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Observations with a 46.5‐MHz radar system at Aberystwyth, Wales (52°N, 4°W) over the period February 1990 to May 1994 have shown a pronounced seasonal variation in the height range of mesospheric echoes. These were observed over the range 64–84 km in all seasons, but those at still greater heights were confined to summer months. Coordinated radar and lidar observations at the same site during 1993 have shown that these intense echoes in summer, analogous to polar mesosphere summer echoes (PMSE), are associated with low mesopause temperatures; the weaker echoes observed in winter seem to be associated with regions of superadiabatic lapse rates; and echoes are observed both above and below the maxima in temperature inversions commonly observed below the mesopause. The types of instability implied in these associations of echo height and temperature structure are considered.

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A study of the ionograms obtained at Kodaikanal (dip latitude 1.7°N) during the period 1954–1966 has shown that the daily variation of the occurrence of the blanketing type of Es (Esb) has a major peak around 1700 LT in the evening and a minor peak around 0800 LT in the morning and is practically inhibited around midday hours when the electrojet currents are maximum. Further, Esb is most frequent during local summer months and in years of minimum solar activity. In individual cases, Esb occurs during the period of very weak or reversed electrojet currents. The ionospheric drifts during these occasions show predominantly a north‐south component. It is suggested that the equatorial Esb is due to the transport and accumulation of long‐lived metallic ions from tropical latitudes to the magnetic equator by equatorward winds during the period when the electrojet currents are very weak and the upward Hall polarization field is thus absent.