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1.
The variability degree of the F 2-layer height, hmF2, from the 1950s–1960s to the 1990s has been analyzed based on the vertical sounding data for a series of midlatitude ionospheric stations. It has been found that the scatter of the hmF2 values (standard deviation) abruptly increases from the earlier decades to the later ones. This increase is more evident in the spring period of the year and is independent of geomagnetic activity. An increase in the scatter of hmF2 apparently indicates systematic changes (trends) in the thermospheric dynamics, the existence of which was suggested in the recent publications of the authors. 相似文献
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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. 相似文献
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V. E. Chertoprud L. N. Leshchenko G. V. Givishvili E. E. Goncharova G. S. Ivanov-Kholodny 《Geomagnetism and Aeronomy》2009,49(4):503-509
The 40-year period of observations of short-term variations (with characteristic times of up to 1–2 days) in the critical frequency of the ionospheric F2 layer (foF2) is analyzed. The continuous (with a step of 1 h) series of fluctuations (F) of the foF2 critical frequency (with eliminated daily variations) has been calculated using the hourly variations in foF2 at Moscow stations. The fractal dimension (FRH) of the fluctuations, characterizing short-term variations in foF2, has been determined and analyzed on a 30-day interval, using the Higuchi method. It has been established that FRH estimates substantially change in time. The 11-year cycle, which is in antiphase with the solar cycle, and the total annual and semiannual variations, similar to the variations observed in the normalized critical frequency of the E region and in the electron density of the D region, are clearly defined in these changes. Thus, the parameters of fast variations in the ionospheric F2 layer are affected by the phase of the 11-year solar cycle and by the position of the Earth in the orbit or seasonal variations in the atmosphere. 相似文献
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Critical frequencies <Emphasis Type="Italic">foF</Emphasis>2 as an indicator of trends in thermospheric dynamics 总被引：1，自引：1，他引：0
The time variations in three parameters during the last decades are considered. R(foF2) is the correlation coefficient between the nighttime and daytime values of foF2 for the same day. Stable trends are found for the minimum (R(foF2)(max)) and maximum (R(foF2)(min)) values of R(foF2) during a year. The foF2(night)/foF2(day) ratio demonstrates both, negative and positive trends, and the trend sign depends on the inclination I and declination D of the magnetic field. The correlation coefficient r(h, fo) between foF2 and the 100 hP level in the stratosphere demonstrates a decrease (in the years of maximum and minimum solar activity) from the 1980s to the 1990s. The trends in all three groups of data are considered under the assumption of long-term changes in the circulation in the upper atmosphere. 相似文献
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On the basis of the F2-layer critical frequency foF2 for the noon at some European stations for 1958–2005, it is found that the geomagnetic activity corresponding to the foF2 median is systematically lower than that averaged over the month; the difference increases with an increase in latitude. Moreover, the dispersion of geomagnetic activity for the foF2 median at relatively high latitudes is lower than at middle latitudes. These regularities are related to the fact that high geomagnetic activity usually leads to a distinct deviation of foF2 from the typical average value, i.e., from the foF2 median, and such deviation is more substantial at relatively high latitudes. That is why the geomagnetic activity for the foF2 median is lower at relatively high latitudes than at middle latitudes. 相似文献
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M. G. Deminov G. A. Zherebtsov O. M. Pirog V. N. Shubin 《Geomagnetism and Aeronomy》2009,49(3):374-380
A method for constructing the empirical model of the F2 layer critical frequency (foF2) under magnetically quiet conditions, aimed at analyzing disturbances of any nature, is proposed. This method has been analyzed, and typical features of regular changes in foF2 of the quiet ionosphere and day-to-day foF2 variability are analyzed using the data from Irkutsk and Slough stations as an example. In particular, it has been obtained that this model differs from the international IRI model, and this difference is mainly caused by the fact that the foF2 values in the IRI model do not correspond to quiet conditions. Therefore, this model gives a larger amplitude of the annual and semiannual variations in foF2 than the IRI model. In addition, this model more accurately reproduces the rate of foF2 annual variations at a fixed local time, especially in equinoxes, when foF2 variations can exceed 1 MHz within one month. 相似文献
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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. 相似文献
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Using the foF2 database obtained from satellites and ground-based ionospheric stations, we have constructed a global empirical model of the critical frequency of the ionospheric F2-layer (SDMF2—Satellite and Digisonde Data Model of the F2 layer) for quiet geomagnetic conditions (Kp < 3). The input parameters of this model are the geographical coordinates, UT, day, month, year, and the integral index F10.7 (day, τ = 0.96) of solar activity for a given day. The SDMF2 model was based on the Legendre method for the spatial expansion of foF2 monthly medians to 12 in latitude and 8 in longitude of spherical harmonics. The resulting spatial coefficients have been expanded by the Fourier method in three spherical harmonics with respect to UT. The effect of the saturation of critical frequency of the ionospheric F2-layer at high solar activity was described in the SDMF2 model by foF2 as a logarithmic function of F10.7 (day, τ = 0.96). The difference between the SDMF2 and IRI models is a maximum at low solar activity as well as in the Southern Hemisphere and in the oceans. The testing on the basis of ground-based and satellite data has indicated that the SDMF2 model is more accurate than the IRI model. 相似文献
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Using model simulations, the morphological picture (revealed earlier) of the disturbances in the F 2 region of the equatorial ionosphere under quiet geomagnetic conditions (Q-disturbances) is interpreted. It is shown that the observed variations in the velocity of the vertical E × B plasma drift, related to the zonal E y component of the electric field, are responsible for the formation of Q-disturbances. The plasma recombination at altitudes of the lower part of the F 2 region and the dependence of the rate of this process on heliogeophysical conditions compose the mechanism of Q-disturbance formation at night. The daytime positive Q-disturbances are caused exclusively by a decrease in the upward E × B drift, and this type of disturbances could be related to the known phenomenon of counter electrojet. Possible causes of formation of the daytime negative Q-disturbances are discussed. 相似文献
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Spatial and time variations in the critical frequencies foF2 before the strong earthquake of August 15, 1963, with the magnitude M = 7.75 are analyzed. The epicenter of the earthquake was located in the vicinity of the magnetic equator in the American longitudinal sector. The data of the topside ionosonde on board Alouette-1 and of the series of ground-based ionosondes has been used for this purpose. The ground-based ionosondes made it possible to detect an insignificant anomalous decrease in foF2 within the zone of earthquake preparation a few days prior to the earthquake. This result confirms the conclusion drawn earlier on the basis of the satellite data. The modification of the ionosphere at the F-region level is more evident in the satellite than in the ground-based data. It is also noted that the character of the time variations in foF2 a day before the earthquake is similar to the so called “quiet time” Q-disturbances in the ionosphere, when the electron concentration at the F region maximum differs from the median values by more than 20% under undisturbed geophysical conditions. 相似文献
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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. 相似文献
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The dependence of the correlation coefficient r(h, fo) between the stratospheric parameter h(100) and critical frequency foF2 revealed in the data of two solar cycles (1979–1989 and 1990–2000) on geomagnetic activity is analyzed. It is shown that the character of the r(h, fo) dependence on limitation on the Ap geomagnetic index is the same in both cycles but depends on the time of day and solar activity level for the given year. It is also found that there is a considerable difference in the absolute values of r(h, fo) between two cycles. 相似文献
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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. 相似文献
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E. V. Liperovskaya 《Geomagnetism and Aeronomy》2008,48(6):807-811
The observations of spread F during the nighttime hours (0000–0500 LT) have been statistically analyzed based on data of Tokyo, Akita, Wakkanai, and Yamagawa Japan vertical ionospheric sounding stations for the time intervals a month before and a month after an earthquake. The disturbances in the probability of spread F appearance before an earthquake are revealed against a background of the variations depending on season, solar activity cycle, geomagnetic and solar disturbances. The days with increased solar (Wolf number W > 100) and geomagnetic (ΣK > 30) activity are excluded from the analysis. The spread F effects are considered for more than a hundred earthquakes with magnitude M > 5 and epicenter depth h < 80 km at distances of R < 1000 km from epicenters to the vertical sounding station. An average decrease in the spread F occurrence probability one-two weeks before an earthquake has been revealed using the superposed epoch method (the probability was minimal approximately ten days before the event and then increased until the earthquake onset). Similar results are obtained for all four stations. The reliability of the effect has been estimated. The dependence of the detected effect on the magnitude and distance has been studied. 相似文献
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The daily samples of the hourly measurements of the foF2 critical frequency, obtained on January 5–21, 1995, at the midlatitude and high-latitude automated ionospheric stations (geographic latitude higher than 60°), are considered. The {fo} sets are transformed into the {δfoF2} sets of relative variations, for which asymmetry (A) and excess (E) are calculated. The selected stations are grouped into 20 pairs of automated ionospheric stations (AISs) located at distances of 200–10 000 km from one another. Sign estimates of the cross-correlation coefficients between the sets of 16 A and E values for different pairs of stations are used. Two types of structures of the statistical invariant spatial distribution are established: the structures with a scale of about 300 km, invariant with respect to latitude, and with a scale of about 6000 km (for only high latitudes). 相似文献
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A. V. Pavlov N. M. Pavlova S. F. Makarenko V. N. Shubin 《Geomagnetism and Aeronomy》2008,48(3):327-336
The structure and dynamics of the ionosphere and plasmasphere at low solar activity under quiet geomagnetic conditions on January 15–17, 1985, and July 10–13, 1986, over Millstone Hill station and Argentine Islands ionosonde, the locations of which are approximately magnetically conjugate, have been theoretically calculated. The detected correction of the model input parameters makes it possible to coordinate the measured and calculated anomalous variations in the electron density NmF2 at the height hmF2 of the ionospheric F2 layer over Argentine Islands ionosonde as well as the calculated and measured values of NmF2 and electron temperature at the hmF2 height over Millstone Hill station. It has been shown that vibrationally excited N2 and O2 molecules almost do not influence the formation of the winter anomaly under the conditions of low solar activity. A difference between the influence of electronically excited O+ on N e ions under winter and summer conditions forms not more than 11% of the N e winter anomaly event in the F 2 layer and topside ionosphere. The model without electronically excited O+ ions reduces the duration of the N e winter anomaly event. It has been shown that the seasonal variations in the composition of the neutral atmosphere form mainly the NmF2 winter anomaly event over the Millstone Hill radar at low solar activity. 相似文献
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A. D. Danilov 《Geomagnetism and Aeronomy》2011,51(3):321-328
This article considers sparse available data on variations in the main parameters of the ionospheric F2 layer foF2(ss + 2) and hmF2(ss + 2) at the end of the 1990s and the beginning of 2000s. It is shown that the monotonous behavior of hmF2(ss + 2) obtained in earlier publications for the period after 1980 is violated. The hmF2(ss + 2) behavior obtains a more complicated nature by time with a tendency towards a decrease in hmF2(ss + 2) at the beginning of a new century. A statistically significant relationship between foF2(ss + 2) and hmF2(ss + 2) is discovered confirming the Rishbeth statement that during the first hours after sunset, the critical frequency
foF2 is governed by dynamical processes via changes in the F2-layer height. It is found that at the end of the interval in question, there is a tendency towards deviations from the above-mentioned
dependence. The latter could be a manifestation of the fact that changes in the aeronomical parameters caused by the general
cooling and contraction of the thermosphere begin influencing the foF2 value. It is found that in the summer months, the foF2(ss + 2) value demonstrated a systematic decline tendency from the “boundary date” towards the beginning of the 2000s. 相似文献