B.H. Briggs

On the variation of radio star and satellite scintillations with zenith angle

Abstract The theory of the variation of the depth of amplitude scintillations with the zenith angle of the source is considered, for radio waves received from a star or a satellite. It is assumed that irregular phase fluctuations are impressed on the wave in travelling through the ionosphere, and that the amplitude fluctuations develop by a diffraction process as the wave propagates in the free space beneath the ionosphere. Two effects are produced by an increase in the zenith angle of the source. The magnitude of the phase perturbations increases, because the thickness of the ionosphere along the line of sight increases, and the geometry of the diffraction process changes. Theoretical results are given, both for Isotropic irregularities and for irregularities elongated along the direction of the earths magnetic field. The advantages of observing simultaneously on two wavelengths are stressed. Some comparisons with available experimental data are made, and suggestions are made for future experiments. Irregularities at different distances from the observer are not equally effective in producing amplitude scintillations, and some examples of the “weighting function” which determines their effectiveness are given. Various measures of “scintillation depth” are discussed and related to each other.

On the analysis of moving patterns in geophysics—I. Correlation analysis

Abstract The analysis of a moving random pattern is considered for the case where the whole pattern can be recorded. The pattern is assumed to have a certain mean velocity, and also to change randomly in form as it moves. It is shown how the velocity of movement, as well as parameters describing the spatial properties and the random changes, can be derived from two observations of the pattern separated by a known time interval. The determination of the velocity does not depend on the assumption that the spatial and temporal correlation functions have the same shape. If several observations of the pattern are made, separated by different time intervals, then the temporal correlation function of the random changes alone can be determined. The results should have applications to radio experiments using large aerial arrays for observations of ionospheric drifts, to photographic observations of moving patterns of light emission from the night sky, and to other similar phenomena.

Radar observations of atmospheric winds and turbulence: a comparison of techniques

Abstract Various radar techniques which can be used to observe wind velocities and turbulence in the troposphere, stratosphere and mesosphere are discussed. It is shown that the Doppler technique and the spaced antenna technique are basically the same, although for practical reasons the Doppler method cannot always be used. The relative advantages and disadvantages of the two techniques are discussed for situations where both are practicable. The use of beam swinging methods for distinguishing between turbulent scatter and stratified layers is discussed and it is shown that such experiments must be interpreted with care.

First results with the Adelaide VHF radar: spaced antenna studies of tropospheric winds

Abstract The first results from a VHF radar of the ST type located at Buckland Park near Adelaide, Australia (35°S, 138°E), are presented. The radar is designed to be versatile and can be used to measure velocities in the lower atmosphere using both the spaced antenna (SA) and Doppler beam-swinging (DBS) techniques. Here studies of irregularities and motions made with the spaced antenna technique are discussed. It is shown that the scale of the diffraction pattern formed by the backscattered radiation varies with altitude, with the mean pattern scale being smaller in the troposphere than in the stratosphere. The observations are consistent with the backscattered energy decreasing as a function of off-vertical angle by 1.5 dB per degree in the troposphere and by about 2.8 dB per degree in the lower stratosphere. An intercomparison of zonal velocities measured with the SA and DBS methods shows good agreement. In May and August 1984 an extensive comparison was made between the velocities measured by the SA method and winds determined from over 80 balloon-borne radiosondes released from Adelaide Airport, situated some 36 km to the south of the radar. The velocities were compared on a statistical basis and showed excellent agreement, although the SA speeds tended to be 1–2 m s−1 smaller in magnitude than the radiosonde velocities. Overall, the rms differences between the two sets of measurements was only 3–4ms−1 throughout the troposphere, a result which is consistent with the random errors inherent in each technique, as well as the spatial separation between the radar and balloon observations. The utility of the SA method for meteorological observations is illustrated by a study of both the horizontal and vertical wind fields during the passage of a cold front made in November 1984. The high time resolution available with the radar allows detailed studies of the development of the pre-frontal jet, the wind convergence into the front and associated vertical motions.

Observations of radio star scintillations and spread-F echoes over a solar cycle

Abstract Observations of the scintillations of the radio source Cassiopeia A were made at Cambridge on a frequency of 38 Mc/s over the period 1949–1961. The results are presented in the form of curves showing the mean diurnal variation of the scintillation index for each month. The variations of scintillation index with solar time, sidereal time and zenith angle are derived and discussed. Seasonal and solar cycle variations are also considered. Ionograms from Slough were analysed to study the occurrence of spread-F echoes for the period 1949–1960, and the results are compared with the observations of scintillations. Special consideration is given to the variations with the solar cycle, which are opposite for the two phenomena; the scintillation effect is greatest at sunspot maximum, but spread-F echoes occur more frequently at sunspot minimum. It is concluded that at sunspot maximum the ionospheric irregularities which cause radio star scintillations must be mainly above the level of maximum ionization of the F-region, and are therefore unobservable by ground-based sounders. For the years near sun-spot maximum, scintillations occur frequently by day as well as by night. Possible explanations of these daytime scintillations are considered.

The variability of ionospheric dynamo currents

Abstract The method of Hibberd is used to study the variability of ionospheric dynamo currents from day to day, with solar rotation and with the solar cycle. The method eliminates many sources of disturbance by using the difference of H at two magnetic observatories having the same longitude but different latitudes. In this way, a measure of the strength of the ionospheric currents can be obtained almost every day, even during magnetically disturbed periods. It is concluded that the currents are produced partly by tidal modes driven by in situ heating in the thermosphere, and that variations in the amplitude of these modes are mainly responsible for the solar rotation and the solar cycle effects which are observed. There are also random day-to-day changes, uncorrelated from one day to the next, and these suggest that upward propagating tidal modes are also important in driving the currents.

Radar measurements of aspect sensitivity of atmospheric scatterers using spaced-antenna correlation techniques

Abstract The theory of spaced-antenna techniques for the measurement of aspect sensitivity of radar backscatter from the atmosphere is considered. Correlation measurements made on the wave field can be used to determine the angular power spectrum of the returned energy in much the same way that aperturesynthesis techniques are used in radio astronomy. After correction for the antenna polar diagrams, this yields the aspect-sensitivity function for the refractive-index fluctuations which are responsible for the scatter. Allowance is made for the possibility that the backscattered power may vary with azimuth angle as well as zenith angle. It is shown that commonly available data from three spaced antennae can give useful information about aspect sensitivity and can also be used to estimate the effective pointing angle of an off-vertical beam. A method is suggested which may provide increased angular resolution, based on the fact that a single antenna can observe a very long cross-section through a moving ground pattern. The method would be analogous to a synthetic-aperture radar.

A test for dispersion in F-region drifts observed by the radio star scintillation method

Abstract Records of scintillations of the source Cassiopeia A made on 38 Mc/s using three spaced interferometers have been analysed to determine whether Fourier components of different frequencies give different values of drift velocity. Such an effect is found to occur, and in most cases the speed of drift increases with frequency, while the direction of drift does not change. This effect is related to a systematic ‘skewness’ of the cross-correlation functions. The dispersion could be produced by the propagation of some type of dispersive wave through the F-region of the ionosphere, but it is considered more likely that it is produced by a variation of drift velocity along the line of sight.

ON THE ANALYSIS OF MOVING PATTERNS IN GEOPHYSICS. II. DISPERSION ANALYSIS.

Abstract A method of analysis is described which enables the velocity of each spatial Fourier component of a two-dimensional pattern to be calculated from records of the pattern obtained at two times. From the results, it is possible to determine whether the velocity is a function of the wavelength. This may be a useful method of analysis when it is suspected that dispersive wave motions play a part in the movements of the pattern. It should have applications to the study of ionospheric movements in experiments which use large arrays of aerials. The analysis may also have applications to other phenomena such as moving patterns of light emission from the night sky, observations of the form of the surface of the sea, and other geophysical phenomena in which moving patterns occur.

THE RELATION OF SPREAD-F ECHOES TO MAGNETIC DISTURBANCES

Abstract The correlation between the occurrence of spread- F echoes at Slough and the magnetic K -index is examined for a complete solar cycle. Cross-correlograms for the two quantities show that variations of occurrence of spread- F echoes lag by 0.5–2 days on variations of the K -index. It is shown that the K -index often shows a marked 27-day recurrence tendency, but that there is no such recurrence tendency for spread- F echoes.