dusty Plasmas The work reviewed above shows that it is theoretically possible for a charged cloud of particles to exhibit characteristics of visibility and motion similar to many UAP reports received. Further, the implication that atmosphere-borne particles must be present strengthens the proven circumstances where UAPs are reported more frequently in dust-laden scenarios, e.g. near volcanoes and earthquakes. For the uninitiated it would not take too much imagination to observe such a cloud, especially one with bright spots and turn this into a UAP, with portholes!

19. As the formation of clouds (of the charged aerosol type) would clearly depend on the presence of atmospherically-carried particles, it may well be worth making correlation studies of sightings of UAPs with geographical locations where dust is produced, e.g. quarries, or where other pollution particles are emitted (e.g. from power stations or factories). It may also be useful to compare the incidence of sightings in areas taken both during and after cessation of work where dust or other particles are produced. For example, where factories have closed or applied dust/smoke control regimes, or where dusty quarries operated but no longer do so. However, a considerable amount of effort would be required to properly investigate this suggestion. The critical information is not available on the database created for this study.

20. The topics of 'dust in plasmas' and 'dusty plasmas' have become of particular importance in the last two to three years. These plasmas can easily be produced under laboratory conditions, but much more research is required to understand all the mechanisms involved. This may well point towards an understanding of such plasmas in the atmosphere.

## ATMOSPHERIC PLASMAS AS REFLECTORS & ABSORBERS

21. The electromagnetic properties of a plasma at atmospheric pressure is an inter-disciplinary topic, combining plasma physics, radio wave propagation, and air chemistry. Plasmas in the atmosphere can either reflect or absorb electromagnetic waves, depending on plasma characteristics. A high reflection co-efficient requires a grazing angle of incidence. A high absorption requires a high collision rate, a low plasma density and a plasma transition of about one wavelength. The power to sustain a plasma in air is high because of the short plasma lifetime unless continuously replenished. The bandwidth, as an absorber, can extend from Metric to I Band. However, absorption peaks at VHF/UHF. In an atmospheric plasma, electrons undergo numerous collisions with atoms and molecules in the background gas(es) and the collision convert the electromagnetic energy which may be feeding the plasma to heat and damp the essential electron motion. The collision rate varies with pressure and temperature. Also water vapour has an effect - so that, for example, 50% relative humidity approximately doubles the sea-level dry air collision rate.

22. Plasma Lifetime The plasma lifetime depends on the electron density and the pressure (altitude). The presence of gases other than those normally in air can change lifetime significantly. For example by a factor of ~10,000 for helium.