Variation Of Critical Frequency Research Articles

Geophysical Effects of the Active Stage of the Stromboli Volcano Eruption on October 9, 2022

The response of the microbaric background and variations in the Earth’s magnetic and electric fields to the paroxysmal activity of the Stromboli Volcano (October 9, 2022, Italy) was analyzed using the results of instrumental observations carried out in the surface atmosphere. The stage of active volcanic eruption was accompanied by the generation of an acoustic signal and pronounced variations in the geomagnetic and atmospheric electric fields at considerable distances both during the period of volcano activity and with the arrival of the infrasonic wave at the observation point. The volcanic eruption was noted to cause a response as critical frequency variations of the F2 layer in the ionosphere.

Validation of F2-layer critical frequency variations in the ionosphere with radio observations of solar bursts

The high-frequency (HF) ionospheric cutoff restricts the possibilities of radio astronomical observations from the Earth, but on the other hand, it allows one to estimate the F2-layer critical frequency of the ionosphere. This effect has been measured using a new active antenna meant for receiving cosmic radio emission in the frequency range 1–40 MHz. The instrument was implemented near the Ukrainian T-shape Radiotelescope, second modification (UTR-2) and serves as a prototype of the HF antenna for a future radio array on the farside of the Moon. We detected directly a storm of solar radio bursts on May 22, 2021 and observed clearly their cutoff due to the ionosphere. For comparison our analysis of the experimental data was supplemented by simultaneous measurements from ionosondes located close to the UTR-2. The distance between the closest ionosonde near Zmiiv (Kharkiv region, Ukraine) and the radio astronomical antenna used in the experiment is 46.5 km. Results obtained from the solar radio records are consistent with the ionosonde data. We show that such an antenna implementation provides us with new opportunities to study the F2-layer critical frequency of the ionosphere.

Joint Ionosonde Studies of F2 Layer Critical Frequency Variations in the Ionosphere Over Kharkiv and Tromsø During Fall Equinox in Quiet and Disturbed Conditions

According to the results of joint ionosonde studies of variations in the ionospheric F2 layer critical frequency over Kharkiv and Tromsø during low solar activity for fall equinox on September 22 – 24, 2020, the features of foF2 variations in middle and low latitudes were investigated for magnetically quiet and magnetically disturbed conditions. On the magnetically quiet day of September 22, 2020, the foF2 values over Kharkiv were found to exceed the foF2 values over Tromsø for the entire time interval of joint observations 02:45 - 16:45 UT. Both over Tromsø and over Kharkiv, a rapid increase in foF2 to its local maximum value was observed after the sunrise. Quasi-periodic variations in foF2 were revealed at high latitudes, which had lower amplitude compared to variations in foF2 over Kharkiv. Over both measuring sites, a pre-sunset local maximum in foF2 was observed. During magnetically disturbed conditions over Tromsø and Kharkiv, quasi-periodic fluctuations in foF2 were observed after the sunrise. Oscillations over Tromsø had lower amplitude than over Kharkiv, and were almost completely suppressed after the onset of a strong magnetic disturbance at high latitudes on September 23, 2020. The foF2 values over Tromsø exceeded its values over Kharkiv in a time interval of 10:45 – 12:15 UT. Comparison of the time variation of foF2 over Tromso on a magnetically quiet day, September 22, 2020, and on a magnetically disturbed day, September 23, 2020, showed that the foF2 value for September 23, 2020 from 10:15 to 15:00 UT exceeded the foF2 values for the same period on September 22, 2020. Comparison of the temporal variations in foF2 over Kharkiv on a magnetically quiet day, September 22, 2020, and on a magnetically disturbed day, September 24, 2020, showed that the foF2 value for September 24, 2020 exceeded its value for September 22, 2020 from 03:00 to 04:45 UT and from 07:00 to 13:00 UT. Magnetic disturbances were found to cause a rapid increase in foF2 values both over Kharkiv and Tromsø, which exceeded foF2 values under magnetically quiet conditions, and also led to a significant increase in the relative amplitudes of traveling ionospheric disturbances over Kharkiv.

Comparative analysis of variations in the critical frequency of the ionospheric F2 layer under different heliogeophysical conditions at the Moscow station

Results of the statistical analysis of variations of critical frequency of a layer of F2 of an ionosphere at various levels of solar and geomagnetic activity are presented by data from the station Moscow from 1957 to 2019. Optimum intervals of division into levels of geomagnetic activity are experimentally found. The need to divide solar activity into low and high in ionospheric research has been proved. The effect of solar and geomagnetic activity on the occurrence of periodic positive and negative deviations of the critical frequency of the ionosphere F2 layer from the median, calculated for 27 previous days in various seasons, is shown. It has been shown that the daily course of deviations at a high level of geomagnetic activity for negative deviations ceases to be pronounced, unlike positive deviations.

Analysis of Relationship Between Ionospheric and Solar Parameters Using Graphical Models

AbstractFor the investigation of time variations of critical frequency (foF2), solar radiation flux at 10.7 cm wavelength (F10.7 index) and geomagnetic activity index Kp, we use conditional independence graphical models which describe and transparently represent the structure of relationships in the time series. We employ multivariate statistic methods applied to daily observational data obtained from midlatitude ionosondes within the period 1994–2009 (23rd Solar Cycle). It is demonstrated that conditional independence graphical models represent a robust method of multivariate statistical analysis, useful for finding a relation between one of the main ionospheric parameters and space weather conditions. This method appears to be more appropriate than correlation analysis between foF2 and the main geomagnetic and solar indices, especially for long‐term data for which the model characteristics may change or time series can be interrupted. We compare the results obtained by the graphical models method with cross‐correlation analysis. We found that the method is suitable for the analysis of the dependence between foF2 values and time‐shifted F10.7 time series. In particular, we clearly identified +0 day shift in all cases, and +1‐, +2–3‐, and +4–5‐day shifts in European, American, and East Asia sector, respectively. The conditional independence graphs method can be applied even in the case when classical parametric methods are not convenient.

Intensity Variations of OI 557.7 nm and OI 630.0 nm Lines before M5.0 and Greater Earthquakes in Tokyo Region, Japan, in 1979 to 1990

The intensity variation of 557.7 nm oxygen green line and 630.0 nm oxygen red line at Kiso observatory, Tokyo, and variations in critical frequency of the ionospheric F2 layer (foF2) and semi transparency coefficient related to the E layer before earthquakes with magnitude ≥ 5 are considered for 1979 to 1990. The intensity of both the lines increases within six days before earthquakes; foF2 and semi transparency coefficient abnormally increase before enhancement of airglow. The superposed epoch analysis of each variable is done for further confirmation. Some physical processes in atmosphere-ionosphere in pre-seismic days enhance the concentration of electron and neutral molecules in the ionospheric F and E regions. These events influence the dissociative recombination processes related to oxygen emission, and so the intensity of two line emissions show some special type of variations during earthquake events.

Global Variations in Critical Frequency of the F2 Layer in Various Models of Solar EUV Radiation

The paper presents the results of modeling ionospheric parameters using the Global Self-Consistent Model of the Thermosphere, Ionosphere, and Protonosphere (GSM TIP) of the Earth. Numerical experiments have been performed for three variants of the solar EUV spectrum data: the Nusinov model, EUVAC, and generalized solar flux measurements (SOLID). The calculations have been carried out on March 22, 2009 (F10.7 ~ 68) and March 22, 2014 (F10.7 ~ 154). The maximum ionization rate and, correspondingly, the critical frequency of the F2 layer have been obtained using the Nusinov model for low and high solar activities. The lowest ionization rates and values of foF2 have been obtained using the EUVAC model. Simulations with the SOLID spectrum show the better agreement of foF2 with the empirical model IRI-2007 at the maximum solar activity. At the same time, at the minimum solar activity, all models of the spectrum provide an underestimation of the values of the critical frequency of the F2 layer.

Planetary Wave Periods in foF2 Time Variations Based on Winter Data from Kaliningrad Station in 2008−2010

The study presents the results of the analysis of the F2-layer critical frequency variations obtained for the winter periods of 2008–2010, during which sudden stratospheric warmings were observed. The data were obtained at Kaliningrad ionospheric station (54.6° N, 20° E) with the Parus digital ionosonde in standard sounding mode. The mean daily foF2 values were used in the analysis. The results of spectral analysis based on continuous wavelet transform showed that, during all of the warmings that occurred in 2008–2010, the foF2 time variations demonstrated the presence of wave processes with periods of approximately 5−6 days, as well as more extended processes with periods of ~10−13 and 23−30 days. These periods coincide with the characteristic periods of planetary waves observed in the mesosphere during sudden stratospheric warmings, while the 13- and 30-day periods can be conditioned by the influence of the Sun.

Diurnal and Seasonal Variation of Critical Frequency in Malaysia from 2005 to 2007

Ionospheric layer plays an important role as the main medium for communication of wireless radio in the High Frequency (HF) band. Critical frequencies are important parameters of the ionosphere. Critical frequencies vary diurnally following the pattern of day and night due to the activities of the sun. The critical frequency for Parit Raja station is determined using an ionosonde at latitude 1° 52' N and longitude 103° 48' E, which are close to the geomagnetic equator at geographic latitude 5° N. This work analyses the dependence of the critical frequency during the solar minimum of Solar Cycle 23. From the observations, the critical frequency observed in 2005 was the highest, corresponding to a higher sunspot number of between 11 and 192. The general trend of the daily critical frequency was 6.8 MHz to 12.0 MHz from 00:00 to 17:30 Coordinated Universal Time (UTC) and decreasing to 7.0 MHz from 18:00 to 23:00 UTC. The average value of the critical frequency for 2005 to 2007 was 7.5 MHz, with a minimum of 1.8 MHz and a maximum of 11.0 MHz. The results showed a slow decrease from 2006 to 2007 for the critical frequencies in 2006 and 2007.

The Planetary Wave Activity in Temperatures of the Stratosphere, Mesosphere and in Critical Frequencies of Ionospheric F2 Layer

A large body of experimental evidence lends credit to the existence of a close relationship between ionospheric parameters and the underlying atmosphere. Vertical-incidence ionospheric sounding data and temperature measurements at stratospheric (30 km) and mesospheric (80 km) heights have been used in investigating the interrelation between the occurrence of fluctuations with periods of planetary waves in temperature variations at different heights and inF2layer critical frequency variations (foF2) under low solar activity conditions for the time interval 2006-2007. The distinctive characteristics of the manifestation of common periodicities of the wave structures under consideration are discussed. The statistically significant periods of stratospheric and mesospheric temperature fluctuations vary from 11 to 49 days, whereas foF2 periodograms show higher-frequency fluctuations with periods of 5, 8, 9, and 10 days. The study revealed a similarity between periodic structures for the variations in the parameters involved.

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 iono� spheric models. The inversion of critical frequency is analyzed with the hourly values of foF2 of seven iono� spheric 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 interseasonal 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.

Diurnal and seasonal variation of F2-layer ionospheric parameters at equatorial ionization anomaly crest region and their comparison with IRI-2001

Diurnal and seasonal variations of critical frequency of ionospheric F2-region ‘foF2’ and the height of peak density ‘hmF2’ are studied using modern digital ionosonde observations of equatorial ionization anomaly (EIA) crest region, Bhopal (23.2°N, 77.6°E, dip 18.5°N), during solar minimum period 2007. Median values of these parameters are obtained at each hour using manually scaled data during different seasons and compared with the International Reference Ionosphere-2001 model predictions. The observations suggest that on seasonal basis, the highest values of foF2 are observed during equinox months, whereas highest values of hmF2 are obtained in summer and lowest values of both foF2 and hmF2 are observed during winter. The observed median and IRI predicted values of foF2 and hmF2 are analyzed with upper and lower bound of inter-quartile range (IQR) and it is find out that the observed median values are well inside the inter-quartile range during the period of 2007. Comparison of the recorded foF2 and hmF2 values with the IRI-2001 output reveals that IRI predicted values exhibit better agreement with hmF2 as compared to foF2. In general, the IRI model predictions show some agreement with the observations during the year 2007. Therefore it is still necessary to implement improvements in order to obtain better predictions for EIA regions.

Spatial inhomogeneity of variations in the F 2 layer critical frequencies under the conditions of winter 1995

It is necessary to study properties of largescale ionospheric inhomogeneities in the F 2 layer in order to construct theoretical models of this phenomenon. An analysis of statistical properties of the sets of relative variations in the {δfoF2} critical frequency is one of the methods of such study. In this case it is of special importance to establish the measure of deviations from the standard distribution for the probability den� sity function of the δfoF2 value. Such deviations indi� cate that structural formations, corresponding to largescale inhomogeneities in this case, are present in the {δfoF2} set. The third and fourth statistical invari� ants (asymmetry A and excess E) are the measures of such deviations (Handbook, …, 1965). In reality, the sample volume is always finite. Therefore, studying the statistical properties of the sets of critical frequency relative variations {δfoF2} has a number of specific methodological features. The main feature consists in a deficit of information, owing to which a standard distribution is mostly used to model the ionospheric statistics. However, it is possible to overcome the limi� tations of the model of δfoF2 statistical variations in the form of a standard distribution, specifically, by using selective asymmetry A and excess E of a set, which are more complex nonlinear operators of the δfoF2 set scalarization and are the parameters of the new mathematical model of the probability density function. The aim of this work is to consider the spatial variability of asymmetry and excess of the relative vari� ations in the critical frequency of the ionospheric F2 layer at middle and high latitudes, using the set of data on the critical frequency variations in the highlatitude F 2 layer in January 1995.

Height and critical frequency variations of the sporadic-E layer at mid-latitudes

Height and critical frequency variations of the sporadic-E layer at mid-latitudes

Height and critical frequency variations of the sporadic-E layer at midlatitudes

The present study concerns variations in height and critical frequency of sporadic E layer over a wide period range of hours to several days, covering tidal and planetary oscillation domain. Besides periodicities in the tidal and planetary range that are known to occur within time series of critical frequencies ( foEs) of sporadic-E layer [Pancheva, D., Haldoupis, C., Meek, C.E., Manson, A.H., Mitchell, N.J., 2003. Evidence of a role for modulated atmospheric tides in the dependence of sporadic E layers on planetary waves. Journal of Geophysical Research 108, Art. No. 1176; Haldoupis, C., Pancheva, D., Michell, N.J., 2004. A study of tidal and planetary wave periodicities present in midlatitude sporadic E layers. Journal of Geophysical Research 109, Art. No. A02302] among others, we evidence the existence of the 4-day planetary wave well developed in the height of sporadic E time serie ( hEs). Moreover it is shown that the central-period of the diurnal tidal component of hEs is not exactly 24 h but it varies between 22 and 26 h at the planetary wave period. At a first glance, this is surprising since the origin of the diurnal tide is a forced oscillation with a 24 h period due to the Earth rotation and the periodic heating of the atmosphere by the Sun. Our interpretation is based on the perturbation of the height of the Es layer imposed by the planetary wave. In this mechanism the Es layer is moved up and down by the planetary wave producing a Doppler effect and resulting in a shift of the central-period around 24 h. With this interpretation, the excursion of the central-period is related to the vertical velocity perturbation of the Es layer due to the planetary wave. For a central period varying between 22 and 26 h the peturbation velocities are 0.026 and - 0.022 m / s , respectively.

Spatial and time variations in critical frequencies of the ionospheric F region above the zone of equatorial earthquake preparation

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.

Decentralized Control System Design for Multivariable ProcessesA Novel Method Based on Effective Relative Gain Array

In this paper, a novel method for design of a decentralized control system for multivariable processes is proposed. On the basis of a new interaction measure, effective relative gain array (ERGA), in terms of energy transmission ratio, loop interactions are quantified by two elements, i.e., relative gain and relative critical frequency. The interaction effects for a particular loop from all other closed loops are analyzed through both steady-state gain and critical frequency variations. Consequently, appropriate detuning factors for decentralized controllers under different interaction conditions can be derived based on the effective relative gain, relative gain, and relative critical frequency. The design method can be effectively used for both normal processes as well as process-loop transfer functions containing unstable zeros resulted from other closed loops. This design method is simple, straightforward, and effective and can be easily understood and implemented by field engineers. Several multivaria...

Short-term critical frequency variations and their predictions in the midlatitude ionospheric F2 region

Short-term variations δf0F2 in the values of the critical frequency of the ionospheric F2 region in middle latitudes due to solar and geomagnetic activities have been investigated. Diurnal and seasonal features of the energy flow from the auroral into midlatitude ionosphere are revealed. It is shown that they could be taken into account if instead of the 3-hour geomagnetic indices or their daily averages a new index is employed which characterizes the average level of geomagnetic activity over intervals of time no less than nine hours usually during the evening and night hours. A technique for short-term predicting δf0F2 in the midlatitude ionosphere is developed which employs the indices of solar and geomagnetic activities, and errors in the predictions are estimated.

Principles and Method of Forecasting the Critical Frequency Variations of the Mid-Latitude Ionospheric F2 Region From Solar and Geomagnetic Activity Indices

Principles and Method of Forecasting the Critical Frequency Variations of the Mid-Latitude Ionospheric F2 Region From Solar and Geomagnetic Activity Indices

On the ionospheric effects of asthenospheric earthquakes

Abstract Characteristic foF2 variations during preparation of the asthenospheric Hindukush earthquake of 12 September 1983 are investigated. Stable effects in F2 layer critical frequency variations are revealed which can be considered as earthquake precursors. The existence of stable enhancement of foF2 night values 1.5–2 days before an earthquake shock is demonstrated, as well as a steep decrease of foF2 evening values 1 day before the earthquake. These phenomena are regionally restricted.