Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 1–19 www.elsevier.com/locate/jastp Variability of the semidiurnal tide due to uctuations in solar activity and total ozone D. Pancheva a ; ∗ , N. Mitchell a , H. Middleton a , H. Muller b a Department of Physics, University of Wales, Penglais, Aberystwyth Dyfed SY23 3BZ, UK b The University of Craneld, The Royal Military College of Science, Shrivenham, Swindon SN6 8LA, UK Received 11 October 2000; received in revised form 6 November 2001; accepted 20 February 2002 Abstract An attempt was made to investigate some variations of the semidiurnal tide observed in the mesosphere= lower thermosphere (MLT) region by meteor radar above the UK in the time interval 1 January 1989–6 May 1993, which are possibly due to the variability in the total ozone and solar activity. The total ozone is used here mainly as a proxy for stratospheric planetary wave information in the extra tropics. A positive correlation between the solar activity and the variability of the semidiurnal tide and total ozone was found for the investigated interval. During winter the semidiurnal amplitude modulations with periods: ∼10, ∼16 and 25–28 days were found to be present simultaneously with similar variations in the total ozone. They strengthen signicantly during stratospheric warmings and the amplication of the stratospheric height wave 1. The amplitude modulations of the semidiurnal tide observed in the MLT region during winter are mainly produced by non-local coupling between the semidiurnal tide and planetary waves in the stratosphere. The inuence of the planetary wave activity in the lower stratosphere on the variability of the semidiurnal tide observed in the MLT region indicates the non-negligible eects of the tropospheric tidal forcing. A strong response of the total ozone and the semidiurnal tide to solar radio ux variations on the time scale of the solar rotation period was found especially in the beginning of 1991. There is some provisional evidence for a response of the semidiurnal tide and the total ozone to the variations in the solar radio ux at intermediate periods of 50–80 days. c  2002 Published by Elsevier Science Ltd. Keywords: MLT region; Tidal variability; Solar activity; Total ozone 1. Introduction The semidiurnal tide in the mesosphere= lower thermo- sphere (MLT) region can reach amplitudes large enough to make it the dominant component of the motion eld at these heights. Experimental and theoretical studies of the semidiurnal tide have revealed to be characterised by a very high degree of variability in amplitude, phase and vertical wavelength occurring over a range of time scales ranging from the short-term (a few days, or periods similar to planetary waves), the seasonal (where a well-established ∗ Corresponding author. E-mail address: ddp@aber.ac.uk (D. Pancheva). seasonal behaviour is observed) to the long-term (including inter-annual and solar-cycle variability). Long-term varia- tions of the semidiurnal tide are thought to arise from two principle causes (Vial, 1989); rstly, changes in the strength and distribution of the tidal forcing, and secondly, changes in the propagation characteristics of the tide as it ascends through the middle atmosphere towards the MLT region from its (predominantly) troposphere= stratosphere sources. The 11-year solar cycle is presumed to be able to inuence the nature of the semidiurnal tide, since the tide is ultimately excited by the diurnal cycle in the absorption of solar radi- ation. In particular, the absorption of infrared radiation by water vapour in the troposphere, the absorption of ultraviolet radiation by ozone in the stratosphere= mesosphere and the absorption of radiation in the Schumann–Runge bands and continuum by molecular oxygen and nitrogen in the lower 1364-6826/03/$ - see front matter c  2002 Published by Elsevier Science Ltd. PII:S1364-6826(02)00084-6