Kenneth Davies

Recent progress in satellite radio beacon studies with particular emphasis on the ATS-6 radio beacon experiment

In May 1974 a new era in satellite radio beacon studies of the ionosphere opened with the ATS-6 Radio Beacon Experiment. The history of radio beacon studies up to that time is reviewed briefly and the particular features of the ATS-6 beacon are discussed together with the basic theory required to interpret the measurements. The main emphasis is on the ATS-6 beacon experiment but other beacon data are discussed which provide the necessary background. The diurnal and seasonal variations of the total electron content and the plasmaspheric content are presented for the U.S.A. and Europe. In winter the plasmaspheric content over the Western Hemisphere maximizes at night while in Europe and the Pacific it appears to peak near noon. This is thought to be caused by flow of plasma from the local and conjugate ionospheres. Night maxima of total electron content are found showing that they do not arise from depletions of the plasmaspheric content. The plasmaspheric content is highly sensitive to solarterrestrial disturbance, it reaches a minimum on the third day of a storm and may take between 10 and 20 days of partial filling and emptying to recover. Traveling disturbances in U.S.A., Europe, and India show similarities of speeds but not of direction. Beacon observations of micropulsations in total content, tropospheric fluctuations and Fresnel diffraction by intense ionospheric irregularities are discussed together with radio wave scintillations and some applications of beacon radio data to communications and navigation.

Ionospheric disturbances by severe tropospheric weather storms

Abstract Radio observations made near Gainesville, Florida, are reported. Wavelike ionospheric disturbances were observed when thunderstorms with tops (radar heights) in excess of about 40,000 ft occurred within a radius of 250 km about the radio reflection (middle) point in the ionosphere. It appears that the nighttime storms are more efficient in exciting the ionospheric waves. These waves have periods ranging between 1 and 5 min and were detected as Doppler shifts in the ionospheric radio echoes. The spectral content of the ionospheric wave varies from one storm to another. Even for a given storm, the spectral characteristics of the ionospheric response varied from hour to hour. Such an hour-hour variability may not be explicable on the basis of the atmospheric filtering action. It is, therefore, suggested that the observed periods were characteristics of the acoustic sources in thundercloud cells. Smearing of the Doppler records was very common during the occurrence of the (summer) afternoon storms. It is suspected that the smearing may be a manifestation of a different form of ionospheric effect of these clouds.

Acoustic waves in the ionospheric F2-region produced by severe thunderstorms

Abstract Radio observations were made during the spring of 1970, near Oklahoma City, to measure the horizontal velocities of ionospheric disturbances associated with severe thunderstorms. The speeds lay in the range from 850 to 2750 m/s. Using the measured velocities and acoustic ray tracing in the U.S. Standard Atmosphere, the sources of the disturbances were located and found to be in the vicinities of thunderstorm cells. The power in the waves usually has peaks at periods near 3.5 and 4.5 min. We estimate that the source power required to produce waves of the type observed is about 20 MW.

A model of ionospheric F-2 region storms in middle latitudes

Abstract The proposed ionospheric storm model is based on a heat source located at magnetic noon on Feldsteins auroral oval. The rotation of the Earth produces an apparent motion of the source which is greater than the speed of the disturbance. This gives rise to a wake or front which sweeps over the globe and determines the onset time of the negative phase which results from a change in chemical composition. At the front, focussing will occur which accounts for the sudden drop in electron density (or contents) sometimes observed. The calculated onset times of the negative phase are compared with observations for a number of storms. The local onset times vary from 12 at the latitude of the source to around 24 at 10° geomagnetic latitude. This model predicts that the onset of the negative phase at a given location, for storm which commence between about 2000 LT to about 1000 LT, is independent of the time of storm commencement.

A comparison of several methods of estimating the columnar electron content of the plasmasphere

Abstract Data from several different observation techniques are used to estimate the columnar electron content of the plasmasphere. The estimate is based upon the subtraction of the ionospheric electron content from the total content along the radio path from ground to the geostationary satellite ATS-6. The Radio Beacon Experiment on this satellite allows a very accurate determination of the total electron content up to the height of the satellite. For the ionospheric electron content (ground to appr. 2000 km) the following techniques are used 1. (a) Faraday effect on the ATS-6 signals, 2. (b) Differential Doppler effect on the signals of the low orbiting NNSS satellites, 3. (c) combination of bottomside and topside ionograms, and 4. (d) extrapolation of bottomside ionograms. It is shown that the combination of Group Delay and Faraday effect data on the ATS-6 RBE signals provide the most reliable estimates of the columnar electron content of the plasmasphere. Data from low orbiting satellites support this conclusion.

Total electron content and F-region electron density distribution near the magnetic equator in India

Abstract Total electron content derived from the group delay measurements of ATS-6 radio beacons received at Ootacamund (India) are compared with the electron-density vs height distributions derived from the ionosonde data of the nearby station Kodaikanal. The daily variation of equivalent vertical total electron content (NTV) does not show the midday bite out which is so prominently present in the corresponding daily variation of NmF2, the peak electron density. The topside electron content (Na) continues to increase from sunrise to a maximum value around 1500LT, while the bottomside electron content (Nb) reaches a maximum value around 0500 LT. Daily variations of these as well as other parameters, e.g. the vertical slab thickness, the bottomside semi-thickness, the height of theF2 peak have been also studied for a geomagnetically quiet and a disturbed day.

Theoretical studies of storm effects in ionospheric total electron content

Abstract Investigations have been made of the effects produced by thermospheric winds, composition changes and magnetospheric electric fields on the ionospheric F-layer during disturbed conditions. The results of the computations suggest that a combination of realistic temperature and electric field changes would explain fairly satisfactorily the observed changes in total electron content.

Comparison of total electron content measurements made with the ATS-6 radio beacon over the U.S. and Europe

Abstract We compare and contrast ATS-6 radio beacon measurements made at three stations in the U.S.A. with similar measurements made in Europe the following year at two locations. It is shown that over the U.S.A. the winter plasmaspheric content reaches its maximum near 0300 LT whereas in Europe the maximum occurs near noon. The plasmaspheric content decreases with increase of magnetic activity in both continents. Winter night maxima are observed in ionospheric content in both hemispheres. Marked differences occured in magnetic storm effects and in the day-to-night ratio of ionospheric electron content.

Electron density and temperature changes in the equatorial ionosphere during magnetic storms

Abstract Storm changes in the height profiles of electron density and electron temperature near the dip equator are presented for five _F2-region storms. In all five storms the electron temperature increased although the electron density may increase or decrease. On the assumption that, at night, the electron and neutral temperatures are equal it is found that the temperatures agree, within 20 per cent with the temperatures calculated from the 1966 U.S. Standard Atmosphere.

Sky Wave Propagation at Medium and High Frequencies

The sections in this article are 1 Spectrum Considerations 2 Propagation Characteristics 3 Vertical and Oblique Propagation 4 Ionospheric Models 5 Propagation on Medium Frequencies 6 Propagation on High Frequencies 7 Communications Disruptions 8 Conclusions