Direct in situ measurements of wave propagation in the neutral thermosphere

Abstract

-The density gauges on the Explorer 32 satellite have measured local variations in the atmospheric density which confirm that waves propagate in the neutral atmosphere. The waves were observed in the northern-hemisphere over the altitude range from 286 km (satellite perigee) to at least 510 km. The waves are most prevalent at the higher latitudes near the auroral zone (orbit inclination is 659 and were observed most frequently in the late evening and early morning hours, but were not limited to these latitudes and times. The virtual vertical half-wavelengths of the waves increase with altitude from 1 km at 286 km altitude to 70 km at 510 km altitude. Four integrally related wavelengths have been observed, a "fundamental" and the second, third, and fourth harmonics.The wave half-amplitudes range from the limit of detectability to the largest observed of 50% of the smoothed density profile value. These waves are identified as free internal gravity waves propagating predominately north-south, or southnorth, with horizontal wavelengths between 130 and 520 km and periods between 10 and 100 min. This interpretation is consistent with observations of large scale traveling ionospheric disturbances by Thome and Georges .

A review of some recent work on mid-latitude spread-F occurrence as detected by ionosondes.

“…NEWTON et al (1969), who reported waves in the neutral atmosphere at F-region heights, states, "Apparently four integrally related wavelengths have been observed, a "fundamental" and the second, third and fourth harmonics," as might be expected if the gravity waves were breaking. Also, ROTTGER (1976) has suggested that harmonics generated by breaking AGWs may account for some of the equatorial spread-F characteristics.…”

“…These events are recorded at times when spread-F occurs, or can be expected. In view of experimental results and theoretical calculations (NEWTON et al,1969;DYSON et al,1970;CLARK and RAITT,1976;HEDIN and MAYR,1987) it seems reasonable to assume that these TIDs are produced by the passage of atmospheric gravity waves (AGWs) through the neutral atmosphere.…”

“…BLANC and RICKEL (1989) have reported ionospheric irregularities at F-region levels which apparently are generated by nonlinear characteristics of acoustic waves propagated from ground level. In view of the foregoing, particularly the fact that higher harmonics are recorded for AGWs at F-region levels (NEWTON et al, 1969) it would seem appropriate that the theoretical work on AGW saturation effects for middle-atmosphere regions be extended to higher levels of the ionosphere. Also quantitative calculations on electron-density depletions, expected for breaking AGWs, would also appear desirable.…”

Short-Term Delays in the Occurrence of Mid-Latitude Ionospheric Disturbances Following Other Geophysical and Solar Events.

“…They find that the wave amplitudes maximize in the midnight and early morning sectors when the UA-NPD reaches a minimum. Earlier Newton et al (1969) observed that these upper-atmosphere waves were recorded "almost exclusively in the late evening and early morning hours". Furthermore they also state "If the energy flux of the wave is considered to be constant then the density perturbation of the wave might be larger in a less dense atmosphere".…”

Auroral pulsations from atmospheric waves

“…, 1961). The picture of neutral waves that emerges from integrating theory and observation (Newton et al, 1969, Reber et al , 1975 Meriwether et al , 1973, Brekke et al , 1974 can Dopp ler sh ift neutral waves to frequencie s between -0 and twice their natural frequency.…”