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The predictability of those ionospheric parameters relevant to ionosphere-reflected communications is considered along with their optimum utilization. Several excellent original articles and review papers which have been published from time to time dealing with the long term and short term forecasting of ionospheric parameters, radio systems, and modelling needs for ionospheric communications, are covered.
The major properties of polar ionospheric main anomalous events are summarized. The monitoring of large scale features of the ionization distribution that are the projections of large scale structural characteristics of magnetospheric plasma on the upper ionosphere is suggested as a basic principle of polar ionospheric condition forecasting. It is concluded that the processes of the magnetosphere/ionosphere interaction appear to play a predominant role in the creation of the polar ionosphere.
The theoretical distribution of helium ions in the ionosphere is studied as a function of such ionospheric parameters as the remaining ion composition and distribution, temperature, magnetic field topology, and ionospheric dynamics. An attempt is made to verify the theoretical predictions of the H(+) distribution in the ionosphere on the basis of observations of 304 A radiation resonantly scattered from He(+).
An important form of coupling between the magnetosphere and the ionosphere occurs through acceleration mechanisms operative in the high altitude ionosphere on magnetic field lines connecting to the auroral zone. Energetic ion mass spectrometer data from within these auroral acceleration regions are presented to illustrate the characteristics of the mechanisms. Observations of ionospheric plasmas in the ring current, the distant plasma sheet, and the magnetotail lobes are shown illustrating the extent of their circulation and the importance of their contribution to the plasma in each regime. Finally the precipitating plasmas in the auroral region and the extent and peculiar effects of the 0(+) component of that precipitation on the ionosphere are illustrated.
The paper deals with radio occultation measurements of Saturn's ionosphere and upper neutral atmosphere, made by Pioneer 11 near the terminator at latitudes of 9.7 deg south and 11.6 deg south. The principal electron density peak (of about 11,400 cu cm), in the ionosphere occurred at an altitude of about 1800 km, with a sharp lower peak of about 9000 cu cm at 1200 km. The scale height above the main peak corresponds to an exosphere temperature of about 1150 K for an H(+) ionosphere. Ionization appears to extend to 30,000 km. The low density of the lower portion of the ionosphere may be explained by ring shadowing and equatorial anomaly. In the neutral atmosphere, measurements were made to a pressure level of about 180 mbar, showing a temperature inversion region with a triple minimum.
The in-situ measurements of the global composition and Venus ionosphere dynamics recorded by the Bennett ion mass spectrometer on the Pioneer Venus orbiter during Dec. 1978-Aug. 1979 are presented. The observations of three plasma regimes show the bowshock-ionosheath region, the thermal ionosphere, and a superthermal flowing ion layer contacting the ionosphere at the ionopause and extending outward to different heights above the planet. An abundant ionosphere dominated by O(+) above 200 km and by O2(+) down to the typical periapsis altitudes of 160 km occur during quiet periods; less disturbed data shows strong day to night changes in the distributions of ions including O(+), O2(+), CO2(+), and N(+). The ionopause is located near the subpolar point at 250-400 km; under disturbed nighttime conditions it may have randomly spaced concentration gradients in the dusk region.
It is demonstrated that the horizontal ionospheric particle pressure gradient across the Venus terminator is the principal body force accelerating the plasm to the observed anti-solar velocity. The large scale horizontal electromagnetic body force is typically an order of magnitude smaller than the particle pressure gradient. The viscous body forces above 300 km - drag or acceleration - are also an order of magnitude smaller than the pressure gradient body force. In the immediate vicinity of the ionopause where the ionospheric plasma density decreases below 1000 ions/cu cm and the magnetic field strength increases, the electromagnetic body force may become significant. The electromagnetic body force may also be significant in the nightside ionosphere. The bulk of the ionospheric flow momentum is not derived from the ionosheath momentum.
Electrical induction in Io's ionosphere, due to the corotating plasma bound to the Jovian magnetosphere, is one possible source for the attainment of the high temperatures suggested by the large scale height of Io's ionosphere. Unipolar induction models are constructed to calculate ionospheric joule heating numerically, whose heating rates lie between 10 to the -9th and 10 to the -8th W/cu m. The binding and coupling of the ionosphere is due to the dense, and possibly ionized, neutral SO2 atmosphere, and there appears to be no need to postulate the existence of an intrinsic Ionian magnetic field in order to retain the observed ionnosphere.
High frequency radiowaves incident on an overdense (i.e., HF-frequency penetration frequency) ionosphere produce electron density irregularities. The effect of such ionospheric irregularities on the phase of UHF-radiowaves was determined. For that purpose the phase of radiowaves originating from celestial radio sources was observed with two antennas. The radiosources were chosen such that the line of sight to at least one of the antennas (usually both) passed through the modified volume of the ionosphere. Observations at 430 MHz and at 2380 MHz indicate that natural irregularities have a much stronger effect on the UHF phase fluctuations than the HF-induced irregularities for presently achieved HF-power densities of 20-80 uW/sq m. It is not clear whether some of the effects observed are the result of HF-modification of the ionosphere. Upper limits on the phase perturbations produced by HF-modification are 10 deg at 2380 MHz and 80 deg at 430 MHz.
The structure of the Venus ionosphere and the major processes occurring within it are summarized. The daytime ionosphere is created by solar EUV radiation incident on the thermosphere; it is in photochemical equilibrium near its peak at about 142 km, where O2(+) is the major ion, and near diffusive equilibrium in its upper regions, where the major ion is O(+). The day-to-night plasma pressure gradient across the terminator drives a nightward ion flow which, together with electron precipitation, contributes to the formation of the nighttime ionosphere. Large-scale radial holes or plasma depletions extending downwards to nearly the ionization peak in the antisolar region are also observed which are associated with regions of strong radial magnetic fields. The ionopause is a highly dynamic and complex surface, extending from an average altitude of 290 km at the subsolar point to about 1000 km at the terminator and from 200 to over 3000 km on the nightside. A variety of solar wind interaction products are observed in the mantle, a transition region between the ionospheric plasma and the flowing shocked solar wind.
Descriptions are given of the Faraday rotation technique used by Seasat to measure the ionosphere and the scheme employed in mapping the measurements to the spacecraft location, exploiting the fact that the effect of the ionosphere on signal speed, and therefore on Seasat radar altimeter measurements, is directly proportional to the columnar electron content of the ionosphere. The altimeter ionosphere correction is evaluated through comparison with independent methods, and it is demonstrated that the correction, whose total value can be on the order of 20 cm, is accurate to the 3-5 cm level.
Instruments on the Pioneer Venus Orbiter have detected a substantial ionosphere on the nightside of Venus during most orbits. However, during some orbits the nightside ionosphere seems to have almost disappeared, existing only as irregular patches of low-density plasma. The solar wind dynamic pressure on these occasions is greater than average. Data from several instruments (Langmuir probe, ion mass spectrometer, retarding potential analyzer, magnetometer, and plasma analyzer) have been correlated for a number of orbits during which the nightside ionosphere had disappeared. The magnetic field tends to be coherent, horizontal, and larger than usual, and the electron and ion temperatures are much larger than they usually are on the nightside. Mechanisms are suggested which might explain the reasons for the disappearance of the ionosphere when the solar wind dynamic pressure is large.
The diurnal modulation of the dust ring current of Saturn's D-ring causes field-aligned Birkeland currents to flow near the dawn and dusk terminators and close across the midlatitude ionosphere. One consequence of this current system is the establishment of a global convection pattern in the equatorial outer ionosphere. Outward motion of the dayside ionospheric plasma as well as the corresponding absorption effect of the inner ring system might be one physical cause of the depletion of the ionospheric content of Saturn.
An assessment is undertaken of current understanding of the physical and chemical processes that control Venus's ionospheric behavior, in view of the data that has been made available by the Venera and Pioneer Venus missions. Attention is given to the theoretical framework used in general planetary ionosphere studies, especially to the equations describing the controlling physical and chemical processes, and to the current status of the ion composition, density and thermal structure models developed to reproduce observed ionospheric behavior. No truly comprehensive and successful model of the nightside ionosphere has been published. Furthermore, although dayside energy balance calculations yield electron and ion temperature values that are in close agreement with measured values, the energetics of the night side eludes understanding.
While most orbits of the Pioneer Venus Orbiter have indicated a substantial nightside ionosphere, this region virtually disappears, existing only as irregular patches of low density plasma, on those orbits during which the solar wind dynamic pressure is large. There observational results are presently interpreted by means of a two-dimensional theoretical model of the Venus ionosphere in which empirical horizontal velocities are adopted. The degree to which the horizontal transport of ions from day to night can maintain the nightside ionosphere is shown to depend on the flow velocities, together with the ionopause height at the terminator. Attention is also given to the role played by electron precipitation in the support of a nightside ionosphere. Indirect evidence is given for an enhanced deuterium/hydrogen ratio on Venus.
It is pointed out that interplanetary navigation at the Jet Propulsion Laboratory (JPL) is performed by analyzing measurements derived from the radio link between spacecraft and earth and, near the target, onboard optical measurements. For precise navigation, corrections for ionospheric effects must be applied, because the earth's ionosphere degrades the accuracy of the radiometric data. These corrections are based on ionospheric total electron content (TEC) determinations. The determinations are based on the measurement of the Faraday rotation of linearly polarized VHF signals from geostationary satellites. Problems arise in connection with the steadily declining number of satellites which are suitable for Faraday rotation measurements. For this reason, alternate methods of determining ionospheric electron content are being explored. One promising method involves the use of satellites of the NAVSTAR Global Positioning System (GPS). The results of a comparative study regarding this method are encouraging.
The instantaneous spatial distribution of auroral emissions is observed with auroral imaging photometers on board the spacecraft Dynamics Explorer 1 (DE 1) as ground-based meridian chains of magnetometers simultaneously detect the magnetic signatures of ionospheric and field-aligned currents flowing at northern polar latitudes. Ionospheric conductivities at nighttime auroral latitudes are estimated from the measured auroral luminosities and used with the measured polar magnetic variations to compute model distributions of ionospheric and field-aligned currents. Temporal resolution for the coordinated observations and model calculations is 12 minutes. Model ionospheric and field-aligned current distributions are overlayed on global auroral images to illustrate spatial relations on a global scale at the maximum epoch of an auroral substorm. Eccentric-dipole-latitude and magnetic-local-time coordinates are used. A model field-aligned current distribution is compared quantitatively with the distribution of field-aligned currents inferred from simultaneous observations by the DE-2 magnetometer.
The columnar electron content of the ionosphere between a spacecraft and a receiver is measured in realtime by cross correlating two coherently modulated signals transmitted at different frequencies (L1,L2) from the spacecraft to the receiver using a cross correlator. The time difference of arrival of the modulated signals is proportional to electron content of the ionosphere. A variable delay is adjusted relative to a fixed delay in the respective channels (L1,L2) to produce a maximum at the cross correlator output. The difference in delay required to produce this maximum is a measure of the columnar electron content of the ionosphere. A plurality of monitoring stations and spacecraft (Global Positioning System satellites) are employed to locate any terrestrial event that produces an ionospheric disturbance.