Ionospheric Vertical Sounding Research Articles

Vertical structure of a gravity wave like oscillation in the ionosphere generated by the solar eclipse of August 11, 1999

The dependence in altitude and time of the ionospheric electron density and its altitude variations during the solar eclipse of August 11, 1999, have been studied. The true height electron density profiles recorded at the Observatori de l'Ebre station (40.8° N, 0.5° E) during a rapid sequence vertical ionospheric sounding campaign have been used. A gravity wave like oscillation in the ionosphere has been characterized after the maximum of solar occultation by the Moon. The event is clearly seen in both the plasma frequency and in the altitude variations of the F region in the altitude range from 150 to 250 km, and it has a dominant oscillating period T=57.5 min. The oscillation activity linked with the gravity wave event is found to exist in the variations of the ionospheric parameters, being coherent with the oscillations characterized in the plasma frequency variations. The source‐origin of the event was located around the transition region between F1 and F2 layers and from that altitude the wave propagates vertically drawing energy upwards and downwards simultaneously. The decreasing/increasing solar ionizing radiation and the cooling/heating processes linked with the solar eclipse are discussed as a possible mechanism to explain the gravity wave event observed in the electron density and dynamics of the ionosphere.

Ionospheric phenomena before strong earthquakes

Abstract. A statistical analysis of several ionospheric parameters before earthquakes with magnitude M > 5.5 located less than 500 km from an ionospheric vertical sounding station is performed. Ionospheric effects preceding "deep" (depth h > 33 km) and "crust" (h < 33 km) earthquakes were analysed separately. Data of nighttime measurements of the critical frequencies foF2 and foEs, the frequency fbEs and Es-spread at the middle latitude station Dushanbe were used. The frequencies foF2 and fbEs are proportional to the square root of the ionization density at heights of 300 km and 100 km, respectively. It is shown that two days before the earthquakes the values of foF2 averaged over the morning hours (00:00 LT–06:00 LT) and of fbEs averaged over the nighttime hours (18:00 LT–06:00 LT) decrease; the effect is stronger for the "deep" earthquakes. Analysing the coefficient of semitransparency which characterizes the degree of small-scale turbulence, it was shown that this value increases 1–4 days before "crust" earthquakes, and it does not change before "deep" earthquakes. Studying Es-spread which manifests itself as diffuse Es track on ionograms and characterizes the degree of large-scale turbulence, it was found that the number of Es-spread observations increases 1–3 days before the earthquakes; for "deep" earthquakes the effect is more intensive. Thus it may be concluded that different mechanisms of energy transfer from the region of earthquake preparation to the ionosphere occur for "deep" and "crust" events.

Group propagation time of quasi‐monochromatic radiation in a plane layer of plasma with random irregularities

The influence of multiple scattering in a plasma layer with random irregularities on the propagation time of a totally reflected pulse signal has been considered. Two approaches have been implemented. The first one is based on an analytical solution of the radiative transfer equation. The second one is based on a numerical simulation in the spirit of the Monte Carlo technique. The general conclusion is that, subject to relative position of a transmitter and a receiver and the irregularity shape, one can observe both a decrease and an increase of the propagation time in comparison with the reflection from the same layer deprived of random irregularities. The numerical estimates made in this paper regard the case of radio sounding of the ionospheric plasma. It has been shown that the magnitude of the considered effect is of the order of the characteristic pulse length applied in the ionosphere vertical sounding.

Tropospheric events and possible related gravity wave activity effects on the ionosphere

Short-term changes of electron concentration in the ionospheric F-region have been studied during a campaign of rapid sequence vertical ionospheric sounding. The wave-like structures occurred under low solar and geomagnetic activity. The most probable source of the observed waves was below the 175 km level and probably not in the auroral zone. The vertical components of propagation characteristics of the observed waves correspond to the theory of acoustic gravity wave propagation in the ionosphere and confirm the position of the source being below the studied heights. Another case studied showed the effects of passage of strong tropospheric cold front under high solar activity during February 1990.

Multiple scattering effects under vertical sounding of equatorial ionosphere

This paper presents a theoretical study of the influence of electron density small-scale irregularities on radio propagation under vertical sounding of the equatorial ionosphere. The sounder is assumed to radiate in the plane perpendicular to geomagnetic field lines. An approximate analytical solution of the equation of radiation energy balance in a plane layer of randomly inhomogeneous plasma has been obtained for this case. Analysis of the results shows that multiple scattering leads to attenuation of signal power and change of the signal arrival angles in the sounder vicinity.

Universal variation of the F2-layer critical frequency and solar activity

Regular variations of the F2-layer in the quiet ionosphere are studied using the universal variation of the F2-layer critical frequency, i.e. the dependence of the instantaneous mean-longitudinal value of foF2 at a fixed latitude on universal time (UT). Algorithms for the universal variation calculation have been developed and are used to process the ionospheric vertical sounding data for 1957–1990. It is shown that the foF2-universal variation forms are es-sentially different for solar minima and maxima years. In the solar minima years, the foF2-universal variation is similar to the known universal variation of the atmospheric electric field near the Earth’s surface which has a maximum at 19 UT. There is no foF2-universal variation seasonal change in solar minimum years. The foF2-universal variation form changes for solar maximum years have a minimum of foF2, at 16–17 UT, and there are small seasonal changes in the foF2-universal variation. Such behaviour of the foF2-universal variation suggests that under the quiet conditions in solar minimum years the electric field penetrates from the lower atmospheric layers up to the F2 layer of the ionosphere.

A global empirical model of effects of large-scale internal gravity waves in the night-time ionosphere

On the basis of an analysis of a large amount of data from the vertical ionospheric sounding stations located in the Northern half of the Eastern hemisphere, a global empirical model of variations of f 0 F 2 and the F 2 layer height associated with large-scale internal gravity waves (IGW) during magnetospheric substorms and the first few hours of magnetic storms is constructed. The model is valid for night hours in the interval from subauroral latitudes to equatorial ones for all seasons and for different levels of solar activity. The model allows one to estimate the probability, delay relative to substorm onset, amplitude and time evolution of the IGW effects in the ionospheric F region. Using this model, the main features of the IGW effect in the ionospheric F region are derived depending on season, latitude, and levels of solar and magnetic activities. It is found that the IGW probability is quite high; it increases with increases of substorm intensity and solar activity. For substorms with AE max > 1000 nT in years of solar maximum, this probability amounts to as much as 100%. The IGW amplitude also increases with increases of substorm intensity and solar activity. The IGW effect amplitude depends on solar activity nonlinearly: it increases appreciably when F 10.7 changes from 75 to 150 and almost does not vary when F 10.7 grows from 150 to 200. At high solar activity, seasonal variations are clearly exhibited: the IGW effect amplitude is noticeably greater in winter and the equinoxes than in summer. In years of solar minimum, seasonal variations are negligible. For all seasons and levels of solar and magnetic activities, the effect amplitude is greatest at subauroral latitudes and then decreases smoothly as the latitude is reduced, with a local maximum near geomagnetic latitude φ = 20 . Interpretations of some features of the effect are suggested.

Gravity waves propagation in the thermosphere observed from electron density profile and total electron content measurements

Ionospheric observations with five minute intervals between ionograms were made during a campaign from 19th to 23rd June 1996 at the Rome station (41.8N, 12.5E). The data obtained from ionospheric vertical sounding have been analysed together with the Total Electron Content (TEC) data obtained by the GPS receiver measurements. Both the apparatus were installed in the same station. Short periodicity phenomena occurring in the considered period were observed and interpreted as resulting from the propagation of AGWs in the thermosphere. TEC and electron density were then analysed during AGWs activity.

Behavior characteristics of the index of middle-latitude F-spread

We present the results of analysis of data on ionospheric vertical sounding in Zimenki (Nizhny Novgorod) over the period from 1964 to 1992. The January ionograms are used for constructing the plots of daily indices of F-spread for an ionospheric layer. The dependence of the average monthly (January) index of F-spread on the solar activity is obtained. The index of F-spread is found to grow with time. The technogenic nature of this increase is assumed.

On the role of large-scale ionospheric inhomogeneities in the formation of middle-latitude F-spread

We present the results of a special experiment carried out in the settlement of Zimenki, Nizhniy Novgorod province, on ionospheric vertical sounding by HF signals under F-spread using practically synchronous measurements of the multiple-ray nature of normal waves reflected from magnetoactive ionospheric plasma with ordinary and extraordinary polarizations at frequencies shifted by ∼ 0.7MHz. The experiment confirms the dominating role of large-scale electron-density inhomogeneities of the moving ionospheric disturbance type in the formation of middle-latitude F-spread.

Frequency correlation of fluctuations of short-wave signals in the theory of refractive scattering of radiowaves in the ionosphere

We consider the problem of frequency correlation of phase fluctuations of HF signals reflecte from a turbulent ionospheric layer with an arbitrary profile of electron density in vertical and oblique sounding of the ionosphere. Expressions for evaluation of the coherence band of HF signals reflected from a randomly inhomogeneous ionosphere are obtained. Calculation results are compared with corresponding calculations of frequency correlation of fluctuations of HF radiowaves in Buker's theory of refractive scattering of radiowaves. It is shown, in particular, that the influence of splitting of rays for waves with different frequencies in an inhomogeneous ionosphere on the frequency distortions of HF signals prevails compared with the phenomenon of frequency decorrelation of fluctuating HF signals propagating in a free space beyond a turbulent ionospheric layer.

Preliminary results on the ionospheric structure at dawn time observed during HRIS campaigns

Several campaigns of ionospheric vertical soundings with short-time interval between them (HRIS) have been recently performed at the ionospheric station in Rome. Some preliminary results concerning periods around local sunrise are presented. A cIear fading effect, due to the defocusing phenomena in the upper ionosphere, has been observed in different seasons

Ionospheric effects of the solar eclipse of September 23, 1987, around the equatorial anomaly crest region

The ionospheric responses to the solar eclipse of September 23, 1987, in the equatorial anomaly crest region have been investigated by using ionospheric vertical sounding, VLF propagation delay time, and differential Doppler shift data observed at Chungli, which is located near the northern equatorial anomaly crest region. It has been found that temporal variations of the F1 layer and D region are mainly controlled by local solar radiation. Quantitative analysis of the variations of the F1 layer critical frequency, ƒ0F1, shows that electrons are removed from the F1 layer through ionic recombination. However, the temporal variations of ƒ>0F2 and electron density above 200 km show that the variations of the F2 layer around the equatorial anomaly region are controlled not by local solar radiation but by solar radiation at the equator. The fountain effect plays an important role even during the solar eclipse. The VLF propagation time delay is controlled by the variations of average path obscuration. Atmospheric gravity waves produced by the moving bow wave front of the solar eclipse are found with a period around 17‐23 min and wavelength about 293 km.

Linear modelling in vertical ionospheric sounding

Applying the methods of computing N(h) profiles to scalar product spaces provides a more general view of the differences between the individual ionospheric models, which enables a better selection of the optimum model.

Spline approximation of height vs frequency characteristics obtained by ionospheric vertical sounding

Spline approximation of height vs frequency characteristics obtained by ionospheric vertical sounding

Daytime F layer trough observed on a macroscopic scale

This paper reports a new technique of observing the high‐latitude ionospheric trough which defines its continuity in space and time on a macroscopic scale not previously possible. Vertical ionospheric soundings provide the observations in the midday to evening sector where the signature of the trough is found to be a “bite‐out” of a factor of 3–10 in the diurnal distribution of the solar produced electron density at the F layer maximum. The data base employed is that produced by the extensive array of ionospheric sounders operating at high latitudes in the northern hemisphere during the International Geophysical Year period of December 1958, hence at solar maximum. The worldwide extent, high density, simultaneity, and continuity of these measurements make possible the description of the trough for the first time as an integral, spatially continuous entity in its entire daytime extent and UT duration. The technique is demonstrated in studies of 4 days which span the range of activity for the month. On the most disturbed day the daytime trough is continuous in magnetic latitude, magnetic local time between (75°, 1030) and (51°, 1630) and remains nearly stationary in this frame of reference for a duration of 14.5 hours UT. A “snapshot” constructed from multiple simultaneous observations on this day shows the trough to have a continuous length of 6 × 10³ km and an area of >4 × 106 km². The trough, which is also stable for similar durations on the other 3 days, is found to rotate clockwise to earlier magnetic local time by 5.5 hours and to expand equatorward in magnetic latitude by 20° as activity increases from quiet to disturbed. Activity dependence and morphology are consistent with other observations that the trough locates rapid westward convection within the dusk convection cell. These properties of the trough appear on each of the 31 days studied but only in a sector which lies primarily in the eastern magnetic longitudes. Elsewhere, the daytime trough can occur principally in the morning to midday sector.

Relationship between statistical parameters of short-wave signals in vertical and oblique ionospheric sounding

Relationship between statistical parameters of short-wave signals in vertical and oblique ionospheric sounding

The main results in the I.G.Y. program of ionospheric vertical soundings in the U.S.S.R.

The main results of the I.G.Y. program of vertical soundings in the U.S.S.R. are summarized and full references are given.

On the mean temporal variations of electron density at a fixed height in theFregion

Monthly mean values of electron density at 280 km over Puerto Rico, obtained from ionospheric vertical soundings, have been studied for the period February 1959 to January 1960. The results are used in the equation of continuity to deduce a movement term, div (NV). It is suggested that one of the features of the variations in deduced div (NV), a marked post-sunrise effect, might be due to large gradients in the east-west drift velocity. An alternative explanation, which does not appeal to drifts, is briefly discussed.

Atmospheric Scale Height as a Modifying Influence on Linear Trend of F-Region Electron Density with Sunspot Number

IN the prediction of maximum usable frequencies a linear increase in foF2 with sunspot number is generally assumed for practical purposes1. However, an analysis of maximum usable frequencies, estimated for a sunspot cycle interval from Christchurch ionospheric vertical sounding data obtained during 1947–58, indicated that the trend of twelve-month running average values of foF2 monthly medians with twelve-month running average sunspot number showed significant departures from linearity especially in the daylight hours and above running average sunspot number 170 at the 1957–58 sunspot maximum. Chun-Ming Huang2 has observed a similar marked departure from linear trend at the 1957–58 maximum in an analysis of vertical sounding stations in the Western Pacific area, the time of onset relative to sunspot number varying with time of day and station latitude.