Ionospheric support of HF radiocommunication using maps of total electron content

The possibility of determining the field of critical frequencies of the ionospheric F2 layer (foF2) using the maps of the total electron content, constructed based on the registration of signals from satellite radio navigation system of the GPS and GLONASS types, is considered. The calculation of foF2 is based on the SPIM (Standard Plasmasphere-Ionosphere Model) model specifying the ionospheric index of solar activity, which is determined at grid points of the map of the total electron content. The proposed method has been verified using the data of the hourly maps of the total electron content in the North American region during September 1–7, 2005. The variations in the critical frequencies for Boulder and Dyess sites, selected from the reconstructed foF2 maps, were compared with the data of the vertical sounding. The average error is ～10% during the entire period of measurements. The conclusion has been drawn that the proposed method can be used as an ionospheric support of HF radiocommunication in the cases when errors of tenths of MHz in foF2 values are permissible.     

Long-term changes in the Delta <Emphasis Type="Italic">foF</Emphasis>2 parameter based on the data of two European ionospheric stations

Analysis of changes in the critical frequency foF2 in recent decades has been performed by determination of “Delta foF2” parameter. These values determine the mean change of foF2 values from the “etalon period” (1958–1980) to later periods. The results are compared with the determination of foF2 trends, which was performed in a series of previous publications of the authors. The data of two most reliable ionospheric stations of the European region, Slough and Juliusruh, are analyzed. The results confirm all principal conclusions obtained earlier, which were based on analysis of the trends. A systematic decrease of foF2 with time occurs (which corresponds to a negative trend), and the character of changes in the Delta values with season and local time on the whole agrees with the character of changes in the trend. It is shown that the results based on the data of both considered stations show good agreement.     

Variations in <Emphasis Type="Italic">foF</Emphasis>2 at the end of the 1990s and the beginning of the 2000s according to the SPIDR system data

Variations in the critical frequency of the F2 layer at 22 ionospheric stations within the period 1990–2010 according to the SPIDR system data are considered. A confirmation of the negative trends in foF2 for seven stations considered by one of the authors earlier on the basis of median data is obtained. It is found that both negative (a decrease in foF2 with time) and positive (growth in foF2 with time) trends of the critical frequency are observed. During the later part of the considered period (after 1997), negative trends dominate. This fact manifests itself, apparently, in an increase in the role of the decrease in the thermospheric neutral temperature in the formation of foF2 trends.     

Relation between daytime and nighttime values of the critical frequency foF2

The relation between the daytime in the nighttime values of the critical frequencies (foF2) of the ionospheric F ₂ layer is considered. The correlation coefficient of foF2 measured at 1400 and 0200 LT of the same day is considered in various seasons of years with different solar activity (during the complete cycle of solar activity in 1979–1989). Special accent is made on the dependencies of the above mentioned correlation on a choice of magnetically quiet days with various limitations on maximal values of geomagnetic index Ap. It has been obtained that a statistically significant negative correlation between the foF2(1400) and foF2(0200) is more pronounced in the periods of high solar activity. The effect increases with increasing limitation of the considered days on value of Ap: the largest values of the correlation coefficient are observed if only very quiet days are considered (Ap < 6). There are preliminary indications that the considered relation between daytime and nighttime foF2 values depends on latitude.     

Relationship between foF2 trends and geographic and geomagnetic coordinates

The possibility of presenting parameters k(foF2) and ΔfoF2 characterizing the change over time in the critical frequency of the ionospheric F2 layer in recent decades was considered. These parameters were previously obtained by the authors as functions of geographic coordinates, latitude and longitude. It was shown that there is such a possibility and the corresponding statistical significances of the obtained dependencies lie within the limits from ～90% to more than 99%. Simultaneously, the dependence of the linear trend k on magnetic coordinates D and I was studied. This dependence should manifest the relation of the trends in foF2 to dynamical processes (horizontal winds) via the vertical drift induced by them and the corresponding change in the F2-layer height.     

Properties of the <Emphasis Type="Italic">F</Emphasis>2-layer critical frequency median in the nocturnal subauroral ionosphere during low and moderate solar activity

Based on an analysis of data from the European ionospheric stations at subauroral latitudes, it has been found that the main ionospheric trough (MIT) is not characteristic for the monthly median of the F2-layer critical frequency (foF2), at least for low and moderate solar activity. In order to explain this effect, the properties of foF2 in the nocturnal subauroral ionosphere have been additionally studied for low geomagnetic activity, when the MIT localization is known quite reliably. It has been found that at low and moderate solar activity during night hours in winter, the foF2 data from ionospheric stations are often absent in the MIT area. For this reason, a model of the foF2 monthly median, which was constructed from the remaining data of these stations, contains no MIT or a very weakly pronounced MIT.     

Observations of large-scale plasma convection in the magnetosphere with respect to the geomagnetic activity level

The data of the ionospheric observations (the daily f plots) at the Yakutsk meridional chain of ionosondes (Yakutsk–Zhigansk–Batagai–Tixie Bay) with sharp decreases (breaks) in the critical frequency of the regular ionospheric F2 layer (foF2) are considered. The data for 1968–1983 were analyzed, and the statistics of the foF2 break observations, which indicate that these breaks are mainly registered in equinoctial months and in afternoon and evening hours under moderately disturbed geomagnetic conditions, are presented. Calculations performed using the prognostic model of the high-latitude ionosphere indicate that the critical frequency break position coincides with the equatorial boundary of large-scale plasma convection in the dusk MLT sector.     

The ionosphere of Europe and North America before the magnetic storm of October 28, 2003

The X17 solar flare occurred on October 28, 2003, and was followed by the X10 flare on October 29. These flares caused very strong geomagnetic storms (Halloween storms). The aim of the present study is to compare the variations in two main ionospheric parameters (foF2 and hmF2) at two chains of ionosondes located in Europe and North America for the period October 23–28, 2003. This interval began immediately before the storm of October 28 and includes its commencement. Another task of the work is to detect ionospheric precursors of the storm or substorm expansion phase. An analysis is based on SPIDR data. The main results are as follows. The positive peak of δfoF2 (where δ is the difference between disturbed and quiet values) is observed several hours before the magnetic storm or substorm commencement. This peak can serve as a disturbance precursor. The amplitude of δfoF2 values varies from 20 to 100% of the foF2 values. The elements of similarity in the variations in the δfoF2 values at two chains are as follows: (a) the above δfoF2 peak is as a rule observed simultaneously at two chains before the disturbance; (b) the δfoF2 variations are similar at all midlatitude (or, correspondingly, high-latitude) ionosondes of the chain. The differences in the δfoF2 values are as follows: (a) the effect of the main phase and the phase of strong storm recovery at one chain differs from such an effect at another chain; (b) the manifestation of disturbances at high-latitude stations of the chain differ from the manifestations at midlatitude stations. The δhmF2 variations are approximately opposite to the δfoF2 variations, and the δhmF2 values lie in the interval 15–25% of the hmF2 values. The performed study is useful and significant in studying the problems of the space weather, especially in a short-term prediction of ionospheric disturbances caused by magnetospheric storms or substorms.     

Long-term trends in the ratio of the daytime and nighttime values of <Emphasis Type="Italic">foF</Emphasis>2

The consideration of the relation between the daytime and nighttime values of the critical frequency F2, foF2 of the ionospheric F2 layer, started in the previous publication of the authors, is continued. The main regularities in variations in the correlation coefficient R(foF2) characterizing this relation are confirmed using larger statistical material (more ionospheric stations and longer observational series). Long-term trends in the R(foF2) value are found: at all stations the negative value of R(foF2) increases with time after 1980.     

Forcing of the ionosphere from above and below during the Arctic winter of 2005/2006

The present paper focuses on planetary wave type responses of the thermosphere/ionosphere system to forcing from above and below during the Arctic winter of 2005/2006. The forcing from above is described by the sunspot numbers, the solar wind speed, the B_z-component of the IMF and the geomagnetic Kp-index, while the forcing from below, i.e. by upward propagating atmospheric waves, is represented by the SABER/TIMED temperatures. The observed global ionospheric zonally symmetric oscillations with periods of ～9, ～14 and ～24–27 days were approved to be of solar origin. The most persistent ～9-day oscillation is linked to a triad of solar coronal holes distributed roughly 120° apart in solar longitude. The ～18-day westward propagating wave with zonal wavenumber 1, observed in the ionospheric currents (detected by magnetometer data), and in the F-region plasma (foF2 and TEC) could be allocated to a simultaneous 18-day westward propagating planetary wave observed in the stratosphere/mesosphere/lower thermosphere region with large (～70 km) vertical wavelength.     

Spectrum of quasi-wave disturbances in the topside daytime ionosphere according to the data of radiosounding onboard the Intercosmos-19 satellite

Quasi-wave disturbances in the topside daytime ionosphere, related to auroral activity, have been detected using the data of radiosounding onboard the Intercosmos-19 satellite on April 28, 1979. A disturbance was caused by an abrupt enhancement of the eastward electrojet, which was not reflected in the variations in the AE and AU indices. According to the estimates, the period of electron density disturbances was about 0.5 h, the velocity was 350 m/s, and the length along the meridian was several hundreds of kilometers, which corresponds to medium-scale traveling ionospheric disturbances (TIDs). The disturbance amplitude was only 30 km in the hmF2 variations and 0.20–0.25 MHz in the foF2 variations but increased to 0.25–0.30 MHz in the plasma frequency variations at satellite altitudes of 520–580 km with increasing altitude. It is impossible to register so weak short-period variations during ground-based sounding. The method for detecting disturbance spatial characteristics has been proposed. The disturbance spectrum including three quasiperiodic structures has been revealed using this method. The optimal estimates have been made for the trend, described by the polynomial of the third degree, and for the expansion of the residuals in terms of three harmonics.     

Planetary statistics of the critical frequency of the ionospheric F 2 region during catastrophic earthquakes

The statistics of the relative variations (δfoF2) in the critical frequency of the ionospheric F ₂ region during catastrophic earthquakes with magnitude M ≥ 6 at 70 vertical sounding (VS) automatic ionospheric stations (AISs) of the global network is considered. Five-day periods including 70 such situations have been selected for 1981 and 1983–1985. Five sets of 24 foF2 measurements from 3 days before to a day after the earthquake instant are formed for each AIS. The number of AISs, for which the fourth statistical invariant is above the significance level (E ≥ 1), has been analyzed. It has been indicated that the histogram deformation at the set of AISs is substantial a day before earthquakes.     

Inversion of the F2 layer critical frequency with the solar zenith angle

Normalization of the F2 layer critical frequency is put forward with a model related to the solar zenith angle at the time of observation and its local noon value. The method implies a physically based replacement of variable representing one of the key controlling parameters of the modern empirical ionospheric models. The inversion of critical frequency is analyzed with the hourly values of foF2 of seven ionospheric stations for 2000–2006 (half the solar activity cycle). The normalized critical frequency fnF2 reveals an improved correlation between the data of different stations and improved inter-seasonal correlation of the data of a particular station as compared with the initial foF2 values. A correlation radius in summer is two times greater for normalized critical frequency than for observations. The analytical model of expansion of the diurnal variation of critical frequency with four hourly values at 00, 06, 12 and 18 h of local time exhibits improved statistical features of normalized critical frequency as compared with observed critical frequency.     

Properties of solar activity and ionosphere for solar cycle 25

Based on the known forecast of solar cycle 25 amplitude (Rz _max ≈ 50), the first assessments of the shape and amplitude of this cycle in the index of solar activity F10.7 (the magnitude of solar radio flux at the 10.7 cm wavelength) are given. It has been found that (F10.7)_max ≈ 115, which means that it is the lowest solar cycle ever encountered in the history of regular ionospheric measurements. For this reason, many ionospheric parameters for cycle 25, including the F2-layer peak height and critical frequency (hmF2 and foF2), will be extremely low. For example, at middle latitudes, typical foF2 values will not exceed 8–10 MHz, which makes ionospheric heating ineffective in the area of upper hybrid resonance at frequencies higher than 10 MHz. The density of the atmosphere will also be extremely low, which significantly extends the lifetime of low-orbit satellites. The probability of F-spread will be increased, especially during night hours.     

Behavior of parameters of the ionospheric F2 layer at the edge of the centuries: 2. Height of the layer

The trends in the height of the ionospheric F2 layer, hmF2, for the period from the end of the 1990s to the beginning of the 2010s are considered. It is shown that for a majority of the considered situations (time of the day plus season) negative trends are obtained for all stations for which series of initial data satisfying the requirements were found. They vary from ?1.2 to ?3.1 km per year. The trends in the daytime are slightly higher than in the postsunset period which apparently manifests the influence of the contraction of the upper atmosphere on changes in the hmF2 value.     

Relationship between variations in the characteristics of the high-latitude ionosphere and magnetic <Emphasis Type="Italic">PC</Emphasis> indices

It is shown in a joint analysis of ionospheric vertical sounding data at the arctic Heiss Island and antarctic Vostok stations and the geomagnetic PC index, which characterizes the geoefficient component of the interplanetary magnetic field, that, during a disturbed geomagnetic period when PC > 2 in years of solar activity (SA) maxima in the winter season, positive phases of ionospheric disturbances are predominantly observed. In the nighttime hours, an increase in the critical foF2 frequencies by a factor of 2–3 can occur. In a disturbed geomagnetic period at the PC > 1.5 level in the summer season, negative phases of ionospheric disturbances are mainly observed. In years of maximum and moderate SA, the decrease in foF2, as compared to their median values, happens at night (∼30%). In years of low SA, the decrease value is much lower. At a substantial decrease in the PC index level, in the region of the geomagnetic pole at the Vostok station, in some cases, a substantial increase in the electron density level in the F region occurs with a delay of 0.5 h. At the same time, a significant correlation (r = −0.57) is observed between variations in the PC index and foF2.     

Relation between changes in foF2 and hmF2 within various time intervals

The relation between the critical frequency foF2 and F2-layer height hmF2 is considered for ten ionospheric stations in the periods before and after 1980. It is shown that in the earlier period the relation between foF2 and hmF2 is well pronounced. In the later period, a distortion of this relation is observed. The statistical characteristics of the foF2 dependence on hmF2 are spoiled. That shows that due to the cooling and contraction of the upper atmosphere the height distribution of the photochemical parameters governing the equilibrium concentration in the layer maximum changes. A larger contribution to this effect is evidently provided by changes in the atom-to-molecule concentrations ratio.     

Increase in the critical frequency of the ionospheric <Emphasis Type="Italic">F</Emphasis> region prior to the substorm expansion phase

Specific variations in the critical frequency of the ionospheric F ₂ layer during magnetospheric substorms have been found based on the data of vertical sounding stations in Europe and North America. Maximal attention has been paid to the positive peaks of ΔfoF2 with a duration of 6–8 h before the beginning of the substorm expansion phase (T ₀). The possible physical mechanisms by which these peaks are formed (related to the impact of fast particles in the foreshock region of the solar wind on the Earth’s magnetosphere and different for middle and high latitudes) have been considered. The positive peaks of ΔfoF2 can be used in a short-term prediction of the ionospheric disturbance onset and space weather on the whole.