P. F. Denisenko

An empirical model for the global distributions of density, temperature and effective collision frequency of electrons in the ionosphere

Abstract An improved ionospheric global empirical reference model (named RIM-85) is established for the distributions of density (Ne), temperature (Te) and effective collision frequency (νe) of electrons between 65 and 600 km. For altitudes above 200 km, at low and middle latitudes, the Ne distribution is based on IRI-82, with some necessary corrections. In comparison with IRI-82 and other models the model RIM-85 has a number of important advantages: (i) it describes in detail the height distributions of Ne and Te below 200 km; (ii) it gives for the first time an adequate description of the high latitude ionosphere, its variations with geomagnetic activity reflecting the occurrence and dynamics of the main large-scale structures of the spatial Ne distributions; (iii) it describes rather correctly the shape of the Ne height profiles at low and high latitudes, in the outer ionosphere and also at high solar activity; (iv) it gives a spatial maximum height distribution of the F-region in the southern hemisphere which is close to the real one and (v) it describes rather correctly the diurnal and seasonal variations of Te. The model RIM-85 is established as a computer program, the general structure of which is similar to the corresponding programs for the IRI-79 and IRI-82 models; it includes some subroutines taken from this model software. Therefore the newly developed model can easily be included into a new version of IRI as a subprogram for Ne, Tes and νe calculations.

Peculiarities of the inverse problems of vertical radio sounding of the ionosphere

Abstract At the interpretation of ionograms (obtained by ground-based or satellite soundings) in terms of electron density, or at that of absorption measurements in terms of an effective electron collision frequency or at that of Doppler measurements in terms of plasma motions, problems arise which might be seen as comparable in character. One can, in fact, resolve these problems by inversion of the first-kind Volterra and Fredholm integral equations. The fact that the first-kind Fredholm equations belong to the class of ambiguous problems necessitates the use of additional information and data which were measured with great precision. Therefore, the main problems in groundbased and satellite diagnostics of the ionosphere is an improvement of the precision in measuring parameters of radio signals and the development of controlling algorithms for the solution of inverse problems.

Errors of ionospheric parameter diagnostics by vertical sounding method

The problem of RMS error determination in ionospheric parameters measured by the method of vertical HF soundings is considered here. This problem is solved for the case of topside ionospheric soundings when the height distribution of electron density is monotonic. An analytical expression is obtained which relates the variance of deduced parameters to the correlation function of errors in measured characteristics of soundings signals. The computing simulation provides for errors in real height determination, in reconstruction of vertical velocities of plasma motion, and in effective electron collision frequencies for the measurements of virtual heights, Doppler frequency shift, and radio wave absorption, respectively. Their dependencies on signal polarization, working frequency range, and geomagnetic latitude of observation point are determined. The obtained results are useful for planning plasma diagnostics experiments using HF sounding technique.

Reconstruction of nonmonotonic height profiles of electron density from vertical sounding ionograms by the regularization method

The problem of reconstruction of daytime non-monotonic N(h) height profiles of electron density from ionograms of ground based vertical sounding is considered. A comparison of the Least Squares Method (LSM) with the Regularisation method (REG) was carried out using computer simulations. It is shown that the application of REG-method allows us to decrease [fie deviation of the restored real height and valley depth from their model values by several times by comparison with LSM-results.The problem of reconstruction of daytime nonmonotonic height profiles of electron density N(h) from ionograms of ground-based vertical sounding is considered. The regularization (REG) scheme has been proposed for determination of valley parameters. A comparison of the least squares method (LSM) with the regularization (REG) scheme was carried out using computer simulations. It is shown that LSM results are very sensitive to random errors in virtual heights. This fact leads to an unpredictably large deviation of the reconstructed N(h) profile from the real one. The application of REG methods makes the results more stable to random errors in measurements and allows us to decrease the deviation of the restored real height and valley depth from their model values by several times in comparison with LSM results.

An empirical model of electron temperature for low and middle latitudes in the 100–200 km height region

An empirical model of electron temperature (Te) for low and middle latitudes is proposed in view of IRI. It is constructed on the basis of experimental data obtained at 100 to 200 km by probe and incoherent scatter methods. Below 150 km the model gives two Te values: one from incoherent scatter data and another from probe measurements. The model can be used for all seasons for quiet geomagnetic conditions (Kp not greater 3) and at almost all levels of solar activity (F10.7 between 70 and 200). It is presented in an analytical form that allows one to calculate Te profiles for different latitudes, longitudes and at any season (day). Depending on geomagnetic latitude and solar zenith angle, electron temperature distributions are presented for two heights along with Te profile variations during the day (at middle latitudes).

An empirical model of electron density for low and middle latitudes below 200 km

Abstract Our empirical model of electron density (n e ) for quiet and weakly disturbed geomagnetic conditions (Kp not greater 4) takes account of comparative analysis of existing models and of experimental data obtained by rockets and incoherent scatter radar. The model describes the n e distribution in the 80 to 200 km height range at low and middle latitudes, and to some extent, in the subauroral region. It is presented in analytical form thus allowing one to calculate electron density profiles for any time. The electron density distribution at 140 km depends on the season (day of the year) and on the solar zenith angle. Profile variations during the day are for one season shown. Different from other models, ours specifies the variations during sunrise and sunset and reflects the particular profile shape at night admitting the occurrence of an intermediate layer.

Correction of Electron Density Profiles in the Low Ionosphere Based on the Data of Vertical Sounding with the IRI Model

The method of correcting the daytime vertical profiles of electron plasma frequency in the low ionosphere from International Refererence Ionosphere (IRI) model in accordance with the measured data of the virtual heights and absorption of signal radiowaves (method А1) reflected from the bottom of E-region at vertical sounding (VS) is presented. The method is based on the replacement of the IRI model profile by an approximation of analytical dependence with parameters determined according to VS data and partially by the IRI model. The method is tested by the results of four joint ground-based and rocket experiments carried out in the 1970s at midlatitudes of the European part of Russia upon the launches of high-altitude geophysical rockets of the Vertical series. It is shown that the consideration of both virtual reflection heigths and absorption makes it possible to obtain electron density distributions that show the best agreement with the rocket measurements made at most height ranges in the D- and E-regions. In additional, the obtained distributions account more adequately than the IRI model for the contributions of D- and E-regions to absorption of signals reflected above these regions.

The resolving power of radio sounding methods for ionospheric parameter reconstruction

The r.m.s. errors in ionospheric parameters studied by the vertical HF-sounding method were determined for a monotonic height distribution of the electron density. Numerical estimates were obtained for errors in real heights, in vertical velocities of plasma motion and in effective electron collision frequencies deduced from experimental measurements of virtual heights, Doppler frequency shift and radio wave absorption respectively. Their dependencies on signal polarisation, working frequency range and geo-magnetic latitude of the observation point were determined.