Particles in the Polar Mesosphere

In the mesosphere heavy particles of volcanic, meteoric or man-made origin may exist. Under the conditions of the extremely cold polar mesosphere they can act as nuclei for the formation of ice particles which can then be seen as Noctilucent Clouds (NLC). Other phenomene associated with these dust or aerosol particles are Polar Mesospheric Summer Echaes (PMSE) detected by ground-based VHF MST radars. A campaign to be conducted from the Andoya Rocket Range (Norway) in Summer 1999 is dedicated to the study of these problems. Novel impact detectors which can differentiate between neutral and charged aerosals will be flown aboard two American sounding rockets which carry as one of the key instrument a full measurement of the electric field. The flights of these two sounding rockets will be coordinated with two other, pre-dominently Europeen payleads which have the emphasis on neutral turbulence measurements. The two salvos of fully instrumented sounding rockets will be accompanied by a number of flights of small Viper Darts; part of them will be of the commercial type with an inflatable balloon from whose fall rate neutral density can be determined, the other will be modified into miniature sounding rockets either to detect heavy particles with a simple impact detector, or carry background plasma density instruments. The Graz hardware involvement is with electron density measurements aboard the two American payloads and also aboard about 10 instrumented Vipers. The latter are technologically challenging because of the high acceleration and the required miniaturisation. A better insight into the conditions conducive to the formation of NLC and PMSE, as well as on the distribution of heavy particles will be obtained. In addition the plasma densities by themselves serve as inputs to empirical D- region models which have their own merits.

The mesosphere is the interface between the thermosphere which correlates with solar activity and the tropo- and stratosphere where no such a connection seems to exist. The top of the mesosphere has the coldest natural temperature anywhere on earth. At high latitudes the mesosphere undergoes a pronounced seasonal variation with a very abrupt transition in spring. Features only found in the high-latitude mesopause are noctilucent clouds (NLC) and Polar Mesospheric (Radar) Echoes (PMSE) which frequently, but not necessarily occur together. Whereas the height of the PMSE can vary and cover several kilometres, the altitude of NLC`s is strikingly constant (830.5 km) ever since this phenomenon was first recorded in the late 19th century. This apparently stable region may actually be the transition between the region where indications suggest a long-term cooling (the thermosphere) and the region where anthro-pogenically induced warming ("Global Warming") is suspected. The largest campaign was conducted from Norway in Summer 1999 and consisted of two big NASA payloads (DROPPS) and one smaller recoverable Norwegian-German payload (MIDAS). For all of these Graz provided instruments to measure electron densities important for the understanding of the generation of the ice particles which form the visible NLC. All flights were in conjunction with optical measurements by the lidar facility and the MST (Mesosphere-Stratosphere-Thermosphere) radar co-located at the rocket range. One of the results of the electron density measurements is that there appears to be a distinct threshold required to initiate PMS. In 2000 two of the MIDAS payloads were flown at the time of the expected transition to summer conditions and were both successfully recovered from sea. In addition a meteorological rocket (Viper IIIa) was successfully used as a sounding rocket. This was technologically particularly challenging because of the extremely large acceleration in excess of 150 g. Hardware of the electron density instrument for three more flight is at the University of Tromsø and will be used as part of a further campaign. The campaign in 2001 was specifically aimed at studying the neutral and ionised atmosphere near summer solstice and consisted of two flights with MIDAS payloads. Interesting results are expected from these flights pertaining to neutral density fine structure and masses of aerosols, both charged and neutral. The eight electron density profiles obtained in the project are welcome additions to the data base of only 106 such high latitude measurements. The enlarged base are used in an ongoing endeavour of establishing empirical models of auroral-zone electron densities.