Frequency Of F2 Layer Research Articles

Opposite Latitudinal Dependence of the Premidnight and Postmidnight Oscillations in the Electron Density of Midlatitude F Layer

AbstractThe dense observation points of the oblique‐incidence ionosonde network in North China make it possible to discover the ionospheric regional variations with relatively high spatial resolution. The ionosonde network and the Beijing digisonde are used to investigate the ionospheric nighttime oscillations in January and February 2011. The electron density enhancements occurring before and after midnight present the obvious opposite latitudinal dependence in the time‐latitude maps, which are composed by the differential critical frequency of F2 layer. The premidnight enhancements (PRMEs) appeared earlier in the north and then moved to south. The postmidnight enhancements (POMEs) did the opposite. The data analysis shows that the PRME was a part of the large‐scale traveling ionospheric disturbance (LSTID), which may be produced by gravity waves. The southward propagation of the LSTIDs is considered to form the positive latitudinal dependence of the wave peaks and troughs. The postmidnight F layer oscillation was composed by a single enhancement and a single decline following it. The westward electric field‐induced E × B drift, which pushed the F layer to lower altitudes with higher recombination loss, was most likely to compress the plasma and produce the POMEs. Along with the continuously dropping of the layer, the recombination loss exceeded the density increase due to the compression effect and then the electron density decline appeared.

A severe negative response of the ionosphere to the intense geomagnetic storm on March 17, 2015 observed at mid- and low-latitude stations in the China zone

In this paper, the critical frequency of F2 layer of the ionosphere (foF2) and the total electron content (TEC) recorded at mid- and low-latitude observation sites near 120°E in the China zone were used to investigate the response to a severe geomagnetic storm on March 17, 2015 (the minimum Dst −223 nT at 23 UT). The results showed that the strong geomagnetic storm caused a massive effect on the ionosphere. The characteristics of foF2 and TEC did not show obvious perturbation during the main phase. Severe depletion of foF2 and TEC was observed at all stations during the storm recovery period. The maximum absolute discrepancy in TEC compared with the past 27-day average value was 78 TECU, and the minimum percentage deviations reached −71% at Fuzhou (26.1°N,119.3°E). The minimum percentage deviations of decrease in foF2 reached −65% at Sanya (18.1°N,109.3°E) and Mohe (53.5°N,122.3°E). This was an infrequent negative effect that foF2 and TEC sustained throughout the day with extremely low level on March 18. The O/N2 rate showed a distinct reduction on March 18 in the China zone, which may be mainly responsible for the severe depletion of foF2 and TEC. The spread-F seemed to be developed at first but was then suppressed to some extent during the main phase. During the recovery phase, the spread-F was suppressed at Sanya, while it developed at Wuhan and Mohe. The disturbance electric fields and thermospheric circulation may contribute to this phenomenon.

Unusual Change in Critical Frequency of F<sub>2</sub> Layer during and Prior to Earthquakes

The unusualness in the critical frequency of different layers of earth’s ionosphere is commensurate to be associated with seismic events. We present study of critical frequency of F2 layer (denoted as f0F2) during some major earthquakes in South American region. We use the semi-empirical Barbier’s theorem of air-glow and define a parameter using the critical frequency and virtual height of F2 layer and named it as “F Parameter”. To investigate the variation of this parameter, we consider five large earthquakes in the junction of Nazca plate and South American plate having magnitude greater than M > 6.5 and study the temporal variation of F parameter during these earthquakes. The F Parameter is measured using the ionograms as recorded from the Ionosonde in Jicamarca Radio Observatory (lat. 11.95°S, long 76.87°W) in Chile which lies within the earthquake preparation zones of these five earthquakes. We examine the F Parameter within a span of ±15 days during earthquakes and observed significant change in the evaluated F Parameter in 12 to 3 days prior to the earthquakes. The increment is over +3σ from the normal variation. We also observe significant changes during aftershock events. The solar geomagnetic indices were found to be low which ensures that these anomalies in F Parameter are due to seismic events.

Multisite Remote Sensing for Tsunami-Induced Waves

In the 2011 Tohoku tsunamigenic earthquake, ionospheric anomalies generated by tsunami-induced gravity waves were observed by many types of instruments. Three digisondes and one Doppler receiver located in East Asia were applied to investigate the far-field ionospheric response to the westward-propagating gravity waves generated by the earthquake. Based on time–period spectrum analysis, oscillations between 20 and 36 min on the $fo\text{F}2$ (critical frequency of F2 layer) curves and Doppler variations of each observation location were identified. The horizontal group speed, generation time, and source location estimated with the ray-tracing method all indicated that the periodic disturbances recorded by the four radio systems were gravity waves induced by the tsunami following the 2011 Tohoku earthquake. The plasma frequency variations at five fixed altitudes in the ionospheric F2 layer over I-Cheon were used to investigate the vertical propagation of the tsunami-associated gravity waves. The measured phase progression in the vertical direction was opposite that of the energy transport, which further confirmed that the recorded waves were atmospheric gravity waves. The use of multisite remote sensing for examining tsunami-induced waves in the ionosphere may open new perspectives in oceanic monitoring and future tsunami warning systems.

Midlatitude ionospheric changes to four great geomagnetic storms of solar cycle 23 in Southern and Northern Hemispheres

AbstractThis paper presents an investigation of ionospheric response to great (Dst ≤−350 nT) geomagnetic storms that occurred during solar cycle 23. The storm periods analyzed are 29 March to 2 April 2001, 27–31 October 2003, 18–23 November 2003, and 6–11 November 2004. Global Navigation Satellite System, total electron content (TEC), and ionosonde critical frequency of F2 layer (foF2) data over Southern Hemisphere (African sector) and Northern Hemisphere (European sector) midlatitudes were used to study the ionospheric responses within 15°E–40°E longitude and ±31° to ±46° geomagnetic latitude. Midlatitude regions within the same longitude sector in both hemispheres were selected in order to assess the contribution of the low‐latitude changes especially the expansion of equatorial ionization anomaly (EIA) also called the dayside ionospheric superfountain effect during these storms. In all storm periods, both negative and positive ionospheric responses were observed in both hemispheres. Negative ionospheric responses were mainly due to changes in neutral composition, while the expansion of the EIA led to pronounced positive storm effects at midlatitudes for some storm periods. In other cases (e.g., 29 October 2003), penetration electric fields, EIA expansion, and large‐scale traveling ionospheric disturbances were found to be present during the positive storm effect at midlatitudes in both hemispheres. An increase in TEC on the 28 October 2003 was because of the large solar flare with previously determined intensity of X45 ±5.

Solar Transients Disturbing the Mid Latitude Ionosphere during the High Solar Activity

We investigate the effect of solar transients on the mid latitude ionosphere during the high solar activity period of solar cycle 23 i.e 2003 and 2004. A mid latitude station, Guangzhou (23.1N, 113.4E) was selected to carry out the investigation. The ionospheric behaviour at the selected station is characterized by considering the critical frequency of F2 layer (foF2) obtained by using the ground based Ionosonde observations. Then we selected two types of solar transients viz. solar flares and Coronal Mass Ejections (CMEs). To quantify the effect of solar flares we have considered the X-ray flux (1-8 Å) and EUV flux (26-34nm). Similarly to quantify the effect of CMEs, we have considered the geomagnetic storms, because during high solar activity the geomagnetic storms are caused by CMEs. From our analysis we conclude that during the geomagnetic storms the value of foF2 decreases as compared to quiet days thereby showing a negative effect. On the contrary we found that during solar flares there is sudden and intense increase in foF2. We also performed a correlation analysis to access the magnitude of association between changes in flux values and peak values of Dst during flares and storms with the corresponding changes and peak values of foF2. We found that a strong correlation exists between the enhancements/decrements in foF2 and enhancements in flux values and Dst. We conclude, while geomagnetic activity suppresses ionospheric activity the flares enhance the same.

Morphological Investigation of Disturbed Ionosphere during Intense Geomagnetic Storms

Geomagnetic Storms are the disturbed magnetic conditions, influenced and induced by Interplanetary Magnetic Field and the Charged Particle's motion around the Earth, respectively, in Geospace. As the ionosphere is woven by the earth's magnetic field it responds to the change in that. During the geomagnetic storms the filled-in plasma between the magnetic field lines, geomagnetic flux tubes, redistributes itself in effect of the magnetic field forcing. In the present study we have done the investigation of the morphology of the ionosphere over the mid and high latitude regions during intense Geomagnetic Storms. We got fairly convincing results; in three cases decrease of the critical frequency of F2 layer (foF2) and in one case enhancement of the critical frequency of F2 layer (foF2) at mid and high latitudes.

Validation of the TEC2F2 model over the African equatorial region

In this paper a statistical method, TEC2F2, of estimating critical F2 layer frequency (foF2) from Global Positioning System (GPS) Vertical Total Electron content (VTEC) is extended from Africa’s mid-latitudes to the equatorial region, and the results validated. The equatorial region is one of the most dynamic yet under-represented over Africa in ionospheric studies. The TEC2F2 method was first considered for the South African region (mid-latitudes). This region is covered by a network of ionosondes that provided a validation platform for the TEC2F2 method before being applied to other parts of Africa. The results show that over the African equatorial region, the TEC2F2 method is a potential candidate in more accurately estimating the foF2 parameter than the most recent version of the International Reference Ionosphere (IRI-2012) model.

Ionospheric variability over Indian low latitude linked with the 2009 sudden stratospheric warming

AbstractIn this paper, we analyze radar observations of E × B drift and plasma irregularities, ionosonde observations of E and F layer parameters including spread F, and magnetic field observations made from Indian low latitudes linked with the 2009 sudden stratospheric warming (SSW) event. E × B drift variations presented here are the first of their kind from the Indian sector as far as the effect of SSW is concerned. Difference of magnetic fields observed from the equator and low‐latitude (∆H) and E × B drift show linear relation, and both show remarkably large positive values in the morning and negative values in the afternoon exhibiting semidiurnal behavior. Remarkable changing patterns in the critical frequency of F2 layer (foF2) and F3 layer (foF3) were observed after the occurrence of SSW. Large variations with quasi 16 day periodicity were observed in ∆H, foF2, and foF3. Both semidiurnal and quasi 16 day wave modulation observed after the 2009 SSW event are consistent with those reported earlier. We also noted quasi 6 day variations in ∆H and foF2 soon after the SSW commencement, not much reported before. During the counterelectrojet events linked with the SSW event, while equatorial Es (Esq) disappeared as expected, there were no blanketing Es (Esb), a finding not reported and discussed earlier. Esb was also not formed at the off‐equatorial location, indicating the absence of required vertical wind shear, but E region plasma irregularities were observed by the ionosonde and radar with a close relationship between the two. Weak F region irregularities were observed in the postmidnight hours, and case studies suggest the possible role of SSW‐related background electric field in the manifestation of postmidnight F region irregularities.

Analysis of ionospheric anomaly preceding the Mw7. 3 Yutian earthquake

: On February 12, 2014, a large Mw7. 3 earthquake occurred in Yutian of Xijiang Province, China. We processed the global ionosphere maps provided by CODE (the Center for Orbit Determination in Europe) and the foF2 (the critical frequency of F2-layer) data of Chongqing ionosonde station to analyze the pre-earthquake ionospheric anomalies. Solar activities and magnetic storm were checked by the sliding inter quartile range method to remove their effects on the ionosphere. A positive ionospheric anomaly with the large amplitude of 20 TECU was observed near the epicenter on February 3 (10th day before the earthquake). In addition, the foF2 at Chongqing station had an unusual increase of more than 40% on the day, which was consistent with the TEC (Total Electron Content) anomaly. The global disturbance represents that the peak of TEC anomaly didn't coincide with the vertical projection of epicenter. The TEC anomalous area was closer to the equator, and it mainly occurred from local time 16 : 00 to 20 : 00. An enhancement of TEC with the small amplitude also appeared in the magnetically conjugated region.

Ionospheric Anomalies before/after the 1960 Agadir Earthquake

This paper focuses on the ionospheric anomalies observed in the F2 layer at Tortosa in Ebre Observatory (40.8oN, 0.49oE), before and after a seismic event in Morocco (Agadir). The data on optimal frequency of F2 layer (f0F2) were acquired using ground based ionosonde. The median-based statistical technique was applied to the data to examine the possible relationship between the earthquake displacement field and the variation of the f0F2 in the ionosphere. The results point to a variation characterized by absence of f0F2 values over four days before the earthquake and continue for almost three days after the day of earthquake. The diurnal monthly median of the f0F2 values also revealed anomalous which has been considered as supporting evidence for the observed correlation.

Geomagnetic Disturbances and Its Impact on Ionospheric Critical Frequency (foF2) at High, Mid and Low Latitude Region

A geomagnetic storm is a major disturbance of Earth's magnetosphere that occurs when there is a very efficient exchange of energy from the solar wind into the space environment surrounding Earth. During solar and geomagnetic activities, critical frequency of F2 layer (foF2) varies in a great extent. In this Study, our main aim is to examine the effect of solar and geomagnetic activities on the critical frequency (foF2) during January 2014 to April 2014 respectively. One magnetic intense storm occurred on 19 February with (Dst -112) and other moderate storm occurred on 12 April 2014 with (Dst -80). In our study, we have analyzed these effects on critical frequency of F2 layer for ionospheric monitoring. We have used ionospheric data at Low, mid and high latitude station. The absorption and ionization of the ionospheric medium depends on solar activity. The value of foF2 increased from their normal value at all the three latitudes. This is due to geomagnetic storms that occurred around the same time. A very interesting feature that can be seen in the figures is that the increase of foF2 at Low latitude is much more intense as compare to high and mid latitude. Comparison among all the latitudes shows that the values of foF2 at high latitude are quite less as compared to low and mid latitude. We have found that the effect of solar and geomagnetic storm disturbances is strongest at the low latitude and weakest at the high latitude during the geomagnetic storm time.

Unusual Enhancements of NmF2 in Anyang Ionosonde Data

Sudden enhancements of daytime NmF2 appeared in Anyang ionosonde data during summer seasons in 2006-2007. In order to investigate the causes of this unusual enhancement, we compared Anyang NmF2's with the total electron contents (GPS TECs) observed at Daejeon, and also with ionosonde data at at mid-latitude stations. First, we found no similar increase in Daejeon GPS TEC when the sudden enhancements of Anyang NmF2 occurred. Second, we investigated NmF2's observed at other ionosonde stations that use the same ionosonde model and auto-scaling program as the Anyang ionosonde. We found similar enhancements of NmF2 at these ionosonde stations. Moreover, the analysis of ionograms from Athens and Rome showed that there were sporadic-E layers with high electron density during the enhancements in NmF2. The auto-scaling program (ARTIST 4.5) used seems to recognize sporadic-E layer echoes as a F2 layer trace, resulting in the erroneous critical frequency of F2 layer (foF2). Other versions of the ARTIST scaling program also seem to produce similar erroneous results. Therefore we conclude that the sudden enhancements of NmF2 in Anyang data were due to the misrecognition of sporadic-E echoes as a F-layer by the auto-scaling program. We also noticed that although the scaling program flagged confidence level (C-level) of an ionogram as uncertain when a sporadic-E layer occurs, it still automatically computed erroneous foF2's. Therefore one should check the confidence level before using long term ionosonde data that were produced by an auto-scaling program.

Comparison of foE and M(3000)F2 variability at Ibadan, Singapore and Slough

The variability, VR, of critical frequency of E-layer, foE, and ionospheric propagation factor, M(3000)F2 at Ibadan (7.4°N, 3.9°E, 6°S dip) is investigated for local time, seasonal and solar cycle variations. Latitudinal influence of these characteristics is sought by comparison with foE VR and M(3000)F2 VR of Slough ( 51.5°N, 359.4°E, 66.5°N dip) in the European sector, and Singapore (1.3°N,103.8°E, 17.6°S dip) in the Asian sector. While the pattern of foE VR is similar to those of other F2 characteristics with characteristic peaks around dawn and dusk, M(3000)F2 VR shows no clear diurnal trend.A lower bound of foE VR is usually 3% while the maximum VR ranges between 8% and13% at post-sunrise and pre-sunset hours at all the epochs, M(3000)F2 VR is however lower during MSA (about 9%) than during LSA and HSA when it is 4% to about 12–14%. Generally, daytime M(3000)F2 VR is greater than that of foE VR by between 5% and 10%. Furthermore, no latitudinal difference is observed in both characteristics during both HSA and MSA. While nighttime M(3000)F2 VR is about half that of nighttime foF2 VR (the critical frequency of F2-layer ) VR, daytime VR of both characteristics are about equal during the three epochs at Ibadan. For Slough, nighttime M(3000)F2 VR and nighttime foF2 VR as well as the daytime VR of both characteristics are about equal. This difference is most likely due to latitudinal effect.

Revealing the classes of ionospheric disturbances on the basis of multiyear data on the critical frequency of the F2 layer

The bases of the classification method of ionospheric disturbances caused by solar-geomagnetic activity on the basis of the critical frequency of the F2 layer are developed. Data for the total solar activity cycle from 1975 to 1986 were used for studying variations in the critical frequency of the ionospheric F2 layer. The critical frequency was measured at the Moscow ionospheric observatory (55°45′N, 37°37′E) at an interval of 1 h. The gaps in the critical frequency values were filled in by the cubic interpolation method. The solar activity level was estimated using the F10.7 index. The geomagnetic disturbance was determined using the Kp · 10, Dst, and AE indices. According to the developed classification, an index of ionospheric activity is introduced. An analysis of the obtained values of the index for years of solar activity minimum and maximum shows that an increase in the absolute values of the index as a rule occurs at an increase in global geomagnetic and/or auroral disturbances. This fact indicates the sufficient information content of the developed index for characterizing ionospheric activity in any season. Moreover, using the sign of the index, one can form an opinion regarding an increase or decrease in the concentration of the ionospheric F2 layer, because the values of the considered index correspond to real oscillations in the critical frequency of the midlatitude ionosphere.

Comparative study of the equatorial ionosphere over Jicamarca during recent two solar minima

It is a critical issue that whether or not the extremely deep solar minimum of solar cycle 23/24 brought serious influences on the Earth's space environment. In this study, we collected and manually scaled the ionograms recorded by a DPS ionosonde at Jicamarca (12.0°S, 283.2°E) to retrieve F layer parameters and electron density (Ne) profiles. A comparative study is performed to evaluate the equatorial ionosphere in solar minima of cycle 22/23 (1996–1997) and 23/24 (2008–2009). The seasonal median values of the critical frequency of F2 layer (foF2) were remarkably reduced in four seasons during the deep solar minimum, compared to those in 1996–1997. It is the first time to find that lower values prevail at most times in 2008–2009 in the F2 layer peak height (hmF2) and Chapman scale height (Hm). The bottomside profile thickness (B0) shows higher values in 2008–2009 than that in 1996–1997 at some daytime intervals, although it also becomes smaller during the rest times. Furthermore, the ionosphere in 2008–2009 is contracted strongly at altitudes above hmF2 and more perceptible in the afternoon hours. The decrease in Ne is strongest in September equinox and weakest in June solstice. The ionospheric responses from solar minimum to minimum are mainly caused by the reduction in solar extreme ultraviolet intensity, and the contribution from dynamical processes competes and is variable. Analysis reveals that semiannual and longer‐scale components are certainly reduced during the deep solar minimum, while shorter scale (e.g., 4 month) components may disrupt the decline picture at some times.

Study of the ionosphere during a magnetospheric storm using data on the ground and in space

For the magnetospheric storm of May 14–16, 1997 geophysical data of satellites DMSP and IMP-8 are compared with data of radio propagation on the high-latitude HF radio path of Heiss Island – St. Petersburg and data from European ionosondes. Peculiarities of variations of the operational frequencies range MOF–LOF (maximum and lowest observed frequencies) on the path were considered. The range has been determined by the method of oblique ionospheric sounding (OIS). The latter is more informative for observations during a magnetic storm compared to the vertical sounding method. Nevertheless, an analysis of variations of the critical frequency of the ionospheric F2 layer from the chain of European ionosondes was carried out. For interpretation of results, data of magnetospheric parameters, AE-indexes and riometer data were used. The variations of both frequency range on the path and critical frequencies of the F2 layer through the ionosondes chain during the disturbed period had certain regularities of behaviour. These regularities are being explained from the physical point of view. The analysis of the satellite DMSP data has showed that a magnetospheric disturbance causes displacement equatorward of precipitation and some growth of its width and energy.

Study of variation in slab thickness of ionospheric F-region during earthquake by wavelet analysis

The present work deals with the anomalies in ionospheric slab thickness of F-region associated with strong seismic activity. In this regard we have considered ionosonde data of Critical frequency of F2 layer (foF2) and Ionospheric Total Electron Content (ITEC) observed for Athens [38°N, 24°E] and Rome [42°N, 13°E] stations. We have considered three cases of earthquake occurring on December 20, 2007, June 08, 2008 and April 06, 2009 and analyzed the ionosonde data of foF2 and ITEC for slab thickness of F-region of the ionosphere in one-hour interval time series record by filtering the geomagnetic disturbances. The result of the study shows that some unusual perturbations were observed in slab thickness of F-region for some days before the main seismic event. It may be due to the generation of seismogenic electric field above the surface of the earth well before the seismic event. This anomalous behavior of perturbations may be used as earthquake precursor.

Análise da ionosfera usando dados de receptores GPS durante um período de alta atividade solar e comparação com dados de Digissondas

Cada vez mais e crescente o uso do GPS (Global Positioning System ) em estudos da atmosfera terrestre. Neste artigo, a atmosfera superior da Terra, denominada ionosfera, foi estudada durante um periodo de alta atividade solar (ano de 2001) usando dados de receptores GPS de dupla frequencia localizados na regiao brasileira, pertencentes a RBMC (Rede Brasileira de Monitoramento Continuo). A partir dos dados GPS foram calculados os valores de TEC (Total Electron Content - Conteudo Total de Eletrons) da ionosfera. Adicionalmente foram incluidos no estudo dados de frequencia critica da camada F2 advinda de Digissondas localizadas em Sao Luis/MA (3oS; 44oW) e Cachoeira Paulista/SP (22oS; 45oW), para fins de comparacao. De uma forma geral, os resultados mostraram maiores valores de TEC durante os meses proximos aos equinocios e menores durante os meses de inverno. Quanto a variacao diaria do TEC, os menores valores foram verificados por volta das 4-6 HL (Hora Local) e os maiores durante o periodo da tarde, com valores um pouco maiores para Sao Luis. O segundo pico da anomalia equatorial foi verificado em Cachoeira Paulista nos meses proximos aos equinocios e verao. Na maioria dos meses, uma alta correlacao linear foi verificada quando realizada a comparacao entre os valores de frequencia critica da camada F2 e os de TEC, principalmente para Sao Luis.

Relation of long-period variations in the critical frequencies of the F 2 layer to geomagnetic activity

The relation of the long-period variations in the midnight and noon values of the critical frequency of the ionospheric F2 layer at three midlatitude stations (Irkutsk, Moscow, and Boulder) to the daily mean index of geomagnetic activity in years of different solar activity has been studied. It has been found that the correlation coefficients between the above parameters depend on time of day, season, and solar activity level. The correlation coefficients are higher at night than in the daytime, especially at low solar activity. The highest absolute values of the correlation coefficient most often appear during equinoxes: April–May and September–October. It has been shown that the variability of the critical frequencies of the midlatitude ionospheric F2 layer depends not only on geomagnetic activity but also (to a considerable degree) on the effect of the lower atmosphere.