Abstract
Abstract. Recent years have shown a continuing interest in studies of the mesosphere-lower thermosphere region at low latitudes, with more than 50 papers dealing specifically with this area published over the past 5 years. Experimental ground-based work has been carried out mainly in South America and the Caribbean, India and the Pacific areas. Subjects of interest include gravity waves, tides and planetary waves, the temperature structure of the mesopause region, with special reference to temperature inversions and the two-level mesopause, sporadic neutral layers and their relationship with ionized layers, the possible effects of the micrometeoroid influx, and long-term trends in the MLT region. Experimental techniques in use include MF, MST and meteor radar, lidar, airglow (including satellite-borne limb-scanning measurements) and rocket-borne instruments. Airglow imaging has shown itself to be a particularly useful technique, mainly for studying gravity wave propagation in the MLT region. This paper will present highlights of recent work and will discuss some of the problems which remain to be resolved.
Highlights
ClimatologyOur knowledge of the global structure and climatology of the MLT region has improved dramatically over the past decade
Our knowledge and understanding of the mesosphere and lower thermosphere has improved considerably during the past decade, there are still some gaps, especially at low latitudes
Much recent work has been aimed at understanding the importance of energy and momentum transport by gravity waves, tides and planetary waves, and their interaction with the mean flow
Summary
Our knowledge of the global structure and climatology of the MLT region has improved dramatically over the past decade. At low latitudes the temperature distribution appears always to be similar in form to the midlatitude winter profile This result is consistent with the lidar measurements of Clemesha et al (1999) and the HRDI data presented by Thulasiraman and Nee (2002). It is interesting to note that there appears to be a discrepancy between the ground-based measurements and the satellite measurements for low latitudes Both the OH rotational temperature measurements of Takahashi et al (1995) and the resonance lidar measurements of Clemesha et al (1999) show considerably higher temperatures than those measured by the HRDI and HALOE instruments. It may be relevant that She et al.’s (2000) winter temperatures for 41◦ N appear to be warmer than those derived from the satellite measurements Traditional climatological models such as CIRA and MSIS do not reproduce well the temperature structure of the MLT region at low latitudes. The existence of data sets such as those from HRDI and HALOE make it possible to update the empirical models, and a new version of CIRA should be available in the near future
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