J. V. Evans

Theory and practice of ionosphere study by Thomson scatter radar

The application of Thomson (or incoherent) Scatter observations to the study of the earths ionosphere is described. Those aspects of theory of Thomson Scatter that have been put to practical use in ionospheric investigations are reviewed briefly and the type of radar equipment constructed for these investigations is discussed. Methods of measuring electron density, electron and ion temperatures, and ionic composition are then reviewed. Other applications of the technique--to the study of the neutral density and temperature of the upper atmosphere, drift motions, the flux density of fast photoelectrons, and the orientation of the earths magnetic field--are also described.

Radar studies of the moon

Radar observation of lunar distance, motion and surface statistical nature, noting enhanced reflectivity associated with craters

Millstone Hill measurements on 26 February 1979 during the solar eclipse and formation of a midday F-region trough☆

On 26 February 1979, the Millstone Hill incoherent scatter radar was operated to observe the F-region over Canada along the path of the total eclipse of the sun. A continuous scanning mode was used with the radar elevation fixed at 4° and the azimuth swept continuously from 350° to 298°. Each scan required 20 min to complete and useful results were obtained at ranges up to 2992 km. The path of totality crossed the center of the region swept by the radar beam. The F-region electron density, ion temperature, electron temperature and ion line-of-sight drift were measured. Electric field components have been extracted from the radar line-of-sight component of the ion drift by assuming that the electric field may be represented by a quasi-static two-dimensional potential with the potential assumed constant along geomagnetic field lines. Electron density decreases were observed in association with the eclipse. At 300 km an ∼50% decrease occurred near the region of totality, but the variations were small above 450 km. However, apparently unrelated to the eclipse, very large electric fields developed in the region under view. Somewhat later a well defined trough formed north of Λ = 70°, whose equatorward edge is extremely sharp and appears to be approaching the radar with time. Large values of the electron and ion temperatures were observed in the trough and the trough formed where the drift was westwards and exceeded 1000 m s−1. It is suggested that the observed through may be due to an increase in the rate of the charge transfer reaction O+ + N2 → NO+ + N in the presence of a very large electric field impressed from the magnetosphere in association with a large magnetic disturbance.

Thermospheric properties as deduced from incoherent scatter measurements

Abstract This paper reviews the results obtained over the past decade using the incoherent scatter technique that have contributed to present understanding of the structure of the thermosphere. Principal among these has been the measurement of the exospheric temperature with considerable accuracy (± 10–20 K) and time resolution (10–30 min) at four widely differing latitudes over periods of several years. These measurements contributed significantly to the development of the best empirical model for the thermosphere presently available, namely the MSIS model. Very recent results suggest that the thermospheric temperature dependence on Kp is not properly represented in the MSIS model for Millstone and that larger amounts of heat are deposited in the summer auroral zone than the winter one. In addition, it has been found from data gathered in France that the inverse scale height parameter s is not constant as assumed in the model, but decreases as F f10.7 or Kp increases. The technique also is being used to study the oscillation of the temperatures, density and winds near the lower boundary of the thermosphere introduced by upward propagating tides. Also, seasonal variations of the composition of the thermosphere have been detected and support the view that atomic oxygen is transported from the summer to winter hemisphere.

High-power radar studies of the ionosphere

Some fifty years ago, the first experiments were conducted that provided unequivocal evidence for the existence of an electrically charged region in the upper atmosphere capable of reflecting radio waves. Appleton and Barnett, in England, and Breit and Tuve, in the United States, conducted expeximents in which the height of the reflecting layer could be reduced. They observed echoes from several heights between 100 and 200 km. Some years later Watson-Watt coined the term ionosphere for this reflecting region. These early experiments led to the development of instruments, known as vertical-incidence sounders or ionosondes, to perform routine sounding of the reflection characteristic of the ionosphere, and a worldwide network of sounders continues in operation. The results these instruments obtained constituted the most important source of information on the properties of the atmosphere above 100-km altitude prior to about 1950, when high-altitude rocket research became practical. In the last 25 years, there has been an explosive growth in our knowledge of the properties of the upper atmosphere raising from in situ measurements made with rockets and satellites. Radio-wave investigations have continued to make very important contributions, however, owing to the development of a new sounding technique known as incoherent scatter. In this method a very-high-power VHF or UHF radar is employed to observe the weak signals backscattered by density fluctuations in the ionosphere resulting from the random thermal motion of the electrons and ions. Unlike the older reflection technique, this permits the variation of the electron concentration to be measured to very great altitudes (≥ 10 000 km); moreover, the temperatures of the electrons and the ions may be determined separately. Over certain altitude ranges the following additional information may be obtained: the concentration, temperature, and horizontal wind velocity of the neutral particles; the intensity and direction of the polarization electric field (established by the neutral wind and/or the interaction of plasma from the sun with the geomagnetic field); the transport of ionization and heat to, or from, the magnetosphere (i.e., the outermost portion of the ionized envelope surrounding the earth); information on the production of photoelectrons by the sun. The paper provides a brief summary of the early reflection experiments, and outlines the principles and practice of the new method. The power of the incoherent scatter technique is illustrated by examples of recent results. In conclusion, current plans to construct large new radars to study the auroral ionosphere are described.

Traveling ionospheric disturbances detected by UHF angle-of-arrival measurements

Abstract From January 1971 to March 1973, the Millstone Hill L-band satellite tracking radar was employed in a study of ionospheric refraction. Satellites of the Navy Navigation Series were tracked smoothly by radar (at 1295 MHz) by commanding the 84-ft diameter antenna to follow the predicted path of the satellite across the sky, and allowing any measured angular errors slowly to close out the offsets. Simultaneously, the apparent position of the satellite was sensed passively by receiving (on the same antenna) the UHF beacon signals. Ionospheric refraction manifested itself during the rising and setting portions of the path as a displacement between the UHF and L-band apparent positions. In addition, quasi-sinusoidal fluctuations in the displacement were seen at intervals along thepath that are believed to be caused by traveling ionospheric disturbances. On 40% of all passes, these waves were present with wavelengths typically in the range 30–60 km. These TIDs were seen equally frequently at all local times and seasons. They did not give rise to significant fluctuations of total electron content along the line of sight to the satellite, as measured simultaneously by a differential-Doppler experiment. This suggests that they are confined largely to the lower thermosphere and manifest themselves in tilting the lower surface of the ionosphere where the density gradients are largest. The source of these TIDs is not known and they appear to have been little studied heretofore.

RADAR SIGNATURES OF THE PLANETS

When radio waves are transmitted by an antenna system, they will be reflected by objects lying along their path. Radar makes use of this principle to determine the presence of and distance to scattering objects from a given point. The first use of radar was not (as is widely supposed) during World War 11, but in 1926 when the American scientists A. Breit and M. Tuve determined the height of the reflecting layer in the Earth’s ionosphere by transmitting short pulses and measuring the length of time for their return. The War did, however, cause a vast amount of development work to be accomplished in an extremely short time. Speculation about the possibility of obtaining radio reflections from the moon or planets had been made as early as 1927, but a t that time the sensitivity of the equipment was inadequate. Immediately after the war, groups of workers in Hungary and America, using modified military equipment, obtained radio reflections from the moon. Claims by other groups to have detected moon echoes a t this time have been made in the press, but not in the scientific literature. We shall outline in Sec. I1 the difficulties involved in obtaining radio reflection from the planets in comparison to the moon. Here we need only say that an interval of some 15 years elapsed after the first detection of moon echoes before echoes were detected from the planet Venus. During this period, a small number of scientists (principally in England and America) made extensive studies of the moon by radar, and these will be described in later sections. It seems that serious study of the planets by radar might not have come about without certain large increases in the sensitivity of available radar equipments, whichstemmed from other needs. The first of these developments can be traced to the military requirement of tracking ballistic missiles, which led in America to the design and construction of the Ballistic Missile Early Warning System (BMEWS). The Millstone Hill radar, operated by Lincoln Laboratory (Section 11-d), was one of the first to detect echoes from Venus and is basically similar to a BMEWS radar. The Arecibo Ionospheric Observatory radar in Puerto Rico (Section II-d) employs a much larger antenna than the Millstone system, but it has also benefited from the military requirement for high-power long-pulse transmitters. The second development was the advent of deepspace investigations using instrumented probes. In order to transmit instructions to these packages, American and Russian engineers built powerful radio systems capable of communicating over vast distances in space. In each *Operated with support from the U. S. Air Force.