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We compute the linear parametric convective amplification of electron plasma waves by an O mode pump propagating vertically into a vertically stratified ionosphere. The upshifted plasma line compares favorably with Arecibo observations, center frequency, spectral width, and possibly amplitude when the radar is aimed in a northerly direction. The dependence of the computed enhanced plasma line shape upon density scale length, electron-to-ion temperature ratio, pump power, density gradient scale length, and backscatter radar incidence angle is investigated.

A 32-element HF-phased array has been used to study the angular spectrum of spread F returns from an ionosphere modified by the powerful radio frequency heater transmitter at Platteville, Colorado. The diagnostic radar is a coherent pulse system that provides an angular resolution of 2.7° at the midband frequency of 7.5 MHz, a range resolution of 30 km, and a Doppler resolution of 0.08 Hz. The results are presented in the form of sky maps (brightness versus azimuth and elevation) of the returns as a function of group delay, Doppler shift, polarization, and sounding frequency. The principal result is that the returns that suffer the greatest group delay at a given sounding frequency come from a direction looking up the earth's magnetic field and exhibit negative Doppler shift. Ray-tracing simulation of the observed dependence of the range and the Doppler on the angle of arrival of the returns supports the view that the heater-generated spread F is caused by field-aligned ducts produced within the heated volume. A simulation run for Arecibo, Puerto Rico, in comparison with that for Platteville, reveals that the group delay spread is about the same and the angular spread is approximately in the same ratio as the heater beam widths at the two locations.

A coordinated program between satellite, VHF radar, and optical auroral observations was carried out in Alaska during the spring of 1972. On 18 days in February, March, and April the positions of the >130-keV isotropic and cutoff trapping boundaries in the evening sector of the oval were determined on satellite 1971-89a. On 9 of these days, simultaneous VHF 50-MHz auroral radar backscatter echoes were observed to lie equatorward of the cutoff trapping boundary regardless of the phase of the auroral substorm. The analysis of these data with respect to substorm phase, Dst, and position of the aurora shows that the motion of the trapping boundary is a dynamic feature of the substorm that can be monitored and approximately located by the VHF radar echoes.

Auroral zone E-region neutral winds have been derived from simultaneous measurements of ion drift velocities in different altitudes by the incoherent radar facility at Chatanika, Alaska, on a quiet day before and during the great magnetospheric storm of Aug. 3-9, 1972. The neutral wind expected for a day-night pressure asymmetry appears to be strongly opposed by ion drag and local pressure gradients in the auroral oval.

Previous work on the second-order contributions of radar sea echo by Hasselmann and Barrick is generalized to include the effect of capillarity (i.e., surface tension) in the hydrodynamic part of the transfer coefficient, the lossy dielectric properties of the sea, and the angle of incidence and the polarization of the electromagnetic radiation. The present theoretical results apply to the backscattering of electromagnetic radiation from water waves that are gravity or gravity capillary waves. The introduction of surface tension in the analysis allows for second-order resonant interactions of the gravity capillary wave components. To include resonant interactions of gravity waves, the analysis must be extended to higher order. In general, resonant interactions produce higher-order contributions in the Doppler spectrum at the frequency of the first-order Bragg line that represent the nonstationary and non-Gaussian properties of the water surface. Furthermore, because of the different dispersion relation of gravity capillary water waves, the secondary lines of the second-order Doppler spectrum now are shifted toward the Bragg frequency and 2½ times the Bragg frequency. Numerical results are presented from our generalized theory for various radar and sea (water surface) parameters.

The plasma fluid equations used by Sudan and his co-workers to explain the small-scale secondary irregularities observed in the equatorial electrojet are applied to the diffuse radar aurora. It is shown that the increased magnitude of the ambient magnetic field in the auroral zone and the decreased ion-neutral collision frequency, due to the higher altitude at which diffuse radar aurora occurs, combine to stabilize secondary two-stream irregularities over the expected range of electron drift velocities and primary irregularity amplitudes. It is also shown that secondary gradient drift irregularities are unstable for electron drift velocities in the vicinity of the ion acoustic velocity and that in addition to primary two-stream irregularities, primary gradient drift irregularities may be generated in the auroral E layer density gradients. These results are used to explain some of the spectral observations of the diffuse radar aurora.

Observations of the behavior of the auroral zone ionosphere were made with the incoherent scatter radar at Chatanika, Alaska, during the total solar eclipse of July 10, 1972. The radar beam intersected the approximate center of the eclipse shadow at a height of 400 km. The electron temperature decreased by more than 1200°K, and the ion temperature decreased by approximately 100°K near eclipse totality. Electron concentrations in the E and lower F regions decreased by 50%. Concentrations above the F layer peak decreased slightly, but there was no net change in ionization at the F layer maximum. Apparently, downward diffusion of upper F region ionization was just adequate to balance the loss processes at the layer maximum. An estimate of the heat input to the electron gas at 340 km showed that this quantity fell to about 25% of its background value during totality. Control radar data for the eclipse observations were obtained on July 9, 11, and 12, 1972, along with supporting vertical incidence ionosonde and magnetometer data acquired near College, Alaska.

The formal relationship is established between the geometric optic approach and the autocorrelation approach to the analysis of lunar and planetary radar echoes for a Gaussian autocorrelation function when the surface has Gaussian height statistics and introduces deep phase modulation on the incident wave. Similarly it is inferred that such an analogy also holds whenever the surface undulations contain structure no smaller than the wavelength of observation divided by the rms slope of the surface. If an appreciable amount of small-scale structure is present, it is shown that (1) the range of scales responsible for the scattering will include an increasing amount of small-scale structure with increasing angle of incidence and (2) no ample relationship appears to exist between the distribution of apparent surface slopes and the power backscattered as a function of angle of incidence.

Forward-oblique ionograms made on the path from Thule, Greenland, to College, Alaska in 1963–1964 were studied to determine whether HF signals propagating through the auroral curtain were adversely affected by scattering or blanketing effects. The location of the aurora was determined by an HF radar at College. Neither scatter from the auroral curtain nor blanketing by the auroral-E layer was found to significantly affect the 1F forward-oblique propagation mode on the Thule-College path. Higher-order modes and spread signals diminished appreciably at the onset of an auroral disturbance and reappeared with the decay of the disturbance. The disappearance or marked attenuation of the 1F mode on the Thule-College path was almost always found to be caused by an increase in auroral absorption and not by blanketing or scattering effects. Although auroral-E critical frequencies near 65° magnetic latitude were frequently above 5 MHz, they were rarely above 2 MHz between 70° and 72°, indicating the presence of a strong southward gradient in the electron concentration of the auroral-E layer between 65° and 72°. In an earlier paper it was concluded that auroral sporadic E caused the observed cutoff of the ground scatter echo at the range of the auroral curtain. This suggestion is discussed in light of the results presented in this paper.

A technique to measure the direction of the geomagnetic field by using incoherent scatter techniques is described. The technique has been used to measure the inclination of the magnetic field at the Jicamarca Radar Observatory with an accuracy of the order of 1 minute of arc in the height range from 200 to 800 km. The corresponding inclinations obtained from the Goddard Space Flight Center (12/66) and the International Geomagnetic Reference Field (10/68) models are found to be about 1° in error.

During the period from December 1963 to April 1964, a large number of combined Faraday polarization and Doppler frequency measurements were made on lunar radar echoes at Stanford, California. The results of the measurements taken while the moon was in the antisolar quadrant show a large spatial variation of the electron content beyond the ionosphere. A first-order model of the earth's magnetospheric wake is deduced, having the following characteristics: (1) the electron density inside the wake is about 200 cm−3 greater than the solar wind electron density; (2) the wave extends at least to the orbit of the moon; and (3) the wake intersects the lunar orbit at about 120° from the earth-sun line.

Measurements related to the nature of field-aligned irregularities in the ionosphere E region are presented. The measurements were performed near the magnetic equator, and the irregularities are attributed to the equatorial electrojet. It is shown that the irregularities are most likely to consist of plane acoustic waves generated by an instability resulting from the flow of current in the electrojet. Reference is made to a recent theory in which the existence of such waves is predicted. The characteristics of auroral radar echoes are shown to be similar to those of the equatorial echoes. It is inferred that the auroral field-aligned irregularities are also caused by the flow of electrojets. (auth)

Langmuir probe and retarding potential analyzer (RPA) data on the electron and ion temperatures Te and Ti obtained from Ogo 6 are compared with Te and Ti values obtained from the incoherent scatter network. The satellite to radar temperature ratio TeS/TeR is 1.15 on the average for these comparisons. This discrepancy is larger than the uncertainties usually placed on the probe and radar Te values. The ion temperature ratio TiS/TiR approximately 1.0, independent of the particular radar examined. This comparison serves as an intercalibration of the incoherent scatter network.

The Faraday rotation of 425-Mc/s radar signals reflected from the moon's surface has been analyzed to yield data on the ionization in the ionosphere. The experimental measurements were taken at Trinidad, West Indies, between January and July 1960. A method is presented which permits resolving the ambiguity of the number of rotations of the plane of polarization when one frequency is employed in radar-lunar studies. The total integrated electron density in a vertical column through the ionosphere inferred from the radar Faraday data is found to undergo a diurnal variation, having a maximum value during the late afternoon. An analysis of other ionospheric parameters, such as the equivalent slab thickness, the scale height, and the ratio of the electron content above the peak of the F layer to that below, is also presented.

Satellite data on precipitating auroral particle fluxes were acquired simultaneously with ground-based observations of auroral radar echoes. Measurements on the auroral particle fluxes were in the energy range from about 0.5 to 50 kev, and the auroral radar observations were made at 23 cm with frequency spectra analysis to investigate the motion of the scattering region. The results for an afternoon event show that strong radar echoes occurred in the absence of observable electron precipitation but were in close time and spatial coincidence with proton precipitation, although the proton contribution to the ionization at the scattering height at 112 km was computed to be negligible compared with the solar ionization. The frequency spectra of the auroral radar echoes were not of the character expected from a two-stream instability.

Backscatter and forward-scatter reflection properties of chemically created electron clouds were measured throughout the HF spectrum. The measurements included radar cross sections, signal fading characteristics, and frequency-time-dependence. These quantities led to certain conclusions regarding the cloud model which describes in general the chemical releases. The model so presented appears to be a limited coherent type of reflector; i.e., it is composed of an assembly of reflectors of coherent nature in a random motion which in turn give rise to a partially incoherent radar scatter signal. This simple model adequately describes most of the cesium releases, but it fails often enough to show that further experimentation is required.

Incoherent ionospherically backscattered radar returns have been observed at 440 Mc/s over an approximate height range of 100 to 800 km. The results of experiments designed to determine the variation with height of both the intensity and the frequency spectrum of backscattered returns are presented. Backscatter signal-intensity measurements at radio frequencies much greater than the F2 critical frequency provide an independent means of estimating electron densities. Height profiles of electron densities obtained from experimental data are discussed. Experimentally determined values of the scattering cross section are approximately equal to the square of the classical electron radius. A typical result of the frequency spectrum measurements at 440 Mc/s indicates a half-power spectrum width of about 11 kc/s at a height of about 300 km during midafternoon of a March day. Spectrum widths observed at several heights and at various times of day and night are discussed.

The phenomenon of incoherent scattering of electromagnetic waves by the free electrons in the ionosphere, initially hypothesized by Gordon [1958], has been experimentally validated as a possible technique for studying the characteristics of the ionosphere [Bowles, 1958, 1961; Pineo et al., 1960a, b; Pineo and Hynek, 1962; Millman et al., 1961; Greenhow et al., 1962; Evans, 1962]. In this note the detection of apparent ionospheric movements by the incoherent backscatter technique is described. The experimental observations were conducted in January and February 1961 at the U.S. Air Force Trinidad Test Site, West Indies (10.7° N, 61.6° W), with a high-powered pulsed radar operating at a frequency of 425 Mc/s.

The ionized medium between the earth and the moon is being studied at Stanford by means of lunar radar echoes. Of prime interest is the electron density beyond the earth's ionosphere, in regions (at distances of several to 60 earth radii) where the interplanetary gas may be dominant. Six related techniques for measuring the total integrated electron density are discussed. In practice the ionospheric part could be determined separately and subtracted from the total. These same techniques could be used to measure by radar the electron density between the earth and various planets, and to measure by one-way propagation the integrated electron density between a space probe and the earth. The moon's radar range exceeds the true range, especially at low frequencies, because of group retardation, which is a measure of integrated electron density. Density measurements based on absolute measures of radar range would require very high peak power (for pulse ranging) or very high equipment stability (for CW ranging); futhermore, the true lunar range is not known accurately. In two of the methods described here, the full average power capability of the transmitter can be used without need for high stability or precise knowledge of the lunar range. In effect, a reference is sent with the measuring quantities and only relative radar range is measured. Doppler frequency shifts and the earth's magnetic field cause only slight difficulties. However, path splitting, electron ‘blobs’ and lunar surface irregularities possibly could affect the waves more than the electrons do. The measurements might then yield information about the lunar surface or about the temporal and spatial variations of the cislunar medium.

A six-frequency radar was operated at Homer, Alaska, to determine the wavelength dependence and aspect sensitivity of radar auroral echoes at 50, 139, 398, 850, 1210, and 3000 Mhz. The lack of dependence of aspect sensitivity on frequency and the nature of the wavelength dependence indicates that the formulation for field-aligned scattering described by Booker is not adequate. It is believed that a more plausible model for the generation of field-aligned ionization is described by Farley. However, the nature of these results and the status of Farley's theory make quantitative comparisons beyond the scope of this paper.

By means of a relatively sensitive 400-Mc radar located at College, Alaska, the variation of Doppler shift of auroral echoes has been determined as a function of the following parameters: (1) azimuth angle of the radar ray; (2) off-perpendicular intersection angle of the radar ray and the earth's magnetic field; (3) elevation angle of the radar ray; (4 ) range of the echoes; (5) altitude of the reflection centers; (6) time of day; (7) number of occurrences; (8) strength of the echoes. These data have been further delineated in terms of the type of echo seen (discrete or diffuse) and whether the data were taken before or after magnetic midnight. An estimate of the spectrum spread of auroral echoes has also been determined by pulse and by CW techniques. A consistent trend in these data has been found which would indicate an east-west motion of the ionospheric irregularities. There is no appreciable variation in the direction of motion with time of day or with respect to magnetic midnight. The mean velocity of the east-west motion appears to be 500 meters per second. These conclusions agree with those of Kim and Currie [1958] but disagree with those of Lyon and Kavadas [1958], Nichols [1957], and Bullough et al. [1957].

The return of an ionospherically propagated radar pulse may be a combination of both specular reflection and diffuse scattering from the earth, and it is of some interest to estimate the structure of the return pulse due to this combination. In particular, it appears that specular reflections may occur between the transmitter and its co-located receiver as a result of pulse reflection from suitably disposed geological and man-made facets and that scanning over these specular areas may give some structure in the backscattered pulse.

The possibility of utilizing irregularities produced in the ionospheric D layer by radio-frequency heating or by intense optical radiation from lasers to scatter radio waves is considered. Intense beams passing through the D region can produce large horizontal gradients in electron density where the beam intensity is falling off rapidly. The mechanism in the case of radio frequency is enhancement of electron attachment rate due to electron heating and in the case of lasers is photodetachment. In these regions radio waves may be scattered from blobs produced by turbulent mixing. A recently discovered anomalous relationship between electron energy and electromagnetic field intensity gives rise to the additional possibility of producing sharp vertical gradients in the propagation characteristics of an intense high-frequency wave in a plasma. Some interesting applications to f orward scatter propagation links and to ionospheric research with the Arecibo radar are suggested.

Power spectra of VHF radar auroral echoes were obtained near Anchorage, Alaska, during October 15–23, 1970. These spectra were obtained concurrently with standard range-time-intensity (RTI) film strips of the echoing region using the same radar system. In this paper, these spectra and the RTI film strips are examined and are compared with previously defined radar auroral classifications. The results indicate that there are at least two, and possibly three, separate types of spectra that correspond to identifiable echo configurations on the RTI film strips. Identification of the two-stream instability as an irregularity-producing mechanism for one of these spectral types is possible but not definitive. Another type of spectrum is similar to the type II spectra seen in the equatorial electrojet. The third spectral type bears no similarity to any results obtained at the equator.

Recent interest in the use of a multifrequency radar as a remote sensor for exploration of the earth and the moon has stimulated this study of the variation of radar backscattering with change in electromagnetic wavelength. The analysis of data has shown that radar cross sections of land and sea surfaces decrease with increasing wavelength and, on the average, follow approximately a λ−1 behavior, although the exponent may vary from +2 to −6 in individual cases. Snow-covered surfaces at all depression angles and sea surfaces at angles above 73° are the ones with positive values for the exponent. The wavelength dependence of the moon and the average values found on the earth in this study are quite similar. The combination of polarization and wavelength dependence versus depression angle may be characteristic features for distinguishing various terrain types by a multifrequency radar.

Long-range, fast-fade radar echoes at 14 Mc/s observed looking north from Ithaca, N.Y., are described. They are explained in terms of backscatter from field-aligned irregularities located in a region where straight ray paths come within 10° of perpendicularity to the earth's magnetic field. A typical echo was spread in range from about 1000 to 2000 km, had a fading rate comparable to auroral fading at the same frequency, and seemed to correlate on an hour-by-hour basis with the occurrence of spread F. The minimum echo range was observed to be dependent on the penetration frequency of the F layer. This would not be predicted by geometry with straight ray paths, whereas bending of the ray paths into strict perpendicularity as a result of refractive effects does account for the observed echoes.

A ‘layered’ exponential model for the vertical distribution of refractivity in the form N = N6 exp [−c4 (z −z4)] is investigated. Here N6 and z4 are the refractivity and height, respectively, at a mandatory pressure level. The parameter c4 is the ‘best fit’ when N = N6+1 is the value at the next level where z = Z6+1. A formula for c4 is derived and is found to give good agreement, layer by layer, with the above model. This model appears to have particular merits for the case of air-to-air radar tracking.

Geomagnetic equatorial ionospheric ion temperature, comparing incoherent scatter radar and OGO-D retarding potential analyzer values

A coherent radar was used to measure the ‘sea clutter’ or backscattering of X-band electromagnetic energy from the sea surface. More than 200 recorded samples of clutter were analyzed to give power spectra of the clutter. Each spectrum was displayed as a function of frequency and of position on the water surface and was also averaged to give the mean spectrum of patches 3750 feet long. Five of the samples showed an anomalous downwind displacement of the clutter by as much as 7 knots. The displays indicate again, as in earlier Control Systems Laboratory measurements, that the upwind edge of the clutter spectrum is smooth for all wind speeds observed, but that the downwind edge, for sea state 3 or above, is broadened in an irregular fashion as a function of range. This irregular broadening implies a considerable variability, from patch to patch, in the downwind side of the probability distribution of velocity of scatterers on the sea surface. The width at half-power of a mean spectrum is proportional, for reasonable assumptions, to the width at half-maximum of the probability distribution of scatterer velocities. The variation of the latter width, Δ0, with sea state can be represented by the equation (expressed in consistent units) Δ0=11H1/3/Tm where the numerical factor is dimensionless, H1/3 is the significant wave height, and Tm is the period corresponding to the maximum of the energy spectrum for the water waves themselves when this spectrum is plotted as a function of frequency. This equation fits the experimental data within about 10 per cent for bandwidths in the range of 2 to 5 knots (64 to 160 cps) and wind speeds in the range of 8 to 19 knots. The bandwidth of the clutter is found to be approximately proportional to the wind speed. The relationships of clutter bandwidths to wave and whitecap velocities, radar depression angle, and wind direction are also discussed.

A series of cesium-seeded explosive bursts in the 90- to 110-km altitude region have been observed with meter and decameter wave radar. Detailed measurements of contact surface expansion, undulation, and stabilization have been used to acquire estimates of the altitude dependence of atmospheric pressure and density in the lower ionosphere. It has been found that events in the development of the contact surface are retarded relative to those predicted by Brode’s blast wave model, and this retardation is interpreted as evidence of magnetohydrodynamic effects upon the blast wave. Comparison of the results of these observations with previous results of Rosenberg (1964, 1965) suggests that a suitable combination of optical and radio-frequency instrumentation can be utilized to study in detail both the neutral gas and the magnetofluid characteristics of the lower ionosphere simultaneously.

The effect of ion-neutral atom (or molecule) collisions on the excitation of ion-acoustic instabilities in the auroral ionosphere is considered. These collisions may play an important role in selecting the wavelength of the perturbations that are most likely to grow at any given height. The results of the calculations are compared with observational evidence obtained from radio star scintillations and radar aurora echoes.

Doubly scattered radiation magnitude as function of heights and aerosol concentrations for bistatic laser radar

The experimentally measured dependence of the mean radar power backscattered from the lunar surface on the angle of incidence (delay time) is compared with the theoretical formula based on a lunar surface consisting of a superposition of several normal random processes of different structures. The theoretically derived curve accounts for shadowing and matches the experimental data over almost the entire range of angles of incidence to within ±1 db. The wavelength dependence is also consistent with the observed data as far as they are available. The analysis yields a value of 11° for the rms slope of the lunar surface.

Three high-explosive charges were placed by rocket in the lower E layer to study the effects of detonation waves on this region of the ionosphere. Observations made by coherent-pulse-Doppler high-frequency radar and an ionosonde indicated that three types of radar targets were discerned: (1) Shock-wave excitation of atmospheric constituents and compression of existing ionization created an overdense reflecting surface, with a probable small additional contribution due to partial reflections, that traveled radially outward quite rapidly and was observed for a few tenths of a second. (2) Shock-induced ionization created as a result of the high temperatures achieved in the region between the first shock wave and the contact surface between the explosion products and the atmospheric fluid persisted as a nearly stationary radar target for several minutes. (3) Sonic waves propagating outward from the detonation were observed to perturb sky-wave transmission paths that passed through the ionosphere many tens of kilometers from the explosion. These effects were detected several minutes after the explosion. It is concluded that explosive thermal excitation of the lower E layer is adequate to produce radar reflections of decameter wavelengths by compression of existing ionization and shock excitation of additional ionization and to deposit by this means long-lasting ionization, which may appear as an artificial sporadic-E layer of several minutes duration. This layer is probably composed of the long-lived ions of alkali metal atoms, which exist continuously in the lower E region and form a detectable persistent radar target when they are excited by a shock wave. This result and other workers' independent evidence for atmospheric acoustic-wave effects in the lower E region indicate that such phenomena may be involved in the growth of naturally occurring sporadic-E layers. The sonic wave created by the explosion also perturbs existing ionospheric regions several tens of kilometers from the burst point, and this perturbation is readily detectable as a phase-path modulation in decameter-wavelength earth backscatter.

Howard et al. [1965] and Yoh et al. [1966] have recently reported on a beautiful set of radar experiments in which group and phase delay introduced by the cislunar medium were interpreted in terms of a mean plasma electron concentration. Electron concentrations found by these techniques appear to be very much higher than those derived by other means, such as from whistlers [Carpenter, 1966], from micropulsations [Watanabe, 1965], or on theoretical grounds [Dessler and Michel, 1966]. Scarf [1966] subsequently attempted to explain the discrepancy as an effect of coherent density fluctuation on the mean propagation constants of the plasma, In what follows we show that the effect discussed by Scarf is probably unimportant and hence that some other explanation of the curious discrepancy between the results of the radar propagation experiments and the currently accepted model of the magnetospheric tail must be sought.

Aspect-dependent radar scattering from ionospheric phenomena such as the aurora is known to be dependent on the three-dimensional wave-number spectrum of the random electron-density fluctuations as well as the mean-square deviation of the fluctuations. In characterizing the wavenumber spectrum, it is convenient to speak of the longitudinal and transverse scale sizes or coherence lengths. L and T. which describe, respectively, the average distances parallel and perpendicular to the geomagnetic field over which collective scattering effects take place. An analysis of the scattering process is presented in which the relationships between ihese ionospheric quantities and those quantities which are actually observed in an experimental situation are determined. With the use of these relationships, it is shown that the values of the experimental parameters impose fundamental limitations on the extent to which the quantities of interest can actually be measured in any given experimental situation. In particular, it is shown that the vertical antenna aperture D sets the limit on the largest value of L that can actually be determined. If D/L < 1. it is shown that the apparent measured value of L will always be very close to D, regardless of the true value of L; some experimental results aremore » considered in the light of this effect. It is shown that an analogous situation can occur if the transmitted pulse width is too long, but, in practice, this limitation is not as restrictive. The correct normalization of measured values of received signal strengths necessary to determine the scattered power per unit volume is also complicated by the aspect sensitivity of the scattering is also shown that this problem can be considerably alleviated wiih the proper choice of experimenial parameters. From these considerations, an ideal antenna configuration for making such measurements is suggested. An extension of the analysis to bistatic scattering is also considered. The measurement of the variance of the electron-density fluctuations is discussed.« less

The theory of enhanced radar attenuation due to nuclear explosions is developed here for radar paths outside the fireball region and for ionization produced by delayed γ rays. The basic equations are discussed, and a number of neglected effects are shown to be small in most cases. Values of parameters entering into the equations are given, and their justification is discussed in detail. It is shown that the radar attenuation can be expressed as a single integral over the radar path. An extensive asymptotic evaluation of the integral is given and is applied to a rough parametric study of the attenuation phenomena. The principal results are given in equations 31, 37, 40, 42, 44, 49, and 50.

Radar-based values of the neutral exospheric temperature Tn obtained at 12°S on 127 nights in 1967 and 1968 are presented and discussed. The dominant effect is an annual variation of approximately ±5%, warmer in summer than in winter. Previous radar-based maps of the diurnal temperature bulge show similar seasonal variations of Tn at night. Model atmosphere maps show smaller Tn variations, but since the Tn value of the model is a parameter defining the average distribution of the neutral density, both the radar and model atmosphere maps may be correct. In addition to the annual variation, a semiannual variation and geomagnetic heating effects are also observed. The data indicate that the electron temperature Te is often less than 20°K above Tn at night, although it appears that (Te - Tn) may occasionally be 50°K or more. Because of the apparent near equality of Te, Ti, and Tn, data presented here are suitable for comparison with satellite and other temperature measurements. Agreement is found with an empirical law for Tn based on incoherent scatter at 45°N. Temperatures measured with a retarding potential analyzer (RPA) on Ogo 4 sometimes agree with and sometimes are ∼50% higher than nearly overlapping radar temperatures presented here. The Ogo 4 data imply that the ion-neutral heating rate is sometimes much higher than that inferred from radar observations. However, Ti values from the RPA on Ogo 6 are consistent with the radar data presented herein.

Results obtained at Ithaca, N.Y., concerning radio reflections from the aurora by both pulse and continuous-wave transmissions are described. At 104 Mc/sec, radar reflections show the following properties: (i) They occur only during auroras having ray structure; (ii) the radar must be directed roughly normal to the rays; (iii) echoes are complex, indicating contributions from many scattering elements mutually interfering in more or less random phase. At various frequencies, from 2.4 to 144 Mc/sec, measurements of fading show that auroral reflections fade at a rate roughly proportional to frequency; rates are between one and two powers of ten faster than would be expected for a quiet ionosphere. These observations are interpreted as indicating that radio reflections from auroras arise by scattering from numerous auroral columns of ionization, somewhat like meteor trails, having the magnetic zenith as radiant. The fading is principally due to a wind-like motion of the auroral columns, much faster than is customary for meteor trails, and apparently much faster than can actually be observed for meteor trails occurring during auroral disturbances. Visual observations of the fine structure of rays with high-power field-glasses and by photometric analysis show that rays often consist of a large number of fine streaks, each of which extends only part of the length of the ray and lasts only about a second. It is tempting to associate these visual auroral streaks with the auroral ionization trails that are required to explain radio reflections from the aurora. Alternative interpretations suggested by other workers are considered, especially that by Harang and Landmark [11]. Reasons are given for rejecting these interpretations in favor of that described above. Although practically all of the auroral echoes observed at Ithaca are interpreted as coming from E-region levels, reasons are given why auroral echoes from F-region levels may be seen, especially by radars in lower latitudes.

The theory of incoherent scattering of radio waves by a plasma predicts that for unequal ion and electron temperatures, the scattering cross section is dependent on the angle between the radar ray and the earth's magnetic field. The magnitude of this dependence has been calculated and verified experimentally for an incident radar frequency of 1290 Mhz.

By using a narrow-beam high-power radar operating at 68 cm it has been possible to detect meteors traveling radially toward the radar. In these observations the antenna is directed at the radiant point of an intense meteor shower, and the receiver is tuned to the expected Doppler-shifted signal. Because the actual Doppler shift can be measured with precision, both the velocity and the deceleration of the approaching meteor can be determined. The behavior of the velocity is compared with the theory for a solid particle entering the earth's atmosphere at heights where no air cap is formed. Anomalous behavior was occasionally observed, but the height at which it appears to begin is too great to be accounted for by cap formation. We conclude that errors in the computed height of the meteor may be responsible for the anomalies. A mean value for the effective ablation energy of 15.4 km2/sec2 was obtained, but individual meteors showed wide departures from this value. The mean mass of the meteors is estimated to be in the range 10−2–10−3 gram, corresponding to a visual magnitude of +5.

The theory of radar reflections from a rough planetary surface is examined for the case of deep modulation where the surface correlation function consists of two components having greatly different structure sizes. It is found that a modified superposition principle holds for the angular power spectrum and that the individual components may be detectable by high-resolution pulse techniques. Recent lunar experiments by Mehuron are found to show the existence of two such components. The theory also leads to the conclusion that the angular power spectrum of the echoes cannot be computed from the results of CW measurements for the type of surface considered.

Radar measurement of differential group delay to moon, showing large differences in total cislunar electron content above 1000 km in solar and antisolar directions

Radio frequency scattering experiments have indicated that irregular fluid motions of tens of meters spatial scale and tenths of seconds temporal scale occur in the E region. It has been suggested that there can also occur in this region a dissipative plasma instability (similar to what has been termed the gradient-drift or E×B instability in previous work) that displays these spatial and temporal scales and that may be of importance in the evolution of sporadic E. This paper describes an attempt to study the circumstances under which such an instability might appear by injecting alkali plasma clouds into the lower E region and observing them with decameter and meter wavelength, coherent-pulse-Doppler radar. After an initial blast-wave-driven expansion phase, an alkali plasma cloud undergoes a period of irregular behavior, in which a number of discrete echoes from seemingly coherent, large, short-lived reflectors appear superimposed on the generally diffuse plasma cloud radar echo. Their large apparent size and spectral discreteness indicate that they may arise from an ordered, possibly wavelike structure, which must involve the entire plasma cloud. Comparison of the observed growth rates and probable spatial scales of the instabilities in the alkali plasma clouds with order-of-magnitude calculations of these quantities based on linear instability analysis confirms that the postulated plasma instability can indeed explain the observed behavior. In one phase of plasma cloud evolution a reflecting layer similar to sporadic E is formed, which permits decameter wavelength radiation, incident obliquely on the cloud, to be reflected back to earth in much the same manner as sky-wave transmission is achieved via sporadic E. The similarity in behavior of an artificial sporadic E patch to the natural variety is evident. Also, a nonlinear, two-dimensional plasma instability analysis is capable of explaining the spectral appearance and temporal variation of the decameter radar data from the alkali plasma cloud. An association is suggested between these results and the mechanism by which temperate latitude sporadic E is formed.

A 139- and 398-Mhz auroral radar located at Homer, Alaska, was used to study the correlation between radar auroral echoes and the precipitating charged particles, as measured by a polar orbit satellite, producing the aurora. On the several satellite passes that showed on significant precipitating particles in the region that could be observed by the radar, no radar auroral echoes were obtained. On the one pass when both precipitating electrons and protons were detected by the satellite, the radar obtained auroral echoes that were most closely correlated in space with the >2-kev electrons, and no echoes were associated with the peak position of the >20-kev proton precipitation. The energy of the electrons measured is sufficient to produce ionization in the height region from which the echoes are obtained. The very close spatial relationship between the auroral echoes and the precipitating electrons indicates that for this particular case the primary generating mechanisms for the echo-producing auroral ionization are precipitating electrons.

The technique for studying the characteristics of the ionosphere by means of incoherent scattering of radio waves by free electrons was first postulated by Gordon [1958]. Experimental verification of this phenomenon has been demonstrated by Bowles [1958, 1961] of the National Bureau of Standards and by Pineo, Kraft, and Briscoe [1960a, b] of the Lincoln Laboratory. In this note, preliminary observations of incoherent scattering conducted at Trinidad, WIF (10.7°N, 61.6°W), with a high-powered pulsed radar operating at a frequency of approximately 400 Mc/s, are described.