Gravity wave signature simultaneously observed in the oxygen atom and electron density profiles in the lower thermosphere

Pergamon PII: SO273-1177(96)‘WJ45-2

A&. Space Res. Vol. 19, No. 1, pp. (1)145-(1)148, 1997 8 1997 COSPAR Printed in Great Britain. All rights cemved 0273-1177/97 $17.00 + 0.00

GRAVITY WAVE SIGNATURE SIMULTANEOUSLY OBSERVED IN THE OXYGEN ATOM AND ELECTRON DENSITY PROFILES IN THE LOWER THERMOSPHERE T. Imamura,

K. Kita, N. Iwagami

and T. Ogawa

Department of Earth and Planetary Physics, Graduate School of Science, University of To&o, Bwtkyo-ku, Tokyo 113, Japan

ABSTRACT The fine structures of lower thermospheric atomic oxygen and electron density profiles obtained by a rocket experiment were investigated, assuming that these structures are due to a quasimonochromatic internal gravity wave. The height variation of the horizontal wind amplitude is similar to that of the intrinsic horizontal phase speed below -105 km, implying an evidence of saturation effects. At higher altitudes, the dissipation due to molecular diffusion was suggested. The wavy variation of the intrinsic horizontal phase speed with height can be explained by the mean wind modulation due t,o an atmospheric tide. The horizontal phase velocity vector inferred is interpreted as a consequence of selective transmission of internal gravity waves in lower atmosphere. 01997 COSPAR. All rights reserved

OBSERVATION The observation was carried out by the sounding rocket S310.21 flown from Uchinoura (31’15’N, 131’05’E) at 1200UT on 28 January 1992. The atomic oxygen densit,y was measured with an improved resonance fluorescence technique /l/. The electron density was retrieved from the height distribution of the magnetic field intensity of VLF waves from broadcasting stations and the DC probe collecting elect,rons /2/. Figures 1 and 2 show the measured atomic oxygen and electron densities, respectively. They were measured simultaneously in the rocket ascent. Both of the profiles have several fme structures, which can be attributed to atmospheric waves. Atomic oxygen can be used as a tracer of atmospheric motions because its photochemical time constant of several days /3/ is much longer than the typical periods of gravity waves in this region /4/. Though the removal time constant of electron is short in the case that molecular ions are dominant, electron density also responds to atmospheric motions due to the response of metallic ions whose removal time constants are long enough (>lOO days). In the present paper these fine structures are assumed to be created by a quasi-monochromatic internal gravity wave. CHARACTERISTICS

OF THE WAVE

The horizontal broken lines in Figure 1 indicate the positions of the peaks and the dips of the wavelike structures. The interval between the adjoining peak and dip represents a half of the vertical wavelength X, at each altitude except at -92 km/105 km where the position of the dip/peak cannot be determined. The distinct structure around 104 km is considered as a spurious peak created by the large dip just below this structure, because the density response equation /5,6,7/ predicts that the density response disappears just above this altituge where the phase reversal occurs. The height profile of the intrinsic horizontal phase speed IC - kh - til is obtained from XZ using the dispersion relation, where C is the horizontal phase speed, i&, is the unit vector of horizontal phase velocity and ii is the horizontal mean wind. (1)145

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number density (1O”cm-“) Fig. 1. Measured atomic oxygen profile. Horizontal broken lines indicate the peak and dip heights. Envelopes are drawn to distinguish wavy structures.

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number density (lo3 cm-“) Fig. 2. Measured electrn showing distinct layers.

density

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An initial profile of atomic oxygen without gravity wave perturbations was inferred as in Figure 3. by smoothing the observed profile above -105 km and by a number of numerical simulations for many model profiles using the density response equation below -105 km. This initial profile responds to a gravity wave almost in the same way as the observed structures according to the numerical calculation with the vertical wavelengths inferred above (see Figure 4). The amplitude for horizontal wind, u’. can be obtained from the amplitude for atomic oxygen density fluctuation using the density response equation. Fig. 5. illustrates the height, profiles of 11’and IC - i, . ii1 obtained. Since u’ is close to IC - &, . ii1 considering error bars below -105 km, the observed gravity wave is likely to be saturated and breaking due to convective instability rather than molecular viscosity at these altitudes. The expected distortion of density response due to the localized intense turbulence associated with saturation effect is discussed elsewhere /8/. Above ~105 km the wave dissipates mainly due to molecular viscosity because an energy loss is implied from u’ < IC - i&. til and the growth rate of 1~’with height. It should be noted that this critical altitude of -105 km has an ambiguity of vertical wavelength (~10 km). If the wave packet distributes over the height range from 80 km to 150 km and C and i& are almost constant, the structured profile of IC - ih . ii1 would suggest that, the mean wind ii is modulated by an atmospheric tide. The wavelike variation of u’ with height below -105 km would be a result of wave-tidal interaction suggested by Fritts and Vincent /9/, who have argued that the amplitude of a gravity wave is limited by the local mean wind modulated by tidal motions. The wavy structure of u’ above -105 km would be a result of the wavelike variation of IC - i& .fiI, considering the WKB solution of the Taylor-Goldstein equation describing the amplitude and vertical structure of gravity waves. The propagation direction of the wave can be determined from both atomic oxygen and electron profiles based on the fact that the response of ionic species to gravity waves is different from that of neutral species depending on the propagation direction. For example, for waves propageting eastward, the region in which ionic layers are formed is located at the phase of about -n/2 N 1r/2 downward from the peak positions of background density fluctuation. For westward propagation, it is located at 1r/2 N 3n/2. Using a simple model, the horizontal phase velocity vector was inferred

Gravity Wave Signature in the Lower Thamarphere

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Fig. 4. Observed atomic oxygen profile (solid) and simulated profile at around 100 km for gravity wave perturbation with A, = 12.8 km (dotted).

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speed (m se&) Fig. 5. Height profiles of intrinsic horizontal phase speed (solid diamonds) and horizontal wind amplitude (open squares).

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The linear saturation theory /lo/ shows that gravity waves cannot pass through the critical level where JC-&h.fil= 0. According to the radiozonde and meteorological rocket observations by Japan Meteorological Agency, the critical level did not exist below -58 km altitude for waves with the horizontal phase velocity inferred above. The CIRA1986 model atmosphere /ll/ suggests that the mean zonal wind is eastward below 100 km altitude at 31”N in January or February, implying that the wave packet can also pass through the mesosphere. These can be interpreted by the selective transmission of gravity waves in lower atmosphere. The meteor radar observations /12/ suggests that the horizontal phase velocity vector inferred in this study is typical in the lower thermosphere at around the observation site in winter. Acknowledgement The authors wish to thank Prof. M. Mambo, Drs. T. Okada and T. Fukami for providing the electron density data. Thanks are also due to Dr. T. Nakamura, Dr. M. D. Yamanaka, Prof. S. Fukao and Dr. M. Yamamoto for helpful discussions. The S310.21 rocket experiment was carried out under the research program of the Institute of Space and Astronautical Science, Japanese Ministry of Education, Science and Culture. REFERENCES 1. K. Kita, T. Imamura, N. Iwagami and T. Ogawa, Rocket observation of oxygen atom and night airglow 1. measurement of oxygen atom concentration with an improved resonance fluorescence technique, submitted to Ann. Geophysicae. 2. M. Mambo,

T. Okada and T. Fukami,

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3. R. R. Garcia, and S. Solomon, The effect of breaking gravity waves on the dynamics and chemical composition of the mesosphere and lower thermosphere, J. Geophys. Res. 90, 3850-3868 (1985). 4. A. H. Manson, Gravity wave horizontal and vertical wavelengths: an update in the mesopause region (~80-100 km), J. Atmos. Sci. 47, 2765-2773 (1990). 5. Y. T. Chiu, and B. K. Ching, The response of atmospheric to gravity waves, Geophys. Res. Lett. 5, 539-542 (1978). 6. C. S. Gardner, wave perturbations,

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8. T. Imamura, K. Kita, N. Iwagami and T. Ogawa, Gravity wave breaking at 90-140 km derived from atomic oxygen and electron density profiles, submitted to J. Geophys. Res. 9. D. C. Fritts, and R. A. Vincent, Mesospheric momentum flux studies at Adelaide, Australia: observations and a gravity wave-tidal interaction model, J. Atmos. Sci. 44, 605-619 (1987). 10. R. S. Lindzen, Turbulence and stress phys. Res. 86, 9707-9714 (1981). 11. E. L. Fleming, functions of latitude,

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J. Geo-

breakdown,

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height

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radar