Low-frequency Coherence Research Articles

Interaction of a Low Frequency Coherent Magneto‐Acoustic Wave with a Turbulent Spectrum of Extra‐Ordinary Waves

AbstractStarting from nonlinear evolution equations of a three‐dimensional extra‐ordinary wavepacket in a magnetized plasma and introducing a correlation function, equations are derived that describe interaction of a coherent low‐frequency magneto‐acoustic wave with a turbulent spectrum of high frequency extra‐ordinary waves. These equations are used to derive the dispersion relation for a low frequency magnetoacoustic wave in the presence of a spectrum of turbulent high frequency extra‐ordinary waves. In the narrow spectrum case, it is found that a small but finite spread in the spectrum of turbulence has a stabilizing influence on the instability which exists for vanishing spectral width. In the wide spectrum case, the effect of a weak turbulent spectrum of extra‐ordinary waves is to produce a slight shift in frequency of the coherent low frequency magneto‐acoustic wave and to damp or excite the wave.

Evidence for equatorial trapped waves at the Galápagos Islands

From June 28 to November 23, 1979, sea level fluctuations at three locations on the western side of the Galápagos Islands were measured with shallow bottom‐moored pressure and temperature gauges. Gauges were deployed at 1°24′N, 0°, and 1°S. The sea level variations were dominated by a highly coherent low frequency fluctuation with a period of the order of 100 days. In addition, high‐frequency spectral peaks were observed with periods of 5.3 and 2.8 days. Temperature fluctuations also showed the dominant low‐frequency change and the high‐frequency spectral peaks; however, the temperature fluctuations were less coherent because of the thermal front in this region. Empirical orthogonal functions of the pressure gauge data showed that most (95%) of the low‐pass filtered data could be explained by the first mode. This mode had a meridional shape consistent with a first baroclinic mode equatorial Kelvin wave. The second mode accounted for about 3.6% of the variance with periods from 15 to 5 days. Its shape was consistent with a first baroclinic mode mixed Rossby‐gravity wave. The higher frequency peaks occur at frequencies where equatorial trapped inertial‐gravity waves have zero zonal group velocity.

Low-frequency cw coherence measurements for long underwater paths

Measurements were made to determine the first-order effects limiting the coherence of continuous-wave signals received on underwater hydrophones. It was found that phase fluctuations among the various hydrophones are the most serious deviations from ideal reception. These fluctuations were found to have a relatively unimportant short-term component and a dominant long-term component with a typical period of about 30 min. The fluctuations increased in magnitude as the spacing of the hydrophones was increased. The coherence computed from the phase data was about 0.5 at 22 ft and 0.15 at 2000 ft. These measurements were made primarily with a 270-Hz bottomed source at Eleuthera and receiving hydrophones on the bottom at Bermuda, approximately 700 nautical miles away.

Low-frequency coherence transverse to the direction of propagation

Experimental evidence is presented which indicates that the coherence of 10 Hz signals in the deep ocean is sufficient to form a passive-transverse synthetic aperture in excess of 1600 m. Subject Classification: [43]30.20.

A polarity-correlation system for fast-reactor noise studies

A system consisting of a detector signal amplifier, polarity sampler and associated gating circuitry has been designed and tested in a fast reactor application to do noise studies by polarity correlation techniques. System versatility and accurate signal processing were emphasized. Noise studies and reactivity measurements have been done with this equipment utilizing the cross correlation and low frequency coherence techniques.

Nonlinear Electrical Effects and Recombination of the Hot Electrons in Compensated Germanium

AbstractSteady state and transient current‐voltage characteristics are studied for Cu‐doped n‐type Ge samples with the upper Cu level (Ec — 0.26) eV partly compensated. The field strength is varied from ∼ 1 V/cm to 104 V/cm. The I‐U curves obtained are sublinear at field strengths above ∼ 100 V/cm. A study is made of the effect of temperature and of the spectrum of the incident light upon both the steady state and the kinetics of the photocurrent. Negative differential conductivity and coherent low frequency oscillations are observed at nitrogen temperatures with high illumination. The nonlinear effects are shown to be due mainly to the reduction of electron concentration in the conduction band. This could be ascribed to the field dependence of α the probability of electron capture by the doubly charged Cu ions. In the strong electric fields the crystals are becoming inhomogeneous due to the formation of strong‐ and weak‐field regions. The possible influence of these inhomogenities on the nonlinear effects is discussed and the change of α in the strong field is estimated.