Layers in the Equatorial Mesosphere (LEMMA)

Radars in the VHF and UHF range frequently observe echoes from the mesosphere (50 to 90 km) which may either be the result of irregularities in the electron density profile, or be due to a structured temperature profile. In turn, these structures are believed to be caused by the breaking of upward propagating gravity waves; in summer in polar regions these echoes are know as PMSE (= Polar Mesospheric Summer Echoes) and have extensively been studied within three earlier FWF projects. The measurements from the rocket range Kwajalein close to the equator (9N, 167E) indeed showed a well pronounced signature of wave breaking, namely a strong temperature inversion at 90 km, a so-called "mesospheric inversion layer". Alternatively, perturbation in the electron density profile can be caused by attachment of electrons to smoke particles, such as produced by meteor ablation. Detectors for such heavy particles were also flown, but the interpretation of their data is still ongoing. Another rocket flight dedicated to the detection of such seed (smoke) particles was carried out in the auroral zone in early winter. The data revealed positively charged particles in the 83 to 86 km region. This somewhat contradicts expectations, but can theoretically be explained by the very quiet condition and the full darkness which prevailed during that flight. From the daytime measurement in the tropics we expect to find negatively charged heavy particles due to the dominance of the much more mobile electrons during daytime. In winter radar echoes from the mesosphere are very rare and occur at significantly lower altitudes (60 to 70 km). Two small instrumented rockets were launched into such a Polar Mesospheric Winter Echoes (PMWE) situation during ionospherically severely disturbed conditions and measured very large electron densities. Model calculations demonstrate that - in contrast to PMSE - their winter counterparts (PMWE) do not require seed particles (smoke, meteor ablation, etc.), and are only detectable given a background of large electron density. Enhanced electron densities caused, by large fluxes of energetic charged particles, are rare; in the lower D-region enhancement furthermore requires daylight to result in enhanced electron densities; at high latitudes this limits the conceivable occurrence of PMSWE to a few hours a day.