Variation Of f0F2 Research Articles

The Impact of Coronal Mass Ejections on the Seasonal Variation of the Ionospheric Critical Frequency f0F2

We investigated the relations between the monthly average values of the critical frequency (f0F2) and the physical properties of the coronal mass ejections (CMEs), then we examined the seasonal variation of f0F2 values as an impact of the several CMEs properties. Given that, f0F2 were detected by PRJ18 (Puerto Rico) ionosonde station during the period 1996–2013. We found that the monthly average values of f0F2 are varying coherently with the sunspot number (SSN). A similar trend was found for f0F2 with the CMEs parameters such as the CME energy (linear correlation coefficient R = 0.73), width (R = 0.6) and the speed (R = 0.6). The arrived CMEs cause a plasma injection into the ionosphere, in turn, increasing the electron density, and consequently, f0F2 values. This happens in the high latitudes followed by the middle and lower latitudes. By examining the seasonal variation of f0F2, we found that the higher correlation between f0F2 and CMEs parameters occurs in the summer, then the equinoxes (spring and autumn), followed by the winter. However, the faster CMEs affect the ionosphere more efficiently in the spring more than in the summer, then the winter and the autumn seasons.

Simulation of long-term variations of the F2-layercritical frequency f0F2 at the northern tropical crest of ionization at Phu Thuy, Hanoi, Vietnam using the thermosphere-ionosphere-electrodynamicsgeneral circulation model (TIE-GCM)

In this work, the long-term variations of the simulated f0F2 by the NCAR thermosphere ionosphere-electrodynamics general circulation model (TIE-GCM) at the northern tropical crest of ionization at Phu Thuy-Vietnam (geographic latitudes 21.030N and longitude: 105.950E) during the period from 1962 to 2002 are examined to evaluate the ability of this model to reproduce the major features of the f0F2 as observed. The TIE-GCM simulates the influences of migrating and non-migrating diurnal and semidiurnal tides at the lower thermosphere, and changes of geomagnetic activity on the long-term variation of the f0F2. It reproduces well the diurnal and seasonal variations. We analyze the diurnal and seasonal variations of the observed f0F2 at Phu Thuy in approximately the same solar activity condition as in 1964, 1996 for the March and September equinoxes and June and December solstices. The local time and seasonal structures of these simulated results correspond well to those that are observed in 1964. On the contrary, the TIE-GCM model does not reproduce the amplitude observed at Thuy Phu in 1996. The TIE-GCM model with the chosen inputs does not yet allow us to explain well the long-term variations observed at Phu Thuy. We also try the different numerical simulations to understand how the long-term variation of the f0F2 is formed, how it relates to the current global system and its relationship with the thermosphere wind. The simulations show that the calculated NmF2 values are lower than the observed values. We find that the modeled contributions of the migrating and non-migrating diurnal and semidiurnal tides may cause them to play a major role in reducing the amplitude of the NmF2. The contributions of the integrated hemispheric power of auroral electrons and the cross polar cap potential seem to play an important role in increasing the amplitude of the NmF2. Keywords: F2 layer; Long-term trends; Ionosphere equatorial ionization anomaly; Ionosphere (ionospheric conductivities, ionospheric currents and electric field), thermospheric tides, Electrodynamics of the ionosphere (ionospheric dynamo).

Long-term trends in f0 F2 over Grahamstown using Neural Networks

Many authors have claimed to have found long-term trends in f0 F2 , or the lack thereof, for different stations. Such investigations usually involve gross assumptions about the variation of f0 F2 with solar activity in order to isolate the long-term trend, and the variation with magnetic activity is often ignored completely. This work describes two techniques that make use of Neural Networks to isolate long-term variations from variations due to season, local time, solar and magnetic activity. The techniques are applied to f0 F2 data from Grahamstown, South Africa (26 E, 33 S). The maximum long-term change is shown to be extremely linear, and negative for most hours and days. The maximum percentage change tends to occur in summer in the afternoon, but is noticeably dependent on solar activity. The effect of magnetic activity on the percentage change is not marked.

Periodicity of solar cycle from diurnal variations of f0F2 at Ibadan

Some features of diurnal variations of foF2 are found to show a better response of solar activity impact than the hourly means of diurnal variation over a sunspot cycle. The ionization build-up-rate and the difference in pre- and post-noon peak are found to give 11- year periodicity for Ibadan (7.4EšN, 3.9EšE, 6EšS dip) station while the hourly means are observed to give different periodicities. There is however, a phase lag between these features i.e. build-up rate of ionisation and the difference in post- and pre- noon peak of ionisation on one hand and the sunspot number on the other hand. The morning depression rate and evening decline do not however show solar cycle trend. For Singapore the solar cycle trend of ionisation build-up-rate and pre- and post noon peaks is slight while for Slough (51.5EšN, 359.4EšE, 66.5EšS dip) another station investigated for similar results, the ionisation build-up-rate, difference in post-and pre-noon peaks, morning depression rate and evening decline do not show any solar cycle trend. No solar cycle trend is observed in morning depression rate and evening decline for Singapore (1.3EšN, 103.8EšE, 17.6EšS dip). Key words: Diurnal, Build-up-rate, Periodicity, and Phase-lag.

The Characteristics and a Possible Mechanism of Semiannual Variation of f0F2

AbstractIonospheric data observed at different geomagnetic latitudes in East Asia and Australia sector and two polar stations during 1974‐1986 are used to analyse the characteristics of f0F2 semiannual variation. A new mechanism of the semiannual variation of ionospheric f0F2 is put forward in this paper. The semiannual variation of the diurnal tide in the lower thermosphere induces the semiannual variation of the amplitude of equatorial electrojet. This causes the semiannual variation of the amplitude of ionospheric equatorial anomaly through the fountain effect. This process induces the semiannual variation of f0F2.

Low latitude ionospheric effects near longitude 120°E during the great geomagnetic storm of july 2000

A great geomagnetic storm occurred on July 15/16, 2000 with a minimum value of about −300 nT in Dst index. Collecting digisonde data from ionospheric stations at Chungli, Wuhan, Kokubunji and Anyang, the ionospheric responses at the low latitudes near longitude 120°E during this storm are analyzed in this paper. There was a strong negative phase storm at low latitudes on July 16. The G-condition in the ionograms was clearly seen on the early first day after the commencement of geomagnetic storm. Those were considered to be caused by the storm-induced increase in the concentration ratios of neutral molecular O2 or N2 to atom O. On July 17 and some days thereafter, a positive phase storm appeared. In addition, anomalous equatorial ionization anomaly (EIA) inhibitions and developments were observed on July 16 and 17. There were also prominent nighttime enhancements in f0F2 during these days, and the diurnal variation of f0F2 was less clear than before.

Universal Diurnal Variation of F2-Layer Critical Frequency as Characteristic of Global Ionosphere Condition.

In this paper the universal diurnal variation of f0F2 is suggested to be used as the characteristic of the global ionosphere condition. The method was elaborated to obtain the universal diurnal variation of f0F2 by means of the Fourier series. This method allows us to use the small number of the observatories for the analysis. The possibility to distinguish the universal diurnal variation of f0F2 by the described method is shown as an example for December 1988 data (the period of the International Program SUNDIAL).

General Features of the Ionosphere at Thumba

The paper describes the characteristics of the ionosphere over the magnetic equator as revealed from the ionograms obtained by an automatic ionospheric sounder at Thumba. The well-known midday biteout of the f0F2 at the equatorial stations is almost absent during the winter months and is most pronounced during the equinoxes. The equinoctial maxima of f0F2 seem to be more pronounced for the midnight than for midday hours. The solar cycle variation of f0F2 for any particular solar hour are inversely proportional to the changes in the F2-layer height during the same period. Another interesting feature of the F2-layer at Thumba has been the development of extremely pronounced and sharp peak of hpF2 around 20 hr during the equinoxes. The absence of ionospheric echoes in the early morning hours is very predominant during the low sunspot years. The spread-F at Thumba is of range splitting type during early part of the night and frequency spread type during early morning hours. The midday f0F2 at Thumba are less ...

North-south asymmetry in storm time variation of f0F2

The study of Dst ( f 0 F2) variations at equatorial and subtropical stations at different longitude zones for 1951–1956, show that at equatorial stations of any in the zones the variation indicates an increase of f 0 F2 during the main phase. The northern subtropical stations in any of the zones indicate a decrease of f 0 F2 during the main phase. The southern subtropical stations show decreased f 0 F2 for Asian and African zones but at American and Pacific zones an increase of f 0 F2 during main phase of the storm is noted.

Lunar tidal variations in f0F2 in the American zone during period of low solar activity

The coefficients of lunar monthly (L1) and semi-monthly (L2) oscillations in f0F2 values at Huancayo for each solar hour are calculated for different seasons of a year of low solar activity. The amplitude of L1 does not have a distinct variation with the time of the solar day and is about 0.1–0.2 Mc./s. for any particular solar hour; when the variation of f0F2 with local lunar time is averaged for all solar hours the amplitude of L1 oscillation becomes negligibly small. The amplitude of L2 oscillation is significantly larger for the daytime than for the night time hours. The L2 amplitude for midday hours is largest during December solstice and least during June solstice. The solar daily variation of the L2 amplitude is shown to be closely connected with the development of the abnormal belt of the F2 region over the magnetic equator during the daytime. A close relationship is found to exist between the lunar tides in f0F2 and the horizontal magnetic intensity H at the magnetic equator.

A Study of Ionospheric Characteristics at Trivandrum during High and Low Sunspot Activity Periods

Analysis of ionospheric data obtained during low sunspot activity period corresponding approximately to the first half of IQSY at Trivandrum is considered in this paper. The diurnal and seasonal variation of f0F2, hpF2 and fEs are described. The N-h profiles obtained have been utilized to study the diurnal and seasonal variations of electron density at different heights and the subpeak electron content of the ionosphere. The true heights of maximum electron-density are compared with those obtained by assuming a parabolic distribution.The above results are compared with those obtained during high sunspot activity and the similarities and differences pointed out.

Observations on Geomagnetic Anomaly during Summer and Winter Months

Geomagnetic anomaly during summer and winter months is studied here. Asian and American zones have been considered separately. The range of latitudes over which the winter anomaly exists is found to be different at different hours. The analysis also shows that the latitudinal variation of f0F2 is asymmetrical with respect to the magnetic equator in both the Asian and American zones.

Some Peculiarities in the Maximum Ionization Density of the F2Layer

In the course of analysis of data on f0F2 at different Indian ionospheric stations on 75°E meridian, two peculiarities have been observed. Firstly, for the equatorial stations showing the bite-out effect, the change in ionization between low and high sunspot activity at the hour of post-noon peak of f0F2 is much higher than the corresponding change at the hour of the pre-noon peak. Such an effect is not observed at middle latitude stations. Secondly, when the variation of f0F2 with sunspot number is considered, the linear relationship which is generally assumed to exist between the two, breaks down at high sun-spot numbers at certain hours of the day. The point at which the departure from linearity takes place depends on the local time and on latitude.These effects appear to be due to abnormal increase in the thickness of the layer during high sunspot activity and meridional transport of ionization away from the equator in the evening hours.

Lunar Tide in the F2 Layer of the Ionosphere near the Geomagnetic Equator

IT is well known that the maximum in the lunar tide variation of f0F2 occurs at about 10 lunar hours at moderate geomagnetic latitudes and at about 04 lunar hours near the geomagnetic equator1,2. Recent analyses of the lunar variations at Singapore, Ibadan and Bombay have indicated non-agreement in the phases of the variation at stations having the same geomagnetic latitudes.

On the lunar semidiurnal variation of theD andF2 layers

The lunar semidiurnal variation of ionospheric absorption at Freiburg and of thef0F2 at, Genova, Freiburg and Leopoldville has been deduced. The results are compared with those obtained by other Authors. Magnetic dip, rather than geomagnetic latitude, is found to be a parameter controlling the morphology of the lunar semidiurnal variation of theF2 layer. Finally, a graphical representation of the amplitude of lunar semidiurnal variation off0F2 as function of the magnetic dip is also given.

MAGNETIC CONTROL ON THE VARIATIONS OF THE CRITICAL FREQUENCY OF THE F2 LAYER OF THE IONOSPHERE

The paper discusses the comparative influence of the true magnetic and smooth geomagnetic latitudes on the diurnal and latitudinal variations of the critical frequency of the F2 layer (f0F2) at low latitudes. The diurnal variations of f0F2 are shown to differ considerably at stations having the same geomagnetic latitude, but the discrepancies disappear when the true magnetic latitude is taken into consideration. The latitudinal variation of noon values of f0F2 is also shown to present discrepancies for low latitude stations in the geomagnetic latitudes plot, but on true magnetic latitude plots the points fall regularly along a smooth curve.

Ionospheric Disturbances associated with Magnetic Storms at Kodaikanal

F2 layer disturbances at Kodaikanal have been analysed for a study of the behavior of the critical frequency and the virtual height during geomagnetic storms. The disturbances have been classified for this purpose into two categories namely, the positive and negative. The chrematistics of Dst and SD variation of f0 F2 and the SD valuation of h' F at night have been discussed. The results have been explained in terms of, the quiet and disturbed day vertical drift velocities.

Geomagnetic Distortion of the F2 Region on the Magnetic Equator (II)

The change that takes place with the sunspot cycle of distortional characteristics in the daily variation of the F2-region on the magnetic equator is investigated herewith using Huancayo data during about one sunspot cycle. It is found that the type of the daily variation of f0F2 shows a systematic change with the sunspot cycle and that this change in type of f0F2 daily variation is also fairly closely connected with the phase shift in the geomagnetic Sq variations. Thus it would be confirmed that the change in type of the daily variation of f0F2 with the sunspot cycle is interpreted mainly by the phase shift of the vertical drift, associated with the geomagnetic variation.

On Anomalous Variations of Maximum Electron Density and its Height in the F2 Region of the Ionosphere

An explanation of anomalous variations of the maximum electron density (proportinal to (f0F2)2) and its height (hpF2) of the F2 region is attempted. For this purpose, the reexamination of the behaviours of f0F2, hpF2 and h′F2 in three seasons during sunspot maximum and minimum, is made and some interesting results are obtained. They are the followings. (i) The geomagnetic control of f0F2 is conspicuous in low and middle latitudes in there seasons. But above middle latitudes the geographic control is remarkable. This is applied above 30° in winter (greatest secχ) and equinox. (ii) f0F2 above 20° in December distributes approximately in such a way as f0F2∝cos1/2χ during sunspot minimum, but its distribution during sunspot maximum deviates from this line. (iii) The greater the zenith angle, the greater the increase of electron density from s. s. min. to s. s. max., and thus the magnitude of the seasonal anomaly of f0F2 is amplified far more during sunspot maximum than the minimum (iv) The value of hpF2 is greatest at geomagnetic equator throughout the year. (v) hpF2 becomes larger during sunspot maximum than the minimum. (vi) hpF2 in December does not increase at higher latitudes and rather decreases.From these facts it seems that the variations of f0F2 and hpF2 are controlled not only by the geomagnetic cause, but also by the cause which relates to the geographic latitude or solar activity. We investingate the geomagnetic distortion of daily variation of electron density due to the vertical electron drift, taking the diurnal and semi-durnal components into consideration, and show that the trough in geomagnetic equator can be explained by this drift. The velocities of two kinds of drift are deduced directly from the data. A reason for geographic control is described. It is shown that the seasonal anomaly of f0F2 cannot be accounted for only by the vertical drift. Therefore in order to explain the seasonal anomaly of f0F2 and other phenomena, (iii)-(iv), mentioned above, we suggest that there is at least the seasonal variation of the height distribution of the particle responsible for the F2 region.

The Solar Cycle and the F2Region of the Ionosphere

This paper presents a method of analyzing F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> -region critical-frequency data in a way that shows in a clear-cut manner the correlation that exists between monthly average values of these critical frequencies for undisturbed days and solar activity as measured by the central-zone character figures for calcium flocculi. Curves are presented which show for each month the expected diurnal variation of f0f2 for two values of solar activity. Other curves show for a number of different hours the expected seasonal variation of f0F2 at a constant time of day for the same values of solar activity.