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The current collection of magnetospheric observations consists almost entirely of in situ measurements at isolated spacecraft positions. A novel remote sensing technique for simultaneously imaging the plasma density N and the magnetic field B is proposed. The imaged region of the magnetosphere can be several Earth radii in extent. Simultaneous measurements of both group delay and Faraday rotation of radio waves propagating through the magnetosphere yield both total electron content (TEC) and Faraday rotation angle, which is the integral of electron density weighted by some known function of the magnetic field and ray path. These data can then be inverted to obtain estimates of the three-dimensional spatial distribution of N and B. The geometry of the ray paths determines the number of components of B that can be resolved. Simple multisatellite simulations and common data processing techniques introduce this new application of tomography: Magnetospheric TEC measurements yield a map of N, which is then combined with Faraday rotation angle measurements to produce a two-component map of B in the plane of observation. The successful tomographic reconstructions provide a straightforward illustration of the practicality of using this technique to map the magnetospheric plasma density and magnetic field in two or three dimensions. Furthermore, simultaneous images of N and B are of great topical interest to geospace science because of their direct and immediate relevance to current questions regarding magnetospheric structure and dynamics.

An extensive series of incoherent scatter studies of the ionospheric D-region was carried out at the Arecibo radar facility during 1978 and 1979. They included several full-day sequences of electron density measurements over a range of altitudes, and also included a sequence during the serendipitous occurrence of a large solar flare. For the solar flare event simultaneous data on solar X-ray fluxes in several wavelength bands were available from the GOES-2 and ISEE-3 satellites.

Measurements of electron density at seven D region altitudes were made with the Arecibo radar during a class X solar flare on August 18, 1979. Measurements of solar X ray fluxes during the same period were available from the GOES 2 satellite (0.5–4 A and 1–8 A) and from ISEE 3 (in four bands between 26 and 400 keV). From the X ray flux data we computed ionization rates in the D region and the associated chemical changes, using a coupled atmospheric chemistry and diffusion model (with 997 chemical reactions and 19 vertical levels). The computed electron densities matched the data quite well after we had adjusted the rate coefficients of three reactions, namely, O−2 + O2(1Δ) → e + O2 + O2, O+2 + O2 + M → O+4 + M, and O+4 + O → O+2 + O3. We also discuss the hierarchies among the many flare-induced chemical reactions in two altitude ranges within the D region and the possible effects of two separate isomeric forms of O+4.

The width of the spectra of signals incoherently scattered from 85 to 93 km at Arecibo shows significant variations during sunrise and sunset periods. After sunset, the width decreases substantially for all these heights, indicating a 20–30% decrease in the quotient of neutral temperature to neutral number density T N ). Some increase in effective positive ion mass could also be possible. Somewhat opposite situations are evidenced during sunrise for heights above 87.5 km. At 85 km, on the other hand, the width decreases during sunrise. This is due to the disappearance of negative ions, which were formed during the night hours.

Seasonally averaged quiet-day F region ionospheric E × B drift observations from the Millstone Hill, St. Santin, Arecibo, and Jicamarca incoherent scatter radars are used to produce a model of the middle and low-latitude electric field for solar minimum 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. This article contains supplementary material.

The Arecibo 430 MHz incoherent scatter radar was used to observe the diurnal variation of electron concentration in the 6–100 km altitude region on 14 August 1977. This report is an evaluation of the technique and includes a fairly complete discussion of errors involved. Although interference remains a serious problem, the results are useful down to about 60 km altitude and a minimum density of about 50 electrons cm−3. Characteristic statistical plus systematic errors indicate that an observed 100 electrons cm−3 value actually lies between 50 and 180 electrons cm−3 assuming no interference. Observed variations of electron concentration include not only those due to basic solar control but also at least one wavelike feature characterized by phase shift with altitude. These results should prove particularly useful as constraints to time dependent models of the D-region chemistry.

For the first time a ground based technique, i.e. incoherent scatter observation of the mesospheric spectra has been utilized in identifying the ‘transition height’ of simple molecular ions to complex cluster ions around 80–90 km in the D-region. This transition height also matches with the electron density ledge. A substantial diurnal variation of this height is observed. The transition height varies by about 10km during the course of a day, the lowest being 80 km, near noon. There is also a strong likelihood that both the neutral temperature and effective positive ion mass vary during the course of a day.

The results obtained with ionospheric modification experiments over the three years preceding the XXI General Assembly of URSI in 1984 at Florence are reviewed. The topics discussed include weak electromagnetic sidebands observed using a single pump frequency, the HF-induced plasma line at 3.175 MHz and its similarity to the plasma lines observed using higher HF frequencies near Tromso, the HF-enhanced plasma line observed with the 50 MHz radar, the HF-induced plasma line with a doublehumped spectrum below threshold. HF-induced plasma line spectra with height discrimination using a new technique, the HF-induced plasma line and ion line spectra obtained with two pumps differing in frequency by a few kHz, narrow features such as the OTSI in the HF-enhanced plasma line and ion line spectra observed by a new technique, the use of such narrow features for measuring the line-of-sight electron drift velocity, the discovery of a radical qualitative change in the spectrum of the HF-induced plasma line as the HF power (CW) is increased or as the duty cycle is changed while pulsing, observations of the temporal development of the enhancement of the thermal plasma line at the peak of the F 2-layer by electrons accelerated during ionospheric heating studies of artificial density stratification resulting from the standing wave nature of the heating wave and strong electromagnetic sidebands generated by two Powerful HF radio waves differing from each other by some tens of Hz. Theoretical work on soliton formation and on VLF generation by HF heating is briefly mentioned, as well as experimental studies of self-focusing by observing the scintillation of extraterrestrial radio sources, direct conversion and studies of short scale field-aligned irregularities by VHF radar backscatter during ionospheric modification.

Incoherent scatter spectral observations at Arecibo have been extended down to 59 km. For the first time vertical velocities in the mesosphere have been observed using this technique. The velocities unambiguously reveal a wavelike structure growing with height and varying in time. Above around 75 km, the vertical wavelength is around 16 km and the time period about 4 hours ± 20 minutes. Vertical velocities near 90 km were as large as 1 ms−1 and the apparent vertical energy flux was reasonably constant above 75 km. Below 75 km the energy flux was substantially lower, suggesting a mechanism where the gravity waves acquire significant energy from the diurnal tidal component trapped below the mesopause. The estimated adiabatic density and temperature changes near 90 km, estimated using simple idealized theories, were around 11%. The observed changes in spectral width were also found to be in harmony with estimated density and temperature changes to within 25%.

Incoherent scatter observations of mesospheric spectra at Arecibo have shown the presence of zones of large negative ions in the altitude range 85–90 km. These large negative ion regions are somewhat sporadic in nature and are observed on roughly about one third of the evenings. On at least one occasion, somewhat similar observations have been made during the noon period.

When the ionosphere is illuminated with two strong HF signals, nonlinear mixing processes can take place near the region of reflection. The first direct experimental evidence of such processes occurring in ionospheric plasma are presented in this paper. The experiments were performed at Arecibo, where the HF ionospheric modifier was split to operate as two pure isolated and independent pumps over the frequency range of 3-8 MHz and the frequency difference between the two pumps was varied between a fraction of a Hz and several kHz. The purity of the trans- mitted signals was always ensured. Strong nonlinear effects were observed when the frequency difference was several Hz. Specifically, the HF signal from the ionosphere contained sidebands which were separated by multiples of the difference (Δf) between the two pumping frequencies. The first sidebands were typically 20-40 db below the pump signal. The strengths of the first sidebands increased, when the difference (Δf) between the two pumping frequencies decreased. Below about 5-10 Hz, the power in the first sidebands varied almost in Δf−n fashion, with n between 1 and 2. The power in the sidebands was also a strong nonlinear function of the incident pump field strength, showing the presence of a threshold. It is suggested that studies of such nonlinear mixing processes would be valuable both for diagnostic and practical usage of ionospheric plasma.

Plasma line observations obtained from incoherent radar backscatter have been used as a ground-based method for deriving information about the size and anisotropy of the ionospheric photoelectron fluxes. In the past, data interpretation has been confined to altitudes above the F2 peak. Measurements below the F2 peak consistently show an anisotropy in the ratio of the downshifted to upshifted amplitudes of 20–50% when it is generally assumed that diffusion processes dominate. We describe calculations of the plasma line intensity which use a multiangle multienergy calculation of the photoelectron distribution function. The calculated electron flux exhibits a small low-altitude low-energy anisotropy which is reflected in the plasma line measurements. Given anisotropic elastic electron-neutral cross sections, the flux anisotropy arises when the local mean free path is of the order of the local scale height. The net effect is conversion of a spatial density inhomogeneity into a velocity distribution anisotropy.

For the first time, vertical electron drift velocities in the F region of the ionosphere have been directly determined using the Doppler shift of the electron component of the incoherent-scatter spectra. Simultaneous measurements of ion drift velocities were also made. A large difference between the electron and ion velocities of the order of 100 ms−1 with corresponding currents of 10−5 Am−2 was observed at sunrise. The most likely source of these currents appears to be the action of the local thermospheric wind on the large conductivity gradients associated with the sunrise line.

Plasma drift measurements taken at Arecibo during the solar minimum period of 1974-1977 are examined to determine their average behavior in the E, F2, and F regions. The drifts are generally diurnal in the E region and semidiurnal in the F1 region. These lower thermospheric drifts are set up by polarization fields generated by propagating and in situ atmospheric tides. In the F region the diurnal component is more pronounced, especially in the zonal direction. The magnitude of the drifts is of the order of 25-30 m/s (or 1 mV/m). Enhanced geomagnetic activity appears to increase the westward component of the drift in agreement with the theory of the ionospheric disturbance dynamo (Blanc and Richmond, 1980). Nighttime drifts appear to be at least partly explained in terms of polarization fields.