A series of measurements of the properties of equatorial ionospheric irregularities were made at Kwajalein, Marshall Islands (M.I.) in August 1977 and July–August 1978. These measurements, sponsored by the Defense Nuclear Agency (DNA), involved coordinated ground‐based and in situ sensors. The ARPA Long‐Range Tracking and Instrumentation Radar (ALTAIR), operated by Lincoln Laboratory, obtained backscatter and transmission data during five nights in August 1977 and eight nights in July–August 1978. This report describes the ALTAIR data from the night of August 11, 1978, which yield direct quantitative measurements of 1‐m and ⅜‐m irregularities and of plasma depleted regions. These plasma depleted regions, previously predicted on the basis of theoretical analysis and in situ data, were observed during the decay phase and not the generative phase of the field‐aligned irregularities.
Evidence is given for intense VHF and UHF radar backscatter during equatorial Spread F resulting from irregularities of 1 meter and 36 cm, respectively. The linear theory for high frequency (ω ≳ Ωi, where Ωi is the ion gyrofrequency) drift waves, generated by the drift‐cyclotron and lower‐hybrid‐drift instabilities, is presented. This linear theory is set forth as a possible explanation for the occurrence of these irregularities below the ion gyroradius. The maximum growth for these instabilities occurs for kre ∼ 1, where k is the wavenumber perpendicular to the magnetic field and re is the electron gyroradius. For these instabilities, the growth rate is γ ≳ (me/mi)¼ Ωi and results in growth times less than a second. For typical equatorial Spread F ionospheric parameters, where (νii/Ωi) (kri)² ≳ 1 (νii is the ion‐ion collision frequency, rI is the ion gyroradius), the lower‐hybrid‐drift instability is dominant.
An experiment was conducted at Kwajalein AtollMarshall Islands to investigate the spatial relationship of 1‐m equatorial spread‐F irregularities to total electron content (TEC) depletions. A high‐power radar was operated (1) in a backscatter scan mode to spatially map the distribution of 1‐m irregularitiesand (2) in a dual‐frequencysatellite‐track mode to obtain the longitudinal TEC variations. We show that radar backscatter “plumes” found in the disturbednighttime equatorial ionosphere are longitudinally coincident with TEC depletions. We suggest that the TEC depletions are probably due to the presence of plasma “bubbles” in the equatorial F layer.
We report preliminary results on 1‐m equatorial spread F irregularities obtained with a high‐power VHF backscatter radar located at equatorial latitudes. The field experiment was conducted at Kwajalein Atoll, Marshall Islands, during August 1977. The examples, which include both incoherent scatter measurements and backscatter from equatorial spread F irregularities, illustrate the usefulness of a fully steerable radar for equatorial spread F studies. The backscatter observations at 155.5 MHz are not consistent with the scattering model proposed by Woodman and Basu (1978), which includes a cutoff near the ion gyroradius. The results obtained thus far also indicate that the collisional Rayleigh‐Taylor instability is probably the principal mechanism producing equatorial spread F.
Measurements have been made which show a dependence of attenuation of ultrasonic waves in several liquid media upon the intensity level at the source of the radiation. Measurements of the velocity showed no observable change at high intensities. The attenuation for distilled water and dilute solutions of acetic acid and sodium acetate was found to increase almost linearly with the acoustic pressure amplitude in the interval from 2 to 7 atmospheres. The attenuation in glycerol showed a similar increase when an unavoidable rise in the temperature of the medium was taken into account. Graphs of data taken at 4, 5, 7 and 9 megacycles/sec will be given and theories of the increase in attenuation will be discussed.
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