Longitudinal structures in lower thermosphere density

Abstract

Thermospheric densities near 200 km from the Satellite Electrostatic Triaxial Accelerometer (SETA) experiment during July and December 1983 are analyzed to reveal two types of longitude structures: (1) the so‐called “longitude/UT” variation which has been noted in higher‐altitude satellite data and incorporated into the mass spectrometer incoherent scatter extension‐90 (MSISE‐90) empirical model [Hedin, 1991] and (2) “higher‐order structures” in the form of longitudinal wave patterns which have not previously been mentioned in the thermosphere‐ionosphere literature. The longitude/UT variation in the SETA densities shares many similarities with that embodied in the MSISE‐90 model, except for some differences in symmetry about the equator. Interpretation concerning the origin of the observed variation is handicapped by the Sun‐synchronous limitations of the satellite orbit and by the convolution of high‐latitude heating effects being mapped into the geographic frame while solar‐driven latitudinal variations are mapped into the geomagnetic frame. An unambiguous separation of these effects is not possible with the present orbital constraints. The space‐time Fourier analysis method of Salby [1982a, b] is applied in an attempt to quantitatively characterize the higher‐order structure in the data. The nature of the satellite sampling introduces ambiguities into the determination of zonal wavenumbers and periodicities. There is evidence of s = 1 and s = 2 stationary features, eastward propagating Kelvin waves, and a number of westward propagating waves with zonal wavenumbers s = 0 to s = 6 and periods between 6 and 10 hours. Meyer and Forbes [1997] have recently shown the latter class of waves, referred to as Lamb waves in an isothermal nondissipative atmosphere, to represent the preferred response to broadband forcing of the thermosphere. The present observations may represent the existence of Lamb waves in the thermosphere.