Earth Planets Space, Vol. 51 (Nos. 7, 8), pp. 543-562, 1999
Alan H. Manson1, Chris E. Meek1, Chris Hall2, Wayne K. Hocking3, John MacDougall3, Steven Franke4, Kiyoshi Igarashi5, Dennis Riggin6, David C. Fritts7, and Robert A. Vincent8
1Institute of Space and Atmospheric Studies, University of Saskatchewan, Canada
2Auroral Observatory, University of Tromsø, Norway
3Dept. of Physics and Astronomy, University of Western Ontario, Canada
4Space Science and Remote Sensing Laboratory, University of Illinois, U.S.A.
5Upper Atmosphere Section, Communications Research Laboratory, Tokyo, Japan
6Colorado Research Associates, Boulder, U.S.A.
7Dept. of Electrical and Computer Engineering, University of Colorado, U.S.A.
8Dept. of Physics and Mathematical Physics, University of Adelaide, Australia
(Received August 7, 1998; Revised July 15, 1999; Accepted July 15, 1999)
Abstract: Observations of winds and gravity waves (GW) by MF radars from the Arctic to the Equator are used to provide frequency spectra and spectral variances of horizontal motions, and information on the predominant azimuthal directions of propagation for the waves. The years used are mainly 1993/4; the height layer 76-88 km; and the GW bands 10 100 min. and 1-6 hrs. The high/mid-latitude locations of Tromsø, Saskatoon, London/Urbana, Yamagawa, generally demonstrate similar behaviour: the monthly spectra have slopes near -5/3 in winter months, but smaller (absolute) slopes at higher frequencies (<2 hrs.) in summer. Corresponding to this, the spectral densities (10-100 min.) are larger for conditions of higher mean background windspeed--this is related by means of a new correlation-vector technique to GW propagating anti-parallel to the mean zonal winds, and the closure of the solstitial mesospheric jets. Also consistent with this, the sizes and orientations of perturbation ovals (fitted to the wind variations), demonstrate strong semi-annual-oscillations (SAO), and generally similar monthly and latitudinal directions. This suggests strong control, especially of the high-frequency GW band, by the dominant zonal wind-structures of the mesosphere. In contrast the low-latitude locations of Hawaii and Christmas Island demonstrate uniquely different behaviours, with indications of significant inter-annual variability. The frequency spectra for all months tend to have smaller slopes at higher frequencies. Also the dependence of spectral density in both GW bands, upon background wind speed, is negative rather than positive, and is shown to be generally consistent with GW propagating parallel to the mean-global winds. This is consistent with weaker vertical shears in the zonal winds (76-88 km), and lower GW momentum depositions. The perturbation ovals reveal much weaker SAO, and more variable orientations, consistent with more dependency upon GW sources, and less control by the mean winds of the mesosphere.