Ionosonde Observations Research Articles

Topside ionospheric effective scale heights (HT) derived with ROCSAT‐1 and ground‐based ionosonde observations at equatorial and midlatitude stations

In this study we propose the assimilation of topside in situ electron density data from the Republic of China Satellite (ROCSAT‐1) along with the ionosonde measurements for accurate determination of topside ionospheric effective scale heights (HT) using an α‐Chapman function. The reconstructed topside electron density profiles using these scale heights exhibit an excellent similitude with Jicamarca incoherent scatter radar (ISR) profiles and are much better representations than the existing methods of Reinisch‐Huang method and/or the empirical International Reference Ionosphere–2007 model. The main advantage with this method is that it allows the precise determination of the effective scale height (HT) and the topside electron density profiles at a dense network of ionosonde/Digisonde stations where no ISR facilities are available. The demonstration of the method is applied by investigating the diurnal, seasonal, and solar activity variations of HT over the dip‐equatorial station Jicamarca and the midlatitude station Grahamstown. The diurnal variation of scale heights over Jicamarca consistently exhibits a morning time descent followed by a minimum around 0700–0800 LT and a pronounced maximum at noon during all the seasons of both high and moderate solar activity periods. Further, the scale heights exhibit a secondary maximum during the postsunset hours of equinoctial and summer months, whereas the postsunset peak is absent during the winter months. These typical features are further investigated using the topside ion properties obtained by ROCSAT‐1 as well as Sami2 is Another Model of the Ionosphere (SAMI2) model simulations. The results consistently indicate that the diurnal variation of the effective scale height (HT) does not closely follow the plasma temperature variation and at equatorial latitudes is largely controlled by the vertical E × B drift.

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.

Latitudinal dependence of the ionospheric response to solar eclipses

In this study, we statistically analyze the latitudinal dependence of F2‐layer peak electron densities (NmF2) and total electron content (TEC) responses to solar eclipses by using the ionosonde observations during 15 eclipse events from 1973 to 2006 and the GPS TEC observations during six solar eclipse events from 1999 to 2006. We carried out a model study on the latitudinal dependence of eclipse effects on the ionosphere by running a theoretical ionospheric model with the total eclipse occurring at 13 latitudes from 0°N to 60°N at intervals of 5°. Both the observations and simulations show that the NmF2 and TEC responses have the same latitudinal dependence: the eclipse effects on NmF2 and TEC are smaller at low latitudes than at middle latitudes; at the middle latitudes (>40°), the eclipse effect decreases with increasing latitude. The simulations show that the smaller NmF2 responses at low latitudes are mainly because of much higher heights of hmF2 at low latitudes and electron density response decreasing rapidly with increasing height. For the eclipse effects at the middle latitudes (>40°), the simulations show that the smaller NmF2 or TEC response at higher latitude is mainly ascribed to the larger downward diffusion flux induced by the larger dip angle at this region, which can partly make up for the plasma loss and alleviate the depression of electron density in the F region. The simulated results show that there is an overall decrease in electron temperature throughout the entire height range at the middle latitude, but for the low latitudes the eclipse effect on electron temperature is much smaller at high heights, which is mainly because of the much smaller reduction of photoelectron production rate at its conjugate low heights where only a partial eclipse with small eclipse magnitude occurs.

Profiling of Ionospheric Electron Density Based on FormoSat-3/COSMIC Data: Results from the Intense Observation Period Experiment

Multi-channel Global Position System (GPS) carrier phase signals received by the FormoSat-3 (F53)/COSMIC program's six low Earth orbiting (LEO) satellites were used to undertake active limb sounding of the Earth's atmosphere and ionosphere via radio occultation (RO). In this study the effects of large-scale horizontal gradients and/or inhomogeneous ionospheric electron densities (n(subscript e)) were considered by developing an iterative scheme to determine ”compensated” total electron content (TEC) values through nearby occultation observations. Using an iterative Abel inversion through ”compensated” TEC values, more than one-hundred and eighty thousand vertical ionospheric n(subscript e) profiles were collected during the intense observation period (TOP) of 20 June-27 September (day number 171 to 270) 2006. Coincidences of GPS RO observations to ionospheric n(subscript e) ionosonde data from forty-nine worldwide stations have been examined. It is found that the iterative scheme gives improved retrievals over the standard Abel inversion. From several thousand matches within the TOP experiment, the root mean square (rms) fo F2 differences between the ionosonde measurements and the F53/COSMIC retrievals are improved from 1.67 to 1.07 MHz. The results also show improvement in the overestimates (underestimates) on low (high) retrieved foF2s, while the obtained hmF2s give better agreement with ionosonde observations.

Simultaneous observations of ESF irregularities over Indian region using radar and GPS

Abstract. In this paper, we present simultaneous observations of temporal and spatial variability of total electron content (TEC) and GPS amplitude scintillations on L1 frequency (1.575 GHz) during the time of equatorial spread F (ESF) while the MST radar (53 MHz) located at Gadanki (13.5° N, 79.2° E, Dip latitude 6.3° N), a low latitude station, made simultaneous observations. In particular, the latitudinal and longitudinal extent of TEC and L-band scintillations was studied in the Indian region for different types of ESF structures observed using the MST radar during the low solar activity period of 2004 and 2005. Simultaneous radar and GPS observations during severe ESF events in the pre-midnight hour reveal that significant GPS L band scintillations, depletions in TEC, and the double derivative of the TEC index (DROTI), which is a measure of fluctuations in TEC, obtained at low latitudes coincide with the appearance of radar echoes at Gadanki. As expected, when the irregularities reach higher altitudes as seen in the radar map during pre-midnight periods, strong scintillations on an L-band signal are observed at higher latitudes. Conversely, when radar echoes are confined to only lower altitudes, weak scintillations are found and their latitudinal extent is small. During magnetically quiet periods, we have recorded plume type radar echoes during a post-midnight period that is devoid of L-band scintillations. Using spectral slopes and cross-correlation index of the VHF scintillation observations, we suggest that these irregularities could be "dead" or "fossil" bubbles which are just drifting in from west. This scenario is consistent with the observations where suppression of pre-reversal enhancement (PRE) in the eastward electric field is indicated by ionosonde observations of the height of equatorial F layer and also occurrence of low spectral width in the radar observations relative to pre-midnight period. However, absence of L-band scintillations during post-midnight event, when radar observed plume like structures and scintillations were recorded on VHF band, raises questions about the process of evolution of the irregularities. A possible explanation is that whereas small scale (~3 m) irregularities are generated through secondary waves that grow on the walls of km scale size irregularities, in this case evolution of the Rayleigh-Taylor instability itself did not extend to irregularities of scale sizes of a few hundred meters that produce scintillation on a L-band signal.

Midlatitude nighttime enhancement in F region electron density from global COSMIC measurements under solar minimum winter condition

Ionospheric electron density profiles retrieved from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites measurements from 6 November 2006 to 5 February 2007 are used to study the ionospheric nighttime electron density enhancements under winter, solar minimum, and geomagnetically quiet conditions. In this work, the peak electron densities of F2 layer (NmF2) derived from COSMIC measurements are found to be in reasonably good agreement with ionosonde observations during the night. Therefore the morphology of nighttime enhancements is investigated at geomagnetic midlatitudes (MLAT 20–60°) in the northern hemisphere using COSMIC observations. The enhancements of electron density are evident near the F2 layer peak at most latitudes and longitudes; however, significant variations in the latitudinal dependence of the occurrence time and net magnitudes of the enhancements are found in different regions. The characteristics of enhancements in the North Atlantic Ocean sector are distinctly different from those in the eastern part of the North American sector and also from those at longitudes from Europe to Asia, in the Pacific Ocean, and in the western part of the North American sectors. The longitudinal variations of the morphology of the electron density enhancements during nighttime are possibly caused by different downward plasma flux, meridional winds, and electric field drifts under the effect of the geomagnetic field.

Meteor‐induced transient sporadic E as inferred from rapid‐run ionosonde observations at midlatitudes

Observations were made with a rapid‐run ionosonde during a 1‐month period from the end of July to the end of August 2002 that included the Perseid meteor shower. Ionograms were obtained at 1‐minute intervals. All the echo traces below a virtual height of 200 km were hand‐scaled for detecting weak and broken traces as well as well‐determined sporadic E. Among the scaled E‐region traces, a distinct meteor echo persisting for 40 min was observed. This long‐duration meteor event was similar to that observed during the Leonid meteor shower in 2001. However the apparent height of the echo traces in the current event first decreased and then increased after equaling the height of a persistent sporadic E layer, while the apparent height of the long‐duration meteor echo trace observed in Leonids 2001 remained nearly constant. The descending rate of the meteor echo was approximately 22 m/s, which was considerably greater than that of the tidal ion layer trapped in a downward‐moving wind shear node. We ascribe the long‐duration meteor echo to a transient and horizontally drifting sporadic E patch generated from meteor trail plasma.

Thermospheric meridional winds as deduced from ionosonde chain at low and equatorial latitudes and their connection with midnight temperature maximum

Multipoint ionosonde observation was conducted in Southeast Asia to study ionosphere‐thermosphere coupling. For this observation three ionosondes were installed along the magnetic meridian at 100°E (Southeast Asia Low‐latitude Ionospheric Network: SEALION); two of them were at magnetic conjugate points and the third was near the magnetic equator. The F layer virtual height, h′F, was scaled from nighttime ionograms obtained from September 2004 to August 2005, close to the solar minimum. The height variations at the three locations were used to derive thermospheric winds in the magnetic meridional plane for transequatorial and convergent/divergent components (with respect to the magnetic equator). During northern winter months, a prominent 6‐hr periodic variation of a transequatorial component was found. During northern summer months, a diurnal or semidiurnal variation was dominant. Convergent winds toward the magnetic equator were associated with the occurrence of the midnight temperature maximum at the latitudes between the two conjugate stations as observed by satellite instruments during the solstices. However, no clear evidence was found of abatement of the wind due to the pressure bulge associated with the midnight temperature maximum.

Midday bottomside electron density profiles during moderate solar activity and comparison with IRI-2001

Bottomside electron density (Ne-h) profiles during midday (10–14 h) are analyzed using modern digital ionosonde observations at a low-middle latitude station, New Delhi (28.6N, 77.2E, dip 42.4N), for the period from January 2003 to December 2003, pertaining to moderate solar activity (MSA). Each individual profile is normalized with respect to the peak height and density (hmF2, NmF2) of the F2-region. These profiles are compared with those obtained from the International Reference Ionosphere (IRI-2001) model. Bilitza [Bilitza, D. International Reference Ionosphere 2000. Radio Sci. 36 (2), 261–275, 2001] using both the options namely: Gulyaeva’s model [Gulyaeva, T.L. Progress in ionospheric informatics based on electron density profile analysis of ionograms. Adv. Space Res. 7 (6) 39–48, 1987] and B0 Tab. option [Bilitza, D., Radicella, S.M., Reinisch, B.W., Adeniyi, J.O., Mosert Gonzalez, M.E., Zhang, S.R., Obrou, O. New B0 and B1 models for IRI. Adv. Space Res. 25 (1), 89–95, 2000]. The study reveals that during summer and equinox, the IRI model with B0 Tab. option in general, produces better agreement with the observed median profiles, while the IRI predictions using Gulyaeva’s option, overestimate the electron density distribution at all the heights below the F2-peak. However, during winter, in general, the IRI model, using both the options, reveals shows fairly good agreement with the observations.

The F-Region Equatorial Ionospheric Electrodynamics Drifts

The ionospheric plasma drift is one of the most essential parameters for understanding the dynamics of ionospheric F-region. F-region electromagnetic drifts are calculated for three seasonal conditions from ionosonde observations acquired during quiet period of a typical year of high and low solar activity at Ibadan (7.4oN, 3.9oE, dip 6oS), Nigeria. The vertical plasma drifts derived from h' (f) ionosonde data are compared with vertical drifts obtained by incoherent scatter radar and AE-E satellite measurements during nighttime periods under similar solar and geomagnetic conditions. We find comparable variability among the ionosonde drifts at Ibadan, Jicamarca VHF radar drifts, and AE-E satellite drifts during high solar flux and geomagnetic quiet conditions at equinox and solstices periods. The equinoctial average evening upward drifts enhancements by the three methods are roughly similar and occur at the same local time (19 LT) for all the seasons. Additionally, the evening reversal time from upward daytime to downward nighttime does not vary much except during the winter months; and occurs earliest in summer and equinox, but least during winter period. Also the data indicate asymmetry of evening reversal times about the dip-equator between the Peruvian, Indian, and the African equatorial regions. Our observations are in conformity with some results obtained at other equatorial ionospheric stations Journal of the Nigerian Association of Mathematical Physics Vol. 10 2006: pp. 27-34

GPS observations of the ionospheric F2-layer behavior during the 20th November 2003 geomagnetic storm over South Korea

The ionospheric F2-layer peak density (NmF2) and its height (hmF2) are of great influence on the shape of the ionospheric electron density profile Ne (h) and may be indicative of other physical processes within the ionosphere, especially those due to geomagnetic storms. Such parameters are often estimated using models such as the semiempirical international reference ionosphere (IRI) models or are measured using moderately priced to expensive instrumentation, such as ionosondes or incoherent scatter radars. Global positioning system (GPS) observations have become a powerful tool for mapping high-resolution ionospheric structures, which can be used to study the ionospheric response to geomagnetic storms. In this paper, we describe how 3-D ionospheric electron density profiles were produced from data of the dense permanent Korean GPS network using the tomography reconstruction technique. These profiles are verified by independent ionosonde data. The responses of GPS-derived parameters at the ionospheric F2-layer to the 20th November 2003 geomagnetic storm over South Korea are investigated. A fairly large increase in the electron density at the F2-layer peak (the NmF2) (positive storm) has been observed during this storm, which is accompanied by a significant uplift in the height of the F2 layer peak (the hmF2). This is confirmed by independent ionosonde observations. We suggest that the F2-layer peak height uplift and NmF2 increase are mainly associated with a strong eastward electric field, and are not associated with the increase of the O/N2 ratio obtained from the GUVI instruments aboard the TIMED satellite. It is also inferred that the increase in NmF2 is not caused by the changes in neutral composition, but is related to other nonchemical effects, such as dynamical changes of vertical ion motions induced by winds and E × B drifts, tides and waves in the mesosphere/lower thermosphere region, which can be dynamically coupled upward to generate ionospheric perturbations and oscillations.

Ionospheric weather: cloning missed foF2 observations for derivation of variability index

Abstract. A techique for filling the gaps of the missing F2-layer critical frequency is proposed and applied for the derivation of the ionospheric weather index, characterizing the degree of disturbance at each particular station. A daily-hourly analysis of ionosonde observations of foF2 for 16 stations at latitude range 37° to 70° N, longitudes of 10° W to 150° E, is performed during the solar minimum, 2006. Missed ionosonde observations are reconstructed by cloning data of another station. The process of gap filling considers hourly values of the F peak density NmF2 (deduced from foF2), normalized to the respective median, and assumes that this ratio remains the same for the parent and cloned data. It is shown that the correlation coefficient between cloned fcF2 and observed foF2 is greater than 0.75 for the positive and negative ionospheric disturbed days during a year at solar minimum, independent of the distance between the stations in high and middle latitudes. The quiet reference is determined as a running daily-hourly median for 27 days, preceding the day of observation calibrated for a seasonal trend with ITU-R foF2 predictions. The hourly deviation DNmF2 is defined as the logarithm of ratio of NmF2/NmF2med. A segmented logarithmic scale of the ionospheric weather index, W, is introduced, so that W=±1 refers to the quiet state, W=±2 to a moderate disturbance, W=±3 to the ionospheric storm, and W=±4 to the extreme or anomalous conditions. The catalog of the ionospheric disturbances for W exceeding ±2 at least during 3 consecutive hours is produced and presented online at the SRC and IZMIRAN web pages. It is found that the moderate disturbance is a prevailing state of the ionospheric weather for all stations. The stormy conditions comprise 1 to 20% of the times which occur more frequently at high latitudes, by night, during equinox and winter.

Prestorm enhancements in NmF2 and total electron content at low latitudes

The enhancement of electron concentrations in the ionosphere before geomagnetic storms is one of the open questions. Using ionosonde observations and total electron content (TEC) from Global Positioning System (GPS) measurements along longitude 120°E, we analyzed three low latitude pre‐storm enhancement events that occurred on 21 April (day 111) 2001, 29 May (day 149) 2003, and 22 September (day 265) 2001, respectively, in the Asia/Australia sector. All three events (and other two cases on 9 August 2000 and 10 May 2002) show quite similar features. The strong prestorm enhancements during these events are simultaneously presented in foF2 and TEC and enhancements have latitudinal dependence, tending to occur at low latitudes with maxima near the northern and southern equatorial ionization anomaly (EIA) crests and depletions in the equatorial region. This is quite different from what reported by Burešová and Laštovička (2007) for middle latitudes. They found no systemic latitudinal dependence in prestorm enhancements over Europe. It is argued that solar flares are not the main drivers for the enhancements, at least for low‐latitude events. Main features of low‐latitude prestorm enhancements do not coincide with the solar flare effects. We postulate that the vertical plasma drift or zonal electric field is a likely cause for the low‐latitude prestorm enhancements. Its existence is supported by the facts of stronger EIA, the latitudinal coverage of the enhancements as well as the lift of the F layer peak height at an equatorward station during the prestorm enhancements. Moreover, the behaviors of hmF2 at low latitudes during the prestorm enhancements may possibly be explained in terms of the coupling nature of parallel and perpendicular dynamics at low latitudes (see, e.g., Behnke and Harper, 1973; Rishbeth et al., 1978).

Ionospheric temporal and spatial variations during the 18 August 2003 storm over China

Abstract The effects of the 18 August 2003 geomagnetic storm on the ionosphere over China have been first studied by combining dual-frequency Global Positioning System (GPS) observation data from the Crustal Movement Observation Network of China (CMONOC) using the computerized ionospheric tomography (CIT) technique. The temporal and spatial variations of the ionosphere are analyzed using a time series of ionospheric electron density (IED) maps, and the ionospheric storm evolution process is revealed. The tomographic results show that the main ionospheric effects of this storm over China are: (1) that positive storm phase effects usually happen in the low-latitude ionosphere, (2) that negative storm phase effects occur in the mid-latitude ionosphere, and (3) that the equatorial anomaly structure can also be found. In contrast to the quiet period of the ionosphere on 17 August 2003, the equatorial anomaly crest moved to the north in the main phase of the storm and then moved back to the original location in the recovery phase on 19 August 2003. We compared the peak density NmF2 and the peak height hmF2 obtained from the ionosonde observations at Wuhan station and those inverted by the CIT technique to confirm the reliability of the GPS-based tomographic technique. The tomographic results revealed that the GPS-based CIT technique can be used to monitor large-scale ionospheric disturbances during geomagnetic storms.

Prompt derivation of TEC from GEONET data for space weather monitoring in Japan

Continuous monitoring of ionospheric conditions is essential to monitoring and forecasting space weather. The worldwide use of global navigation satellite systems like the Gobal Positioning System (GPS) makes it possible to continuously monitor the total electron content (TEC) of the ionosphere and plasmasphere up to a height of about 20,000 km. We have developed a system for deriving the TEC from GEONET data rapidly and we use the TEC distribution over Japan in the daily operations of the Space Weather Forecast Center at NICT (RWC Tokyo of ISES). Using instrumental biases from a few days before enables us to drastically shorten the processing time for deriving TEC. The latest TEC values (with a delay of about 1 h) are obtained every 3 h, and most of the values are within 2 TEC units of the actual TEC. We have found our system for deriving TEC rapidly to be useful for continuously monitoring the progress of ionospheric storms under any ionospheric conditions, even those under which the usual ionosonde observations are unable to obtain F-region profiles.

Diurnal and seasonal variation of B0,B1 parameters during high solar activity period at low mid-latitude and their comparisons with IRI-2001 model

Diurnal and seasonal variations of bottom side electron density profile shape parameters B0, B1, representing the bottom side F2-layer thickness and shape, are examined using modern digital ionosonde observations at a low-middle latitude station, New Delhi (28.6 °N, 77.2 °E, dip 42.4 °N) for high solar activity (HSA) (2001–2002). Median values of these parameters are obtained at each hour during different seasons and compared with the predictions of the latest version of the international reference ionosphere (IRI), IRI-2001 model using both the options namely: IRI (Gulyaeva) and IRI (B0 Tab.). Results show in general, a large variability in B0, and B1 parameters during all the seasons, the variability is larger during nighttime than by daytime. The diurnal variation of median B0, in general, show more or less similar trends with diurnal maximum occurring around noontime, except during summer, when it occurs between 09 and 10 LT. Variation pattern of B1 in general, is identical in all the seasons with lower values of B1 by daytime than by night. Comparative studies of B0 with those obtained with the IRI model show that in general, IRI (B0 Tab.) option reveals better agreement with the observations during all the seasons for local times from about 10 LT to about 16 LT, while outside this time period IRI (Gulyaeva) matches well with the observations. The predicted B1 parameter, using IRI (B0 Tab.) is close to observations in terms of diurnal variation, while B1 using IRI (Gulyaeva) option, assumes a fixed value of 3 at all local times irrespective of season.

Comparison of ionospheric measurements made by digisondes with those inferred from ultraviolet airglow

In this work we compare ionospheric measurements made at several ionosonde stations with parameters inferred from airglow measurements made by the Global Ultraviolet Imager (GUVI) instrument aboard the Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED) spacecraft. The purpose of this comparison is twofold. First, our study provides a validation of the technique of using TIMED/GUVI airglow measurements to infer nighttime electron densities in F region of the ionosphere. Second, we use the global characterization of the ionosphere provided by TIMED/GUVI to identify potential biases in individual ionosonde station records. We have found for peak electron densities over the range 4.0–11.0×105cm−3 that the GUVI measurements are consistent with the ionosonde observations. We have also found some station to station and regional variations that warrant further study.

Comparison of two IRI electron-density plasmasphere extensions with GPS-TEC observations

Comparisons of two model results with Global Positioning System, GPS-TEC, measurements have been carried out for different latitudinal, solar activity, magnetic activity, diurnal, and seasonal conditions. The models evaluated are the Global Core Plasma Model (GCPM-2000) and the IRI extension (IRI∗) with the Russian plasmasphere model. Data from 23 observatories providing GPS-TEC and ionosonde data have been used. It is shown that the IRI∗ plasmasphere electron densities are greater than the GCPM results by an order of magnitude at 6370km altitude (one Earth radius), becoming two to three orders of magnitude larger at the GPS satellite orbital altitude of 20,200km. Another source of model and GPS-TEC differences is the selection of the ionospheric F2 layer peak parameters driving the models, either with ITU-R (former CCIR) maps or ionosonde observations. The plasmasphere model extension of IRI improves the accuracy of the TEC model predictions taking into account the plasmasphere contribution to the total electron content which could vary from 10% during daytime under quiet magnetic conditions to more than 50% during the night under storm-time conditions.

FoF2 variability over New Delhi during different solar cycle conditions and comparison with IRI-2001 model

In this paper, the variability of F2-layer critical frequency foF2 is examined, using modern digital ionosonde observations at a low-middle latitude station, New Delhi (28.6°N, 77.2°E, Dip 42.4°N). Data used for the present study are the hourly values of foF2 for the periods of high (2001–2002) and low (2004–2006) solar activity. To investigate the foF2 variability, the values of median, lower quartile and upper quartile are estimated at each hour for different seasons, during both the solar activity periods. The variability index (henceforth referred to as VI) of foF2 is then obtained by using inter-quartile difference and its percentage ratio to the median. Results show that VI is in general, lower during daytime hours than by night during high solar activity for all the seasons, while during low solar activity, VI behavior is irregular during all the seasons. A solar activity dependence of VI is found with maximum values during periods of low solar activity in all the seasons except for winter during pre-dawn and post-sunset hours. Comparisons of observations with IRI-2001 model [Bilitza, D. International Reference Ionosphere 2000. Radio Sci. 36 (2), 261–275, 2001] show in general good agreement during both the solar activity periods, with model values lying within inter-quartile range.

Applying artificial neural network to derive long‐term foF2 trends in the Asia/Pacific sector from ionosonde observations

An artificial neural network (ANN) method is first used for deriving long‐term trends of the F2‐layer critical frequency (foF2) at 19 ionospheric stations in the Asia/Pacific sector. It is found that the ANN method can eliminate the geomagnetic activity effect on foF2 more effectively than usual regression methods. Of the selected 19 stations, there are significant long‐term trends corresponding to a confidence level ≥90% at 14 stations and 12 of these stations present negative trends. An average trend of −0.05% per year in the selected area can be obtained if the 12 stations with significant negative long‐term trends be considered. No pronounced diurnal and latitudinal effects in trends and no uniform pattern of seasonal variation in most stations are detected. The long‐term trends for low latitude and equatorial stations differ from other stations suggest that some special dynamical processes may take effects in the equatorial anomaly region. Many factors which can influence ionosphere, such as the greenhouse effect, solar and geomagnetic activity, and neutral background gas, might contribute to the trend.