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While Appleton (1946) originally linked the Equatorial Anomaly with the real magnetic inclination in the ionosphere the radio communication forecasters, misled by a name, replaced “dip” by “geomagnetic latitude”. As late as 1967 the Comite Consultatif International des Radiocommunications (CCIR) finally decided to replace that misleading mathematical construct by the modified dip (Modip) latitude which has an equator that is identical with the dip equator. In order to check the equatorial symmetry properties low and mid-latitude peak electron densities as given by CCIR or URSI's numerical maps were plotted against geomagnetic and Modip latitudes. For low and high solar activity NorthβSouth symmetry is found not around the geomagnetic equator but around the equator common to Modip and dip. This is true in the ionosphere including the position of both crests. Electron Content up to 1000 km, ECion, was determined with the International Reference Ionosphere (IRI) while the content of the plasmasphere (above 1000 km), Ecpl, was determined with an extension of IRI up to the plasmapause using the Standard Model of the Ionosphere (Chasovitin et al., 1998). Data from the ISSb topside sounder were compared with the IRI model. NorthβSouth symmetry was found for Ecpl (Ecion) with respect not to the dip equator but to the geomagnetic one. This finding reflects the fact that the magnetic field acts like a dipole at greater distances, but not near Earth. Since the plasmasphere contribution to the total electron content is small, symmetry appears not with geomagnetic but with Modip latitude.

Applying the fading method to field strength records of distant LF transmitters, the Collm observatory determines night by night a steady drift and a solar tide rotating once every 12 h, and also the apparent reflection height of these waves (found slightly above the mesopause). Distinguishing summer and winter, our statistical analysis using the ‘superposed epochs’ method reveals a remarkable influence of the lunar phase with amplitudes of up to 2 m·s−1, depending on the characteristic and the season. Our analysis covers the years 1992 through 1996.

The history of the IRI has shown that establishing a satisfying empirical model requires quite a few data distinct from those that are currently measured. Another important requirement are mathematical tools that are adapted to existing geophysical constraints. The family of Epsteine functions has shown its adequateness for the purpose of representing vertical profiles. Rediscovered, data-linked mathematical procedures promise better performance at mapping, at least regionally. The classical distinction of three latitude zones is geophysically well founded and should be more systematically taken into account.

At least in daylight conditions the distribution of the electron population is not maxwellian in most of the ionosphere since a high-energy tail results from degrading photo-electrons. This supra-thermal tail may influence propagation and absorption of waves in the plasma. An observed typical energy spectrum shows that a concept in which two maxwellian populations are combined is unable to yield an adequate description. Another one is proposed which admits a continuous transition.

A set of regional maps of peak electron density in the northern polar cap was deduced from aeronomical computations for southward IMF executed at Utah State University (USU) /1/. Each of the 12 maps holds for invariant latitudes greater than 50° N, covering 24h both in magnetic local time and in UT. The maps for two parameters are analyzed in terms of Empirical Orthogonal Functions (EOF). The dependence on other parameters is described by continuous functions. In the northern polar cap, these maps might be used as a partially substituting supplement of the world wide maps recommended by the CCIR/2/.

Constraints are needed when applying (e.g. in IRI) the analytic profile description using LAY-functions. The reliability of these is tested experimentally. Some have a well-defined geophysical meaning. Others have been suggested which can be used to study the morphology of the terrestrial ionosphere. The data needed to provide these are indicated, gaps are identified.

Neglect of ionisation in the E-F valley region causes appreciable errors in the F-region heights calculated from ionograms. In order to minimize such errors, one-component analysis methods use heuristic relations, e.g. between the real and virtual height of a ‘key point’ above the valley. A considerable reduction of the errors is obtained by a combined analysis of o- and x- data. The analysis can determine only one parameter, reflecting basically the valley width, so some model must be used for the shape and depth of the valley. When the real ionosphere deviates from the ideal model, ionospheric irregularities give different variations in the o- and x- components. Such contradictory information cannot be combined in a joint solution. Increased technical accuracy cannot ‘circumnavigate’ this difficulty. Thus for further progress we must use carefully calibrated rocket and backscatter data to obtain reliable, median models for valley size and depth under different conditions.

For synthesizing ionospheric maps, it is proposed to use the method of “empirical orthogonal functions” instead of the inversion of the analysis schedule. For interpolating between grid points splining is not the only way. In particular, longitudinal interpolation by a low order Fourier development of the different eigenvectors components appears to be advantageous. Compared with the present synthesis method, important economies in storage capacity and computing time can be achieved.

Since plasma interchange in the F-region is field-aligned, it is not possible to describe adequately the spatial structures occurring at low latitudes when admitting that they depend exclusively on the vertical coordinate. It is therefore proposed: (1) To replace in the topside description the vertical coordinate by the length along the magnetic fieldline (in a local dipole approximation) and (2) to replace, in the transition range to the plasmasphere, Rycroft's non-analytic formula by a fully analytic function of almost equal shape.

While briefly describing available in-situ and ground-based data sources, a critical discussion of Langmuir probe measurements in the D-region is presented. A new method of validating the long term data sets via the solar cycle dependence is proposed. This method is applied to available rocket measurements in U.S.A and India and to ground based measurements in Europe. The shortcomings and limitations of existing data are recognised and recommendations are made for future in situ measurements and for the interpretation of ground based measurements.

The routinely reduced ionospheric parameter M(3000)F2 [M] is inversely related with the peak altitude (Shimazaki, 1955; Bilitza et.al., 1979). It undergoes quite important diurnal, day-by-day, seasonal and solar cycle variations. A potential secular change, though apparent in the original data for midnight is by day drowned in the regular seasonal variation. We show that it can even by day be identified after elimination of all shorter term variations i.e. after suitable averaging. Doing so with the monthly median data of a british station at (51.5o N, 0.6o W), and comparing the data of solar cycles 22 and 18 (i.e. over an interval of about 40 years) we find a clear effect by day as well as by night. This change is, however, far from being uniform, since it depends of the phase in the solar cycle. Long-term changes of the peak altitude between 0 km and more than 40 km were found with smaller values by day than by night. Particularly small secular changes occur during the period just after the maximum of the solar cycle.

Observable effects of semidiurnal tidal character caused by the Sun (radiation and gravity) and Moon (gravity only) seem to be influenced by the planet Jupiter and, during approach, by Venus. These effects can be identified during summer for suitably selected planetary configurations. The importance of the lunar effect is found to depend on the solar hour, season and planetary constellation.

Short wave forecasting began in the late 1930s with rather crude, more global methods. The main steps of the development until the middle of the 1950s are described. Analytical methods have now replaced the original ones. (auth)

A fully analytical profile is obtained by suitably combining three height ranges in each of which an independent description by a linear combination of Epstein LAY-functions is arranged. The present IRI description of the topside is directly reformulated in terms of two such functions. Three to four functions will be needed in the middle, up to three in the lower ionosphere (excluding a C-layer). Indications are given, how the different parameters could be determined. Part of these would a priori be given by suitable geophysical relations, another part could be determined in the programme itself with suitable empirically established constraints.

By means of a plane grating spectrometer aboard the German-U.S. satellite Aeros A, solar EUV fluxes have been measured in the wavelength range from 16 to 106 nm monitoring the emission from the full solar disk from December 1972 through August 1973. The instrument was calibrated before flight by means of the radiation produced by an electrons synchrotron. The sensitivity of the multipliers was continously monitored in flight. EUV indices are presented characterizing the most important solar emissions of aeronomic interest orbit by orbit. The data clearly show a long-term decrease with smoothed sunspot numbers superposed by a kind of solar rotation modulation, both effects being more important for higher ionized emissions. By comparing both variations with those of the classical solar activity indices it is found that the 27-day period appears more consistently in the Covington index than in EUV emissions. The Zurich relative sunspot number, however, shows much larger fluctuations. Both traditional indices are therefore not ideal for characterizing solar-terrestrial relations, in particular in aeronomy.

Recent progress made in modeling the electron density profile in the topside ionosphere is reviewed. The results of different F2 peak models are addressed in the light of the data, and the outlook for further progress in this area is discussed. Efforts made toward determining the topside profile shape are reviewed and assessed.

Problems encountered during efforts to reformulate the IRI description of the electron density profile are examined. Consideration is given to Booker's (1979) proposal that the unique, analytic profile functions should cover the entire ionospheric height range. The IRI topside model is reviewed and the electron density profile of the middle and lower ionosphere are discussed. Rawer's (1983) procedure for combining the topside, middle, and lower ionospheric profiles into one analytic profile is reviewed.

With the two aeronomy satellites AEROS-A and B simultaneous observations of the main upper atmospheric and ionospheric parameters as well as of the solar ionizing radiation were conducted. The data basis is described with respect to orbit parameters, performance criteria of the spacecraft and experiments, and the time coverage of the various parameters measured. An outline is given on the various aspects and the major results of data analysis which is performed on an integrated scale.

Quite a few authors have analyzed electron density profiles of the middle ionosphere, others made efforts to determine that of the lower ionosphere. Such data have been used with different additional assumptions in a synthetic effort to achieve model profiles. It is found that the presently available data base is insufficient in some respects and needs completion.

Carefully designed probes of the retarding potential analyzer (RPA) type allow electron temperatures to be correctly measured. When compared with incoherent scatter stations longitudinal structures sometimes appear.

Methods are described by which the desired analytical representation of the whole profile might be reached while enforcing the most important observed physical features. An outline of future work in this connection is given.

Different data sets containing experimental lower ionosphere profiles were considered and fitted with LAY-functions. Since the individual reported profiles show large differences, it is concluded that median profiles should be used for the purpose of the IRI. For the day-time profile, an inflection point near 80 km, as assumed in the present IRI, should continue to be taken as a typical feature which must be represented. At twilight and night, a description with only one or two LAY-functions should be sufficient until consistent data are produced for the levels below 80 km.

A study of empirical ionospheric models was undertaken to improve the accuracy of the ionospheric correction that is needed in satellite orbit determination. Our evaluation concerns the two most widely used empirical ionospheric models lBent and international reference ionosphere lIRIrr and suggests improvements based on comparisons with satellite and groundhbased data. In both models, the topside electron density profiles decrease too sharply with increasing altitude. Our improved IRI topside formula compares well with AEROS satellite data and incoherent scatter measurements of the Jicamarca, Peru, radar facility. For the bottomside, a model for the effective parabolic thickness is introduced, based on the detailed IRI predictions. Recent progress in modeling the F2 peak altitude and electron density is reviewed. The increase in prediction accuracy compared to older F2 models is evaluated with Ionosphere Sounding Satellite b results lfor densityr and incoherent scatter measurements from Millstone Hill, Massachusetts lfor altituder. The plasmaspheric electron content is also taken into account. Finally, results of a METEOSAT tracking exercise are used to check the ionospheric delays calculated with the different models. The changes indicated above were found to be helpful and resulted in an overall improvement.

While describing the formulation of IRI-79, in so far as it is applicable to the sub E-peak electron density profile, an attempt is made to compare its predictions with the available experimental evidence. The improvements needed for D-region electron density modelling are summarized. The current D-region IRI modelling effort is illustrated with a typical example.

With an appropriate system of higher order approximations the Faraday-effect is calculated for instantaneous ionospheric models the data of which are derived from the results of vertical soundings. Only one parameter which corresponds to the thickness of the outer ionosphere is left open; it is determined by global comparison with simultaneous Faraday-observations of satellite emissions. The thickness parameter shows various forms of variation with latitude; an increase towards north is the rule for quiet daytime conditions. Comparison with local methods shows considerable differences due to the indetermination of the absolute number of rotations.

L’introduction precise (1) le but du travail entrepris, les liaisons utilisees et les observations faites. Les auteurs exposent (2) ensuite la technique des observations, depouillent (3) des enregistrements, donnent (4) les resultats concernant la statistique des parcours en particulier pour les liaisons U. S. A.-A. O. F., Europe-A. O. F. et Atlantique Nord-Europe. Ils deduisent (5) d’observations effectuees entre deux stations europeennes distantes de 1216 km que la MUF a une valeur superieure a celle que l’on calcule par les methodes classiques. Ils terminent (6) par des mesures d’intensite des echosLoran.

Ionospheric observations near Adelieland show many anomalies. F 2 -echoes are very often scattered, sometimes to such an extent that they almost disappear, and most F 1 and E -echoes also show some signs of scattering. High values can be attained by selective E-absorption. Es-echoes are frequently received from several levels at the same time, and echoes, similar to that of Es, are observed from very high altitudes (1000 km); all these traces undoubtedly being due to ionic clouds at the E-level. Most of the phenomena observed may be attributed to the proximity of the magnetic pole, which is only 470 km away. Nevertheless, it is only rarely that the quasi-longitudinal z-branch has been observed.

Les resultats donnes ici sont provisoires, il serait sans doute fort utile, d’effectuer un grand nombre d’observations s’echelonnant sur une intervalle de temps beaucoup plus court et d’etendre les methodes statistiques mises au point par Alpert [18] et Beamley [19].

Some observations that have been obtained with variations of the classical echo-sounding method must be interpreted as resulting from perturbation and/or oscillation phenomena occurring in the outer ionosphere. The vertical velocity component of ‘traveling disturbances’ coming from outside and propagating through the ionosphere is determined as 115±35 m/sec. Oscillation-like phenomena have a large range of quasi-periods, between 1/4 and 12 hours.

Amplitude and doppler records of the 20 Mc/s transmissions of 1958 δ2 (Sputnik III) were simultaneously made at six radio observatories in Europe. Several distinct phenomena were noted. A sharp decrease in amplitude, which lasted from 1 2 −1 1 2 min and was followed by an abrupt return of the signal, occurred at the same time at all stations. This phenomenon has been termed “dropout”, although indications of low-level signals were usually present, and is explained as a sudden decrease in transmitter power of the satellite perhaps when the satellite passed through a region of high electron flux. Another type of decrease in the signal was attributed to highly localized absorbing or scattering regions. A third phenomenon, recorded by techniques of simultaneous measurements, was observed in the concurrence of scintillating signals at some stations and Faraday fading at others. The overlap of the propagation path to each station recording the scintillations defines a vertical slab of ionospheric region responsible for the scintillations. The region is about 400–600 km in the horizontal plane but includes small scale irregularities of less than 1 km.

A regular interference model of the fading pattern is considered as opposed to the usually accepted purely random model. It would appear that some of the observed features are well explained by a model which is intermediate between these two types insofar as regular interference perturbations are concerned, but one in which a finite lifetime for the irregularities is supposed. It appears that with the “similar fade” analysis there is a marked tendency to select those periods where the interference is quite regular and the rather small angular dispersion found in most sets of observations can thus be explained. The observed big dispersion of the speed is at least partially due to a small vertical component of the movement.

Observations of ionospheric absorption made on different frequencies reveal an important contribution of selective absorption occurring in the E -layer. This influence can be calculated with the model of a parabolic E -layer and an exponential variation of the collision number. With the results of this calculation a new evaluation of the measurements of Slough and Freiburg has been made for 24 months. A graphical method is proposed, which gives separately the contributions of E - and D -layer. These values for D -absorption are, of course, not as high as can be obtained by the usual method, where the influence of the E -layer is neglected. In spite of a considerable dispersion this evaluation leads to the adoption of a rather constant value of the influence of the E -layer. It is proposed to use this value for future reduction of absorption measurements. Finally this constant leads to a new determination of the number of collisions in the centre of the E -layer, which is certainly below 10 4 [s 1 ](6,5 × 10 3 [s −1 ]) . This value, lower than those of other observers, is in good accordance with recent temperature and pressure data at 125 km.

Die von der Laborphysik erheblich abweichenden Bedingungen von Weltraumexperimenten werden grundsätzlich diskutiert und am Beispiel der Messung der Plasmadichte erläutert. Die behandelten Problem-Stellungen sind: Meßgeräte, Meßgrößen, störende Einflüsse, Experimentier-Bedingungen. *

It seems of interest to characterize in detail the diurnal variation of the F2 layer when “effects of longitude” are inquired into. For this purpose, we use the maxima and minima of critical frequencies, characterizing the heights by the factor “M3000.” Conditions seem to be more complicated than former analysis has revealed.

Apart from explanations in terms of physical models, the average results of multifrequency absorption measurements may be described by empirical expressions. These can be rather simple in the variables ƒ (frequency) and ƒ 0 E , the latter being used as a parameter characterizing both, solar activity and season together. Summer conditions are described by a sum of three terms, two of which give the overall behaviour with ƒ and ƒ 0 E as coordinates while the third one is the selective peak near ƒ 0 E . In winter a fourth contribution characterising the winter anomaly must be added.

Beobachtungen der Drift-Richtung in derE-Region aus dem Herbst 1956 und 1957 wurden auf vermutete solare und lunare 12-Stunden Komponenten untersucht. Beide sind bei Tag und Nacht vorhanden; ihre Amplituden sind von gleicher Grossenordnung.

1. 1. A general discussion of the usual method of absorption measurements reveals some difficulties of physical interpretation. 2. 2. With the absorption measurements of Slough the following three results have been obtained: The seasonal variation follows a law with cos χ 3 4 , but only in summer (Fig. 2). The influence of the solar cycle is important (Fig. 3). A considerable fluctuation exists from one day to another. 3. 3. Such daily values measured at Slough and Freiburg (distance 400 km) have been compared (Fig. 4 and 5). They mostly show a positive correlation, but the correlation coefficient is only of the order of 0·4. Some of the difference seems to be brought about by local phenomena, but this conclusion is not yet certain, because the method of measurement is somewhat different. A new measure of correlation, which is rapidly calculated, is proposed in the appendix.

Maps of the factor (M 3000)F2 which is a parameter corresponding to the inverse of the altitude of theF2-layer have been prepared for America between the magnetic equator and high magnetic latitudes. Common features of all maps are a daily minimum on the magnetic equator, a sunrise-maximum and a daily maximum at a geographic latitude of about 40° N. This latter is less pronounced in summer than in winter. With increasing solar activity we find a general decrease ofM-values the equator daily minimum becoming relatively deeper.