A study of the nocturnal Flayer over Arecibo, Puerto Rico, reveals that substantial spatial variations in the height of the layer are frequently present. The variations are deduced from electron density profiles measured by the incoherent radar in which the radar beam is offset 17 ø in zenith and rotated continuously in azimuth. This scanning procedure shows that the ionosphere above Arecibo often has well-defined bands where the F region as a whole is alternately high and low. The transition between adjoining regions is very sharp. For example, electron density profiles taken at points only a few degrees apart in azimuth, which corresponds to a spatial separation of less than 10 kin, can show variations in hmax of up to 70 kin. It is suggested that these bands are manifestations of the so-called 'Perkins' instability.

Seasonally averaged quiet-day F region ionospheric E x B drift observations from the Millstone Hill, St. Santin, Arecibo, and Jicamarca incoherent scatter radars are used to produce a model of the middleand low-latitude electric field for solar m'mimum conditions. A function similar to an electrostatic potential is fitted to the data to provide model values continuous in latitude, longitude, time of day, and day of the year. This model is intended to serve as a reference standard for applications requiring global knowledge of the mean electric field or requiring information at some location removed from the observing radars. 1974; Kohl, 1976], and by influencing the magnitude of ion iations of the electric field, which are not incorporated into drag through redistribution of ionization [e.g., Anderson and Richmond's [1976] model. The present model thus supersedes Roble, 1974]. Knowledge of the global ionospheric electric that earlier one, field is also useful in providing an upper boundary condition on calculations of middle atmosphere electric fields such as those performed by Roble and Hays [1979]. Substantial collections of ionospheric electric field data exist for the incoherent scatter radar observatories at Millstone Hill, St. Santin, Arecibo, and Jicamarca. In spite of the large day-to-day variability of the electric fields, earlier analyses were able to determine clear average daily variations [e.g., Woodman, 1970, 1972; Evans, 1972; Carpenter and Kirchhojf,, 1974; Kirchhoff and Carpenter, 1976; Richmond, 1976; Blanc et al., 1977]. More recent analyses have quantified the seasonal DATA BASE Data are used from the four incoherent scatter radar stations listed in Table 1. These radars measure the ion velocity, whose component v perpendicular to the geomagnetic field B is related to the electric field E by v = E • B/B • (•) At each station, F region ionization drifts were typically measured on a few days each month within the interval indicated. Depending on the mode of operation, projections of v onto one or more station-dependent axes can be determined at a given time. The measured components of v are averaged in height to improve accuracy, a process justified by the expectation that E and v vary only slightly with altitude within the F region, owing to the near-equipotentiality of magnetic field lines and to the large-scale nature of the global electric field. The mean altitude of the measurements is roughly 300 kin. All stations except St. Santin determine vector velocities by combining line-of-sight velocities measured at different directions from the transmitter/receiver, corresponding to different volumes of the ionosphere. The assumption is then made that the ionospheric drift vectors are the same for all volumes of measurement from a particular station. For further information about the measurement techniques, see Paper number 80A0414.

Vector measurements of the ion transport velocity in the F region above Arecibo (18øN, 50 ø dip angle) are reported for two daytime and five nighttime periods. From these vector measurements both the electric fields and neutral winds iri the F region can be found. Daytime results indii•ate eastward electric fields during the morning, changing to westward near 1400. The most Outstanding feature of the nighttime results is a large (•3 mv/m) westward electric field at 0400. The overall diurnal electric field pattern suggests that an 8-hour tidal mode dominates the wind fields in the dynamo region. Daytime neutral wind measurements indicate the importance of ion drag in determining the neutral winds; the poleward winds are observed to be strongest at •1000, though, according to satellite drag data and evidence presented here, the N-S neutral pressure gradient maximizes in the afternoon. At night the neutral winds are first seen to blow strongly equatorward but then abate and often blow poleward by 0200. Application of the neutral wind and electric field measurements to F region morphology indicates that (1) during sunset strong downward ion transport is the result of poleward neutral winds; (2) the generally occurring (at Arecibo) sudden drop in the height of the F layer near midnight is the result of a reversing wind from equatorward to poleward; and (3) the rise of the F layer often seen at •0400 at Arecibo is the result of I• x B drifts. Finally, there is a strong tendency for the ion motion to be horizontal.Possible reasons for this behavior are discussed. Observatory (18øN) since November 1969. The ion velocity component along the magnetic field is related to the •'rne•idional neutral winds, whereas the components across the magnetic field are due to E g B drifts and thus are direct measurements of the ambient electric fields. Measurements of meridional neutral wirids at the incoherent scatter station of St. Santin have been reported by Vasseur [1969], while ambient E-W electric fields at Jicamarca have been reported by Woodman [1970]. A comprehensive review of ionospheric drift measurements made using incoherent scatter is given by Evans [1972b]. The Arecibo radar has the capability of measuring both winds and electric fields. Thus wind-field interactions can be studied directly. The relative contributions of the winds and the electric fields to various F region morphological phenomena can also be monitored. Since Arecibo is approximately midway in latitude between St. Santin or Millstone Hill and Jicamarca, the Arecibo radar can provide valuable information on the latitudinal variation of the meridional winds and the E-W electric fields. F region ion velocity measurements have been made at the Arecibo

A joint alert campaign was organized during the month of October 1980 by the incoherent scatter radars in the American sector: namely, Jicamarca, Arecibo, Millstone Hill, and Chatanika. The campaign, which met with success, was designed to study the behavior of the ionospheric electric field as a function of latitude during magnetically active conditions. The Arecibo data in this campaign support present and previous observations at Jicamarca that suggest that when the convection E field suddenly decreases, the Alfvén layer shielding field becomes unbalanced and penetrates the plasmasphere. While this type of observation is reasonably convincing, others are more difficult to categorize. We suggest that, beside the high‐latitude electric fields, time‐varying auroral conductivity models will have to be considered in order to understand the morphology of the low‐latitude E field disturbances. We present the first correlation analysis and determination of the amplitude ratio of the disturbed zonal electric field at 30° geometric latitude (Arecibo) to the field at 0° (Jicamarca). Other highlights of the paper are a discussion of DP2, which may help clarify the controversy surrounding it, and a discussion of the sensitivity of low‐ and mid‐latitude radars to disturbances of magnetospheric origin. We show that this sensitivity maximizes at the magnetic equator.

Incoherent scatter radar measure-neutral velocity vectors have equal components in ,, ments made in 1981 and 1982 are used to determine both directions perpendicular to B. the polarization field is fully developed with a well-developed polarization field so long as Ep ---uxB and j--0. At this stage the ion and electrostatic fields originating in the conjugate hemisphere are taken into account.

Self-focusing of high-frequency electromagnetic radiation is observed to produce largescale plasma striations in the ionosphere. Development of a new observational technique has allowed the first detailed study of the instability scale sizes and associated plasma movement. Experimental results are shown to support the theory of wave self-focusing through differential electron heatings Natural density fluctuations cause small variations in the index of refraction of a plasma, resulting in a slight focusing and defocusing of an electromagnetic wave as it propagates through the medium. The electric field intensity increases as the incident wave refracts into regions of comparatively underdense plasma. Ohmic heating 1 and the electric-field ponderomotive force 2 then drive plasma from these focused regions, amplifying the initial perturbation. This selffocusing instability continues until hydrodynamic equilibrium is reached, creating field-aligned striations within the plasma.The study of self-focusing waves in plasmas is motivated by its relevance to ionospheric modification research, 3 laser fusion-plasma heating, 4 and microwave-ionosphere interactions associated with solar-power satellite systems. 5 Development of a new diagnostic technique in conjunction with a recent ionospheric modification experiment has resulted in the first detailed observations of individual self-focused striations, as well as striation maps of the entire waveplasma interaction region. Measured striation scale sizes are in good agreement with the predictions of thermal self-focusing theory. Additional plasma effects can also be identified.Intense high-frequency (hf) electromagnetic radiation incident on an overdense ionospheric plasma is known to excite parametric instabilities, enhancing electron plasma oscillations observable by incoherent backscatter radar. 6 These instabilities continue to be the subject of intense experimental study. Of importance here is the fact that above instability threshold the strength of the enhanced plasma waves directly depends on the local power of the pump electric field. In addition, because of exact frequency and wavenumber matching conditions for both the parametric wave-plasma interaction and the radar incoherent backscatter process, these enhanced waves are detected at only one altitude. As a result, systematic scanning of the narrow radar beam across the interaction region of the enhanced plasma waves yields a two-dimensional cross-section characteristic of the local electric field intensity. These maps of electric field strength clearly show self-focusing striations and large-scale structuring of the illuminated plasma. Because the direction and rate of the radar scan are experimentally controlled, the cross-sectional dimensions of the individual striations are easily measured. Alternatively, if the radar is fixed, the irregularities follow a slow natural (EXB) drift through the beam, allowing a detailed study of the small-scale structure within individual striations. Once the irregularity size is d...

An observation scheme is described in which it is possible to measure the three velocity components of plasma drift in the ionosphere by using a single monostatic incoherent scatter facility with a steerable antenna. The scheme involves a least‐mean‐squares fitting of the line‐of‐sight velocity observed in several different directions to what one would compute from a given set of three orthogonal velocity components. The method is applied to data gathered at the Arecibo Observatory and is shown to be capable of providing velocity information with rms errors of better than 10 m sec−1 for all three components with a time resolution of 20 min.

Thomson radar returns from the ionosphere subjected to a step-function excitation of electromagnetic waves show the immediate rise of electrostatic waves near the resonant height oo p ^ co 0 . The observed secular temporal variation E <* t agrees with a theorybased on direct conversion of electromagnetic waves to electrostatic waves by preexisting density inhomogeneities. The rise time is at least an order of magnitude smaller than previously reported. The frequency spectrum shows this conversion is characterized by frequencies at the pump frequency co 0 .PACS 52.25.Ps, 52.35.Fp, 52.35.Hr There have been several ionosphere-modification experiments 1 in which an electrostatic wave spectrum consistent with parametric decay instabilities 2 has been observed. This process has been well documented in the laboratory 3 and consists of the decay of the incident electromagnetic (EM) wave of frequency co 0 and wave number 5 0 into electrostatic (ES) modes. The decay modes have short wavelength (compared to 27r/|5 0 l) and correspond to a Langmuir (o; z ,E z ) and an ion acoustic (wi^i) wave. The decay process has the following signatures: (1) matching conditions (x) 0 = oo l + a? i? ^0 = ^! +5^; (2) exponential growth in time, i.e., exp(yt); and (3) a power threshold must be exceeded, i.e., it depends nonlinearly on EM amplitude. In this Letter we report the observation of a new process which also gives rise to the excitation of ES modes in the ionosphere. The underlying physics differs from the parametric process and its existence has important consequences for the interpretation and planning of future ionosphere-modification experiments. In addition, the new process reported can be used as a diagnostic tool that samples the properties of preexisting density inhomogeneities in the ionosphere, as well as the local amplitude of the EM wave.The essential physics behind the process investigated, which we refer to as "direct conversion," consists of the resonant pumping of a Langmuir wave by the electric field of the EM wave. The resonant excitation occurs below the ES cutoff, i.e., at a height such that QJ P 0 ; and (4) the amplitude scales linearly with the EM field amplitude and does not exhibit a threshold. These properties are si...