A numerical simulation study of the thermospheric winds produced by auroral heating during magnetic storms, and of their global dynamo effects, establishes the main features of the ionospheric disturbance dynamo. Driven by auroral heating, a Hadley cell is created with equatorward winds blowing above about 120 km at mid-latitudes. The transport of angular momentum by these winds produces a subrotation of the mid-latitude thermosphere or westward motion with respect to the earth. The westward winds in turn drive equatorward Pedersen currents which accumulate charge toward the equator, resulting in the generation of a poleward electric field, a westward E × B drift, and an eastward current. When realistic local time conductivity variations are simulated, the eastward mid-latitude current is found to close partly via lower latitudes, resulting in an ‘anti-Sq’ type of current vortex. Both electric field and current at low latitudes thus vary in opposition to their normal quiet-day behavior. This total pattern of disturbance winds, electric fields, and currents is superimposed upon the background quiet-day pattern. When the neutral winds are artificially confined on the nightside, the basic pattern of predominantly westward E × B plasma drifts still prevails on the nightside but no longer extends into the dayside. Considerable observational evidence exists, suggesting that the ionospheric disturbance dynamo has an appreciable influence on storm-time ionospheric electric fields at middle and low latitudes.

The ionospheric disturbance dynamo

The paper presents some results of backscatter observations of the F region irregularities made with the large 50-MHz radar at Jicamarca, Peru, during a few days of observations. The results were obtained by using three observational techniques: the modified range-time-intensity technique, the digital power mapping technique, and the digital raw data recording technique. Backscatter intensity maps as a function of altitude and time are presented, which can be interpreted as radar pictures of F region irregularities. A classification of spread F spectral signatures resulting from approximately 30,000 spectra obtained in sets of 64 simultaneous heights under a variety of conditions is also given.

Radar observations of F region equatorial irregularities

The phenomenon of scintillation of radio waves propagating through the ionosphere is reviewed in this paper. The emphasis is on propagational aspects, including both theoretical and experimental results. The review opens with a discussion of the motivation for stochastic formulation of the problem. Based on measurements from in-situ, radar, and propagation experiments, ionospheric irregularities ate found to be characterized, in general, by a power-law spectrum. While earlier measurements indicated a spectral index of about 4, there is recent evidence showing that the index may vary with the strength of the irregularity and possibly a two-component spectrum may exist with different spectral indices for large and small structures. Several scintillation theories including the Phase Screen, Rytov, and Parabolic Equation Method (PEM) are discussed next. Statistical parameters of the signal such as the average signal, scintillation index, rms phase fluctuations, correlation functions, power spectra, distributions, etc., are investigated. Effects of multiple scattering are discussed. Experimental results concerning irregularity structures and signal statics are presented. These results are compared with theoretical predictions. The agreements are shown to be satisfactory in a large measure. Next, the temporal behavior of a transionospheric radio signal is studied in terms of a two-frequency mutual coherence function and the temporal moments. Results including numerical simulations are discussed. Finally, some future efforts in ionospheric scintillation studies in the areas of transionospheric communication and space- and geophysics are recommended.

Radio wave scintillations in the ionosphere

A theory of the two-stream ion wave instability in a plasma is developed that takes into account both the effect of collisions of the ions and electrons with neutral particles and the presence of a uniform magnetic field. Applying the results to the ionosphere, it is found that irregularities of ionization density should arise spontaneously in regions in which a sufficiently strong current is flowing normal to the magnetic field lines. These irregularities will be strongly aligned with the magnetic field and may have a wide range of wavelengths. The various predictions of the theory are in agreement with the observed characteristics of certain field-aligned irregularities found in the equatorial ionosphere that are associated with the equatorial electrojet. Similar irregularities often appear in the polar ionosphere during auroral displays; it seems very likely that these are caused by the auroral electrojet. (auth)

A plasma instability resulting in field‐aligned irregularities in the ionosphere

1. Introduction.- 1.1 Data on the Structure of the Ionosphere.- 1.2 Features of Nonlinear Phenomena in the Ionosphere.- 1.2.1. Nonlinearity Mechanisms.- 1.2.2. Qualitative Character of Nonlinear Phenomena.- 1.2.3. Brief Historical Review.- 2. Plasma Kinetics in an Alternating Electric Field.- 2.1. Homogeneous Alternating Field in a Plasma (Elementary Theory).- 2.1.1.Electron Current-Electronic Conductivity and Dielectric Constant.- 2.1.2.Electron Temperature.- 2.1.3.Ion Current-Heating of Electrons and Ions.- 2.2. The Kinetic Equation.- 2.2.1. Simplification of the Kinetic Equation for Electrons.- 2.2.2. Transformation of the Electron Collision Integral.- 2.2.3. Inelastic Collisions.- 2.3. Electron Distribution Function.- 2.3.1. Strongly Ionized Plasma.- 2.3.2. Weakly Ionized Plasma.- 2.3.3. Arbitrary Degree of lonization-Concerning the Elementary Theory.- 2.4. Ion Distribution Function.- 2.4.1. Simplification of the Kinetic Equation.- 2.4.2. Distribution Function.- 2.4.3. Ion Temperature, Ion Current.- 2.5. Action of Radio Waves on the Ionosphere.- 2.5.1. lonization Balance in the Ionosphere.- 2.5.2. Effective Frequency of Electron and Ion Collisions-Fraction of Lost Energy.- 2.5.3. Electron and Ion Temperatures in the Ionosphere.- 2.5.4. Heating of the Ionosphere in an Alternating Electric Field.- 2.5.5.Perturbations of the Electron and Ion Concentrations.- 2.5.6. Artificial lonization of the Ionosphere-Heating of Neutral Gas.- 3. Self-Action of Plane Radio Waves.- 3.1. Simplification of Initial Equations.- 3.1.1. Nonlinear Wave Equation.- 3.1.2. Nonlinear Geometrical Optics of a Plane Wave.- 3.2. Effect of Nonlinearity on the Amplitude and Phase of the Wave.- 3.2.1. Self-Action of a Weak Wave.- 3.2.2. Self-Action of a Strong Wave.- 3.2.3. Self-Action of Waves in the Case of Artificial lionization.- 3.3. Change of Wave Modulation.- 3.3.1. Weak Wave.- 3.3.2. Change of Amplitude Modulation of Strong Wave.- 3.3.3. Phase Modulation.- 3.3.4. Nonlinear Distortion of Pulse Waveform.- 3.4. Generation of Harmonic Waves and Nonlinear Detection.- 3.4.1. Frequency Tripling.- 3.4.2. Nonlinear Detection.- 3.5. Self-Action of Radio Waves in the Lower Ionosphere.- 4. Interaction of Plane Radio Waves.- 4.1. Cross Modulation.- 4.1.1. Weak Waves.- 4.1.2. Strong Perturbing Wave.- 4.1.3. Resonance Effects near the Electron Gyrofrequency.- 4.2. Interaction of Unmodulated Waves.- 4.2.1. Interaction of Short Pulses.- 4.2.2. Change in the Absorption of a Wave Propagating in a Perturbed Plasma Region.- 4.2.3. Generation of Waves with Combination Frequencies.- 4.3. Radio Wave Interaction in the Lower Ionosphere.- 4.3.1. Cross Modulation.- 4.3.2. Fejer's Method.- 4.3.3. Nonstationary Processes in the Interaction of Strong Radio Waves.- 5. Self-Action and Interaction of Radio Waves in an Inhomogeneous Plasma.- 5.1. Inhomogeneous Electric Field in a Plasma.- 5.1.1. Fundamental Equations.- 5.1.2. Distribution of Density and Temperatures in Plasma.- 5.2. Kinetics of Inhomogeneous Plasma.- 5.2.1. Kinetic Coefficients. Elementary Theory.- 5.2.2. Kinetic Theory.- 5.2.3. Fully Ionized Plasma.- 5.3. Modification of the F Region of the Ionosphere by Radio Waves.- 5.3.1. Modification of the Electron Temperature and of the Plasma Concentration.- 5.3.2. Radio Wave Reflection Region.- 5.3.3. Growth and Relaxation of the Perturbations.- 5.4. Focusing and Defocusing of Radio Wave Beams.- 5.4.1. Nonlinear Geometrical Optics.- 5.4.2. Defocusing of Narrow Beams.- 5.4.3. Mutual Defocusing.- 5.4.4. Thermal Focusing in the Lower Ionosphere.- 6. Excitation of Ionosphere Instability.- 6.1. Self-Focusing Instability.- 6.1.1. Spatial Instability of a Homogeneous Plasma.- 6.1.2. Instability in the Wave-Reflection Region.- 6.2. Resonant Absorption and Resonance Instability.- 6.2.1. Langmuir Oscillations in an Inhomogeneous Plasma.- 6.2.2. Excitation of Plasma Waves.- 6.2.3. Resonance Instability.- 6.2.4. Absorption of Ordinary Radio Waves.- 6.3. Parametric Instability.- 6.3.1. Langmuir Oscillations of a Plasma in an Alternating Field.- 6.3.2. Parametric Excitation of Langmuir Oscillations.- 6.3.3. Parametric Instability in the Ionosphere.- 6.3.4. Dissipative Parametric Instability.

Nonlinear phenomena in the ionosphere

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)

Field-aligned E-region irregularities identified with acoustic plasma waves

Although the two-stream instability theory, discussed by Farley, has been quite successful in accounting for many observed features of the plasma-wave irregularities produced in the equatorial electrojet, increased equipmental sensitivity and improved experimental techniques of observing the power spectrum of radio echoes from the electrojet have resulted in the resolution of weaker irregularities not predicted by that theory. In particular, such irregularities are observed both when a VHF radar located near the magnetic equator is directed vertically and when the radar is directed obliquely in the equatorial plane. In the latter case, the irregularities have lower Doppler shifts than are suggested by the theory. Experimental spectra are presented that demonstrate features consistent with the first-order two-stream theory for observations of a well-developed electrojet at oblique angles of incidence. However, the relative contribution of the weaker irregularities can be resolved in spectra for incidence angles approaching the zenith or, when the electrojet is comparatively weak, at most angles. The spectral width obtained at vertical incidence is shown to be narrow for a weak electrojet and to be broadened as the electrojet becomes stronger. The weaker irregularities probably result from the nonlinear interaction among the stronger irregularities, as is demonstrated by Dougherty and Farley in a companion paper. Experimental results are presented that indicate an asymmetry not predicted by either theory; i.e. the upward irregularities (observed at angles of incidence west of the zenith) are stronger than the downward irregularities (observed at angles of incidence east of the zenith). This asymmetry may be accounted for by the variation with height of the physical properties of the region of instability.

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Secondary Irregularities in the Equatorial Electrojet

During the International Geophysical Year, under the joint auspices of the United States National Committee for the IGY, the Voice of America, and the National Bureau of Standards, we operated a chain of 50 Mc/s VHF forward scatter circuits near the magnetic equator in South America [Bowles and Cohen, 1957]. An intense mode of VHF propagation associated with equatorial sporadic E was found to occur and was demonstrated to be closely identifiable in time-variation and height with the equatorial electrojet. (A brief account of these findings was published by Gates [1959] and a more comprehensive report will be submitted shortly for publication.) As a result of Stanford University IGY backscatter studies at Huancayo, Egan (Stanford University Radio Propagation Laboratory Tech. Rept. 1, December 30, 1959, unpublished) confirmed our expectation [Bowles and Cohen, 1957] that the radio echoes from irregularities in the vicinity of the equatorial E region exhibit a magnetic-field related aspect sensitivity similar to that of radio echoes from auroral ionization.

Radio echoes from field‐aligned ionization above the magnetic equator and their resemblance to auroral echoes

The field-aligned irregularities responsible for the ‘equatorial sporadic E’ configuration on equatorial ionograms have been established to be plane waves of electron density aligned with the magnetic field but moving perpendicularly to it. It has been generally realized that there is some connection between these irregularities and the equatorial electrojet. This paper demonstrates that the irregularities are spatially and temporally correlated with the equatorial electrojet. First, it is shown experimentally that the irregularities occur in the height and latitude region in which the electrojet flows; i.e., they are embedded in the electrojet. Second, it is shown that there are temporal correlations between the intensity of radio waves scattered from the irregularities associated with the electrojet and the horizontal intensity of the earth's magnetic field, a measure of the electrojet current. The scattered intensity is imperceptible until a certain ‘threshold’ of electrojet current is reached. As the electrojet current increases above the threshold, a scatter signal suddenly rises above the noise, increasing abruptly at first, then gradually. This threshold suggests that the plasma wave irregularities are generated by the current stream. According to the theory of Farley such a threshold can be interpreted as the onset of a plasma instability in the equatorial electrojet, leading to the formation of plasma waves. It is shown that by means of the intensity of the signal scattered from the electrojet irregularities, together with magnetometer measurements,it may be possible to estimate from ground observations the daytime field variation resulting from disturbance currents at great distances from the earth.

The association of plane-wave electron-density irregularities with the equatorial electrojet

Ionospheric propagation via scattering from the F region was sought at 50 Mc/s over a transequatorial path (with midpoint near Huancayo, Peru) employing a 2580-km transmitter-receiver separation. Propagation via F scatter was present over this path about 10 per cent of the time, though only at night. A condition closely related to the occurrence of F scatter was the presence of equatorial spread-F configurations on the Huancayo ionograms. The height of the propagation medium supporting this F scatter was usually identifiable with the lowest height of the associated equatorial spread F on the ionograms. On the basis of an interpretation of experimental results, equatorial spread F is shown to arise from scattering by relatively thin sheets of irregularities in electron density which occur at the bottom of the F layer or as much as 100 km lower. The thickness of these patches is estimated at the order of 50 km. The scattering irregularities comprising these patches are found to be elongated along the earth's magnetic lines of force. Electron irregularities of scale 10 meters or smaller measured in at least one dimension transverse to the magnetic field lines, and 1000 meters or longer measured along the magnetic field lines, are shown to exist as high as 450 km in the nocturnal equatorial F region during equatorial spread-F conditions. Equatorial spread-F echoes observed at Huancayo are demonstrated to be arriving at all elevation angles in the magnetic east-west plane. The geographical extension of a given scattering sheet in the magnetic east-west direction was at times as great as 1000 km. An observational procedure is suggested for distinguishing two fundamental varieties of spread-F echoes appearing on the equatorial ionograms. A necessary condition, that the contours of mean electron density be parallel to the magnetic lines of force, is proposed as a controlling factor for the occurrence of spread F in the equatorial ionosphere.

Robert Cohen

Papers

Field-aligned E-region irregularities identified with acoustic plasma waves

Secondary Irregularities in the Equatorial Electrojet

Radio echoes from field‐aligned ionization above the magnetic equator and their resemblance to auroral echoes

The association of plane-wave electron-density irregularities with the equatorial electrojet

On the nature of equatorial spread F