Small-scale structure observed in-situ during MAC/EPSILON T. A. BLIX,*E. V. THRANE,* D. C. FRITTS,? U. VON ZAHW,: F.-J. LOBKEN,% W. HILLERT,~ S. P. BLOOD,$ J. D. MITCHELL,~ G. A. KOKIN!~ and S. V. PAKHOM~V//~ *Norwegian Defence Research Establishment, P.O. Box 25, N-2007 Kjeller, Norway; tGeophysica1 Institute and Department of Physics, University of Ahiska, Fairbanks, AK 997750800, U.S.A. ; SPhysikalisches Institut, Universitit Bonn, NuDallee 12, D-5300 Bonn I, F.R.G. ; §Communications and Space Sciences Laboratory, Department of Electrical Engineering, Penn State University, University Park, PA 16802, U.S.A. ; /ICentral Aerological Observatory, 141700 Dolgoprudny, Moscow, U.S.S.R. Abstract- We determine in this paper the level of turbulence activity in the middle atmosphere observed during the MAC/EPSILON campaign using different zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJ h-situ techniques. The observations show a minimum of turbulence in the height region 75580 km, with increasing activity below and above. Based upon spectra of density fluctuations covering height intervals of typically 8 km. a unified spectral description is developed. It is shown that the buoyancy scale La and inner scale I, of turbulence, as calculated from the power of Ructuations in the inertial subrange, agree with the visually observed breakpoints in the spectra. Spectra of ion density fluctuations are also seen to be consistent with spectral amplitudes predicted by linear saturation theory. The relationship between the level of turbulente in the middle atmosphere and the gravity wave en~~irollrnent in which the turbulence zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONML occurs is investigated in detail. Our results suggest that strong turbulence occurs oreferentiallv in or near those phases of the wave motions predicted to be most unstable on the basis of linear gravity wave theory. I. INTRODUCTION AND OL’TLINE The dynamics of the middle atmosphere has been the subject of intense investigations in recent years. One of the goats OF these studies has been to dctcrmine the level of turbulence activity as well as the associated gcncration mechanisms and energy sources in the mesosphcrc and lower thermosphere. Up to a decade ago, littlc was known about turbulence in this height region. With the dcvclopmcnt of new and improved rocket and ground-based techniques, the potential for studies of the middle atmosphere has increased con- siderably. The prime objectives of this paper are to study-. in detail, the relationship bctwccn turbulence and gravity wave saturation in the mcsosphcrc and to prcscnt a unified spectral description of the wave and turbulence fields. It is known that different forces dominate different parts of the spectrum. For large scales (> I km), buoyancy is the primary restoring force, and for the very small scales (for some tens of merres or less) viscosity will be the dominant force. In between these two extremes, the inertial zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCB subrange, where homogeneous isotropic turbulence may exist. is located. This paper uses data obtained with several instrument types to cxaminc the characteristics of. and the transitions between, the different regimes. The esperimental techniques used in this study can be divided into three types : (I) in-sift measurements of irregularities by means of positive ion probes (PIP), clcctron probes (nose tip probes ; NTP), neutral mass spectrometers (BUGATTI) and ionization gauges (TOTAL); (2) radar tracking of passive falling spheres and foil clouds; and (3) sodium lidar. In Section 2 the structure of the campaign and salvo launches will be described briefly. The differ-cm cxper- imental techniques mentioned above will not be described in this paper, but will be the subject of a forthcoming paper (BLOOD, 1990, private cotn- munication). The turbulence parameters (energy dis- sipation rates, eddy diffusion coefficients, scales, etc.) which can be derived from the observed field of ion, electron or neutral density irregularities are presented in Section 3. A brief discussion of atmospheric stab- ility is also presented in Section 3. In Section 4, a unified spectral description of the wave and tur- bulence fields will be given. The experimental results are discussed in the light of the hypothesis that tur- bulence is generated through dynamic and convective instabilities resulting from gravity wave sdtu~dtion in Section 5. Our conclusions are presented in Section 6. 2. CAMPAIGN STRUCTURE AND SALVO LAUNCHES The MAC/EPSILON campaign was planned as a case study of middle atmosphere turbulence by means 835