Thirty-one researchers from twelve countries met in the 'Fair City of Perth' to collate, discuss, and plan their research into noctilucent clouds [NLCs] and polar mesospheric clouds [PMCs]. The variety and sophistication of observational techniques has expanded greatly in the last ten years and there has recently been a spate of less-focussed meetings. With the help and sponsorship of IAGA, this meeting was advertised widely in 2001 among the 'publishing research community'.

The topics for discussion were

1. Observations and measurements from ground level
2. Observations and measurements from orbit
3. Clouds in 'odd' places and 'odd' times
4. The relevant meteorology
5. The relevant physics
6. The relevant chemistry
7. The formation and development of the clouds

Note that the word 'aeronomy' does not appear but if one knows that the clouds are nucleated at the mesopause and are seen fully-formed at 82 km, topics 4 - 7 will be seen simply as a subset of 'aeronomy'.

A useful session under topic 1 was achieved by inviting several amateur observers to present their work and to discuss methods of observation. It was perhaps a revelation to professional researchers to discover the skill and dedication that the amateur brings to his observing. It was also sobering to learn that one observer (Ole Jensen, Denmark) has a photographic record of NLCs, unbroken, from 1959 to 2001.

There have been a number of suggestions that a noticeable increase in the annual frequency of NLCs has taken place in the 1970s and 1980s. This increase has been related to global climate, either through the increase during the last century of atmospheric methane concentration (which has doubled in that time) or through a change in mean temperature at the mesopause. Although much is thought to be known about how NLCs are formed, the data show that statistics of the annual frequencies are dominated by a 10.4-year oscillation. This oscillation is unlikely to be the immediate result of photodissociation of water by solar Lyman-a because the anticorrelation shows a two-year slip on the F10.7 flux; this phase slip has been neither modelled nor explained. Mike Stevens introduced the meeting to his suggestion that the European data on NLC frequencies show the influence of thermospheric seeding by USSR (and lately, Russian) rocket launches ["PMCs formed from Space Shuttle Exhaust: the water from above"].

Ulf von Zahn presented a comprehensive model (developed with Berger) to explain the morphology of the "Summer Upper-Mesosphere Ice Cloud" and other speakers from IAP, Kühlungsborn, spoke on their lidar observations from Northern Norway. Those of us who date back thirty or more years could but envy the youngsters for the thrilling instrumentation they have developed for mesospheric studies.
PMCs over very high latitudes are studied principally by using satellite-borne instruments which are bringing on-stream a wealth of new data. The presence of ice-crystal clouds has a clearly-seen impact on the electron density in the mesosphere. Observations of 'polar mesospheric summer echoes' (PMSEs) - and also PMWEs - were discussed. Use of colocated lidars and VHF radars show the somewhat complex relations between PMSEs and PMCs (or are they NLCs?).

Two papers deserve special mention; the paper dealing with the effect of ice surfaces upon atomic oxygen concentration (Ben Murray) had an impressive slide that demonstrated the wild excitement of laboratory research ….. and Peter Dalin finished the meeting most appropriately with a short PowerPoint show, set to music, of NLC photographs taken from Moscow.

In broad summary, this meeting provided a valuable four days of concentration on a comparatively well-defined topic. The Proceedings, issued just five weeks after the end of the meeting, are available on CD-ROM [ISBN 0-902749-14-5] from the British Astronomical Association, London*, at a cost of 10.50 pounds sterling (including cost of packing and postage). It is a matter for some regret that although participants were asked to bring a disc with them containing a file of what they were going to speak about, authors found themselves unable to provide for publication at least an abstract and some slides for 12 out of the total 34 papers presented at the meeting.

M. Gadsden
*http://www.britastro.org/.