Topside Ionograms Research Articles

Topside sounder studies of corpuscular radiation effects on the ionosphere during quiet and disturbed conditions

Studies of electron scale heights obtained from topside ionograms lead to the conclusion that energetic particles do not usually affect the ionospheric plasma temperature at heights of 400 or 500 km. During magnetic disturbances, however, the electron scale heights in the region of the South Atlantic magnetic anomaly, but not at other longitudes, appear to be abnormal, possibly as the result of particle dumping. It is suggested that some of the localized ionization abnormalities occurring in the South Atlantic area at certain L-values may be caused by particles dumped there and evidence of a possible association between spread- F ionization and energetic particles is presented. Unfortunately no direct proof exists of the reality of the particle effects discussed.

Comparison of irregular features appearing on ionograms recorded by topside and ground-based sounders

The simultaneous occurrence of irregularities in the topside ionosphere and in the region below h max has been examined using Alouette I ionograms and records from ground based ionosondes. There is good agreement between the occurrence of travelling disturbances on ground based records and refractive irregularities occurring on topside ionograms. Spread- F was found to occur more frequently above h max .

Magnetic field-aligned irregularities at mid-geomagnetic latitudes

The conditions are examined under which radio waves may be trapped and guided along magnetic field lines by irregularities in electron density. From the analysis of topside ionograms recorded by the Alouette I satellite, it has been found that field-aligned irregularities can exist in the form of sheets which may be up to 30 km in extent in the E-W direction. The results of ray tracing using simple models for irregularities are also presented.

Topside Refractive Irregularities and Travelling Ionospheric Disturbances

Munro (1950) deduced from ground-based observations that traveling ionospheric disturbances (T.LD.'s) are large-scale distortions of the electron density contours, which travel through the ionosphere, and Calvert and Schmid (1964) attributed extra traces and distortions of the main echo traces, which occur near the critical frequency of topside ionograms, to refractive effects associated with large-scale irregularities. In an examination of topside and sub-peak ionograms recorded close in position and time, it was found that on a number of occasions refractive effects occurred on topside ionograms, when manifestations of T.LD.'s were observed on sub-peak ionograms (Dyson 1967b).

ELECTRON DENSITIES OF 8–100 ELECTRONS CM−3 DEDUCED FROM ALOUETTE II HIGH-LATITUDE IONOGRAMS

Alouette II topside ionograms recorded at high latitudes (L ≈ 6 or greater) and high altitudes (1 500–3 000 km) show an interference pattern between the electron gyro and the electron hybrid resonance spikes, when the electron density at the satellite is very low.This interference pattern can be used to determine the electron number density at the satellite, to an accuracy of better than 10% when the electron density is less than about 100 electrons cm−3. Preliminary results show that electron densities less than 30 electrons cm−3 occur over the region north of about 60 °N. geomagnetic latitude and that the southern boundary of the region where the very low electron densities are observed shifts south by about 8° between noon and midnight.

On the Topside Ionosphere Over the American Continents

Preliminary results of an extensive investigation of the topside ionosphere over the North and South American continents are presented. The required data are obtained through the reduction and analysis of a large number of Alouette I topside ionograms. Illustrative examples of typical latitudinal variations (from 80° N to 80° S geographic latitudes or 90° N to 60° S dip latitudes) of electron density (N) and corresponding scale height (Hu) are presented. The characteristic features of equatorial, midlatitude and polar ionospheres are described. Assuming the topside ionosphere is in a state of diffusive equilibrium above 500 km, the scale height of electron distribution along magnetic field lines (Hs) is calculated. The physical significance and interpretation of the computed scale height, (Hs), curves are discussed.

A NOVEL IONOSPHERIC CYCLOTRON RESONANCE PHENOMENON OBSERVED ON ALOUETTE I DATA

Characteristics of an extraordinary wave trace recently identified on Alouette I topside ionograms, and called the remote resonance trace by Hagg (1966), are discussed in detail. From the observations it has been deduced that the radio energy associated with the production of this trace propagates along an ionospheric magnetic field-aligned wave guide. The electric field of an h.f. wave propagating along such a guide would have a gradient normal to the earth's magnetic field, and because of this gradient the wave could excite an electron–cyclotron resonance in a region of the ionosphere that includes the height where the wave frequency is equal to twice the electron gyrofrequency. At some time delay after the wave has passed through the resonant region, the resonating electrons generate an extraordinary wave, which propagates upward along the guide and is subsequently received by the satellite. This wave is responsible for the production of the remote resonance trace. The equation for the time delay is given and the resonance trace computed, using this equation, is shown to be in good agreement with the observed resonance trace for a specific ionogram.

Remote Cyclotron Resonance Phenomenon observed by the Alouette Satellite

A NOVEL cyclotron resonance phenomenon associated with the second harmonic of the electron gyrofrequency 2fH is sometimes observed on Alouette topside ionograms. The phenomenon occurs remote from the satellite and will consequently be called the ‘remote resonance’. The cyclotron resonances which occur in the vicinity of the satellite, and which are responsible for the cyclotron spikes, have been discussed by previous authors1,2.

Structure and variation of the topside ionosphere close to the Fort Monmouth longitude

Ionograms of the topside ionosphere obtained from the Alouette satellite during a period of more than a year have been reduced to ‘real height’ profiles. Data obtained between September 1963 and January 1964 have been studied in detail. The effective radio horizon of Fort Monmouth enables a study of the upper ionosphere between 15° and 65°N geographic latitude and 50° and 100°W longitude. Curves of plasma frequencies and of electron density at fixed altitudes have been prepared as a function of latitude, longitude, and time of day. Special attention has been given to the effects of magnetic disturbances as revealed on the contours. During the observed magnetic storms, the measured altitudes for specific plasma frequencies were decreased by more than 200 km. Another parameter, the integrated electron content in the topside ionosphere, has been compared with ƒoF2. The assistance of five bottomside sounding stations located within the radio horizon of Fort Monmouth and computer programs for reducing topside and bottomside ionograms to real height profiles made it possible to study the difficulties in matching topside and bottomside profiles. Many topside and bottomside profiles have been found to match closely when the observed ƒoF2 values of topside and bottomside ionograms differ by not more than 0.3 Mc/s. Matching was observed for differences of 1° to 2° of the geographic positions of the ground station and the subsatellite point. Discrepancies on topside and bottomside ionograms (different geographic locations, nonmatching ƒoF2) are exhibited.

The reduction of ionograms from the bottomside and topside

A method is discussed for reducing ionograms to electron density height profiles on a modern digital computer by a representation in terms of parabolic laminations. This provides good accuracy without an excessive number of scaled points. Since no precomputed coefficients are used, ionograms may be scaled at any frequency and from any location. It is shown how a least squares joint ordinary and extraordinary analysis enable a correction to be made for underlying ionization, and reduces sensitivity to scaling errors. Some special features of topside ionograms are discussed. Formulation in terms of segments parabolic in log (plasma frequency) is especially efficient in terms of matching layer shape without discontinuity of slope. Techniques for incorporating the variation of gyrofrequency with altitude are discussed. It is particularly emphasized that significant errors in the deduced electron scale height can be made if this is not done adequately.

On the harmonics of the gyrofrequency observed on topside ionograms

The plasma resonances appearing on topside ionograms at the plasma frequency, at the ‘hybrid frequency’ ωM and the harmonic 2ωM, and at the gyrofrequency ωH and the harmonics p · ωH are discussed on the basis of the kinetic theory (linearized Vlasov equation), including the thermal motion of the electrons. The results are compared with those of the dispersion relation for a cold plasma (magnetoionic theory).

Studies of planetary atmospheres: 1. The distribution of electrons and ions in the Earth's exosphere

The factors which govern the distribution of electrons and ions in a planet's exosphere under diffusive equilibrium are discussed. The theory takes into account the effect of the electric field that arises from charge separation, the centrifugal force arising from the rotation of the planet, and the effect of the planet's gravitational field. It is assumed that the charged particles are constrained to move only along the direction of the planet's magnetic lines of force. The modifications that result in the electron and ion distributions when a temperature variation is assumed along a line of force are also considered. The results predicted by the theory are compared with actual experimental observations of the electron density distribution in the earth's exospheric plasma which have been obtained in recent years from whistler data and from topside ionograms made by the Alouette satellite.

Spread-Fobservations by the Alouette Topside Sounder Satellite

Ionograms recorded by the Alouette topside sounder satellite exhibit a variety of spread-F echo configurations. The observed echoes are found to be consistent with three qualitative radio-propagation models involving aspect-sensitive scattering by thin magnetic-field-aligned irregularities, ducting along broad irregularities, and refraction within large-scale reductions of electron density. A large number of Alouette ionograms were examined for the three types of spread F to determine their frequency of occurrence. Spread F appeared on 54 per cent of the topside ionograms; the three types exhibit markedly different patterns of occurrence in geomagnetic latitude and local time.