The accuracy of data from ionosondes for the estimation of hmF2 and the identification of global change in the ionosphere

A long temporal series of simulated ionograms was generated with a superimposed secular variation of −14 km/century on the h_mF2 parameter. These ionograms were interpreted by the automatic scaling program Autoscala. By applying four different empirical formulas, four artificial series of h_mF2 were generated and then processed with the same methods used by other authors for real data sets. Data analysis of the simulated ionograms revealed the artificially imposed long-term trend. These results lead to the conclusion, that regardless of the empirical formula used, the accuracy of h_mF2 from ionosonde measurements would be adequate to observe a long-term trend of −14 km/century.     

Improvements in bottomside electron density definition in the Autoscala program

Some improvements introduced in the Autoscala program are presented. They include improvements in E valley modeling of the electron density profile N_e(h), and in the link between the E valley and bottom-side F regions. An abrupt variation in N_e(h) generated by the previous version of Autoscala under night conditions has been eliminated.A series of ionograms recorded by the Millstone Hill digisonde (42.6°, 288.5°) were automatically interpreted by the previous version of Autoscala and by the new one. Data from Incoherent Scatter Radar (ISR) were used to comparatively assess the performance of the two versions. For this purpose, the root mean square errors (RMSEs) of the N_e(h) provided by Autoscala were calculated relative to the corresponding values provided by ISR.A more accurate overall modeling of N_e(h) was achieved by the new Autoscala version (RMSE = 0.51 MHz for the new version against RMSE = 0.67 MHz for the previous one).     

Large-scale traveling ionospheric disturbances of auroral origin according to the data of the GPS network and ionosondes

The intensity of large-scale traveling ionospheric disturbances (LS TIDs), registered using measurements of total electron content (TEC) during the magnetic storms on October 29–31, 2003, and on November 7–11, 2004, had been compared with that of local electron density disturbances. The data of TEC measurements at ground-based GPS receivers located near the ionospheric stations and the corresponding values of the critical frequency of the ionospheric F region f_oF2 were used for this purpose. The variations of TEC and f_oF2 were similar for all events mentioned above. The previous assumption that the ionospheric region with vertical extension from 150 to 200 km located near the F-layer maximum mainly contributes to the TEC variations was confirmed for the cases when the electron density disturbance at the F region maximum was not more than 50%. However, this region probably becomes vertically more extended when the electron density disturbance in the ionospheric F region is about 85%.     

Development of a campaign to study equatorial ionospheric phenomena over Guam

The United States Air Force Academy (USAFA) is in the process of developing a series of ground-based and space-based experiments to investigate the equatorial ionosphere over Guam and the southern crest of the Equatorial Appleton Anomaly over New Guinea. On the ground the Digital Ionospheric Sounder (University of Massachusetts, Lowell DPS-4 unit) and a dual-frequency GPS TEC/scintillation monitor will be used to investigate ionospheric phenomena in both campaign and long-term survey modes. In campaign mode, we will combine these observations with those collected from space during USAFA’s FalconSAT-3 and FalconSAT-5 low Earth orbit satellite missions, which will be active over a period of several years beginning in the first quarter of the 2007 calendar year. Additionally, we will investigate the long-term morphology of key ionospheric characteristics useful for driving the International Reference Ionosphere, such as critical frequencies (f_oE, f_oF1, f_oF2, etc.), the M(3000) F2 parameter (the maximum useable frequency for a signal refracted within the F2 layer and received on the ground at a distance of 3000 km away), and a variety of other characteristics. Specific targets of investigation include: (a) a comparison of TEC observed by the GPS receiver with those calculated by IRI driven by DPS-4 observations, (b) a comparison of plasma turbulence observed on-orbit with ionospheric conditions as measured from the ground, and (c) a comparison between topside ionospheric satellite in situ measurements of plasma density during an overpass of a Digisonde versus the calculated value based on extrapolation of the electron density profiles using Digisonde data and a topside α-Chapman function. This last area of investigation is discussed in detail in this paper.     

Variability of foE in the equatorial ionosphere with solar activity

This research examined the variability of f_oE in the equatorial ionosphere with solar activity within the equatorial ionospheric anomaly region. Ionosonde data recorded at Ouagadougou (lat. 12.4°N, long. 1.5°W and magnetic dip 1.43°N) were engaged to study the transient variations of the critical frequency of the E-layer (f_oE) and its dependence on solar activity. The study revealed that f_oE increases with the increase in solar intensity of the sun. The variability of the f_oE decreases with increases in the solar activity. The maximum value of the f_oE is at local noon when the ionosphere is stable; the variability at this local time is minimal. The minimum value of the f_oE is at sunrise and sunset, at this period on local time the equatorial ionosphere recorded its maxima variability. Irrespective of the degree of solar activity, f_oE is observed to be maximum in June solstice, followed by the equinoxes and minimum in December solstice. Equinoctial asymmetry occurred in the variation of the relative standard deviation of f_oE with maximum in September/March equinox for low/high solar activity.     

Using the radial basis function neural network to predict ionospheric critical frequency of F2 layer over Wuhan

Neural networks (NNs) have been applied to ionospheric predictions recently. This paper uses radial basis function neural network (RBF-NN) to forecast hourly values of the ionospheric F₂ layer critical frequency(f_oF₂), over Wuhan (30.5N, 114.3E), China. The false nearest neighbor method is used to determine the embedding dimension, and the principal component analysis (PCA) is used to reduce noise and dimension. The whole study is based on a sample of about 26,000 observations of f_oF₂ with 1-h time resolution, derived during the period from January 1981 to December 1983. The performance of RBF-NN is estimated by calculating the normalized root-mean-squared (NRMSE) error, and its results show that short-term predictions of f_oF₂ are improved.     

Comparing IRI and a regional model with ionosonde measurements in Pakistan

We have used the technique of expansion in Empirical Orthogonal Functions (EOFs) to develop regional models of the critical frequencies of E and F₂ layers (f_oE, f_oF₂), peak height (h_mF₂), and semi-thickness of F₂ layer (Y_mF₂) over Pakistan. In the present study levels of solar activity specified by Smoothed Sunspot Number (R) from 10 to 200 are taken into account. The magnetic dip angle for the model ranges from 30° to 60°. We have compared the regional model and the International Reference Ionosphere (IRI) with measurements of three ionosondes in Pakistan. The model parameters f_oE and f_oF₂ are found overall comparable to the observed hourly median values during daytime at Karachi (geographic latitude = 24.95°N, longitude = 67.13°E, magnetic inclination = 37°), Multan (30.18°N, 71.48°E, 45°) and Islamabad (33.75°N, 73.13°E, 51.5°) during the years 1988, 1996 and 2000. For h_mF₂ the computed values by regional and IRI model for the year 1995 are found close to each other. However, for Y_mF₂the results are better during daytime as compared to nighttime.     

Planetary wave coupling of the atmosphere–ionosphere system during the Northern winter of 2008/2009

This paper presents the global spatial (latitude and altitude) structure and temporal variability of the ∼23-day ionospheric zonally symmetric (s = 0) planetary wave (PW) seen in the Northern winter of 2008/2009 (October 2008–March 2009). It is shown that these ∼23-day ionospheric oscillations are forced from PWs propagating from below. The COSMIC ionospheric parameters f_oF2 and h_mF2 and electron density at fixed altitudes and the SABER temperatures were utilized in order to define the waves which are present simultaneously in the atmosphere and ionosphere. The long-period PWs from the two data sets have been extracted through the same data analysis method. The similarity between the lower thermospheric ∼23-day (s = 0) temperature PW and its ionospheric electron density response provides valuable and strong experimental evidence for confirming the paradigm of atmosphere–ionosphere coupling.     

Effect of vertical plasma transport on ionospheric F2-region parameters at equatorial latitude

The variability of the F2-layer even during magnetically quiet times are fairly complex owing to the effects of plasma transport. The vertical E × B drift velocities (estimated from simplified electron density continuity equation) were used to investigate the seasonal effects of the vertical ion drifts on the bottomside daytime ionospheric parameters over an equatorial latitude in West Africa, Ibadan, Nigeria (Geographic: 7.4°N, 3.9°E, dip angle: 6°S) using 1 year of ionsonde data during International Geophysical Year (IGY) of 1958, that correspond to a period of high solar activity for quiet conditions. The variation patterns between the changes of the vertical ion drifts and the ionospheric F2-layer parameters, especially; f_oF₂ and h_mF₂ are seen remarkable. On the other hand, we observed strong anti-correlation between vertical drift velocities and h′F in all the seasons. We found no clear trend between N_mF₂ and h_mF₂ variations. The yearly average value of upward daytime drift at 300 km altitude was a little less than the generally reported magnitude of 20 ms⁻¹ for equatorial F-region in published literature, and the largest upward velocity was roughly 32 ms⁻¹. Our results indicate that vertical plasma drifts; ionospheric F2-layer peak height, and the critical frequency of F2-layer appear to be somewhat interconnected.     

Upgrading CCIR's foF2 maps using available ionosondes and genetic algorithms

We have developed a new approach towards a new database of the ionospheric parameter $f_{o}F2$. This parameter, being the frequency of the maximum of the ionospheric electronic density profile and its main modeller, is of great interest not only in atmospheric studies but also in the realm of radio propagation. The current databases, generated by CCIR (Committee Consultative for Ionospheric Radiowave propagation) and URSI (International Union of Radio Science), and used by the IRI (International Reference Ionosphere) model, are based on Fourier expansions and have been built in the 60s from the available ionosondes at that time. The main goal of this work is to upgrade the databases by using new available ionosonde data. To this end we used the IRI diurnal/spherical expansions to represent the $f_{o}F2$ variability, and computed its coefficients by means of a genetic algorithm (GA). In order to test the performance of the proposed methodology, we applied it to the South American region with data obtained by RAPEAS (Red Argentina para el Estudio de la Atmósfera Superior, i.e. Argentine Network for the Study of the Upper Atmosphere) during the years 1958–2009. The new GA coefficients provide a global better fit of the IRI model to the observed $f_{o}F2$ than the CCIR coefficients. Since the same formulae and the same number of coefficients were used, the overall integrity of IRI’s typical ionospheric feature representation was preserved. The best improvements with respect to CCIR are obtained at low solar activities, at large (in absolute value) modip latitudes, and at night-time. The new method is flexible in the sense that can be applied either globally or regionally. It is also very easy to recompute the coefficients when new data is available. The computation of a third set of coefficients corresponding to days of medium solar activity in order to avoid the interpolation between low and high activities is suggested. The same procedure as for $f_{o}F2$ can be perfomed to obtain the ionospheric parameter M(3000)F2.     

A different method to update monthly median hmF2 values

The height, h_mF2, and the electron density, N_mF2, of the F2 peak are key model parameters to characterize the actual state of the ionosphere. These parameters, or alternatively the propagation factor, M3000F2, and the critical frequency, f_oF2, of the F2 peak, which are related to h_mF2 and N_mF2, are used to anchor the electron density vertical profile computed with different models such as the International Reference Ionosphere ( Bilitza, 2002), as well as for radio propagation forecast purposes. Long time series of these parameters only exist in an inhomogeneous distribution of points over the surface of Earth, where dedicated instruments (typically ionosondes) have been working for many years. A commonly used procedure for representing median values of the aforementioned parameters all over the globe is the one recommended by the ITU-R ( ITU-R, 1997). This procedure, known as the Jones and Gallet mapping technique, was based on ionosondes measurements gathered from 1954 to 1958 by a global network of around 150 ionospheric stations (  and ). Even though several decades have passed since the development of that innovative work, only few efforts have been dedicated to establish a new mapping technique for computing h_mF2 and N_mF2 median values at global scale or to improve the old method using the increased observational database. Therefore, in this work three different procedures to describe the daily and global behavior of the height of the F2 peak are presented. All of them represent a different and simplified method to estimate h_mF2 and are based on different mathematical expressions. The advantages and disadvantages of these three techniques are analyzed, leading to the conclusion that the recommended procedure to represent h_mF2 is best characterized by a Spherical Harmonics expansion of degree and order equal to 15, since the differences between the h_mF2 values obtained with the Jones and Gallet technique and those obtained using the abovementioned procedure are of only 1%.     

On the time of minimum ionization in the F2 region

Using measurements of the critical frequency of F2 region (f_oF2) the validity of the International Reference Ionosphere model to predict the time of minimum ionization is checked. Data obtained at different ionospheric stations have been considered. The CCIR and URSI options are used to model calculations. For CCIR option the results show that good predictions were obtained for about 40% of the considered cases. For the rest of the considered data, the model predicts the minimum at times earlier than that observed in the measurements. The percentages of good predictions obtained with URSI option are lower than those corresponding to CCIR one.     

Investigation of the relationship between ionospheric foF2 and earthquakes

Variations of the ionospheric F2 region critical frequency (foF2) have been investigated statistically before earthquakes during 1980–2008 periods in Japan area. Ionosonde data was taken from Kokubunji station which is in the earthquake preparation zone for all earthquakes. Standard Deviations and Inter-Quartile Range methods are applied to the foF2 data. It is observed that there are anomalous variations in foF2 before earthquakes. These variations can be regarded as ionospheric precursors and may be used for earthquake prediction.     

Bottom side profiles for two close stations at the southern crest of the EIA: Differences and comparison with IRI-2012 and NeQuick2 for low and high solar activity

Bottom side electron density profiles for two stations at the southern crest of the Equatorial Ionization Anomaly (EIA), São José dos Campos (23.1°S, 314.5°E, dip latitude 19.8°S; Brazil) and Tucumán (26.9°S, 294.6°E, dip latitude 14.0°S; Argentina), located at similar latitude and separated by only 20° in longitude, have been compared during equinoctial, winter and summer months under low (year 2008, minimum of the solar cycle 23/24) and high solar activity (years 2013–2014, maximum of the solar cycle 24) conditions. An analysis of parameters describing the bottom side part of the electron density profile, namely the peak electron density NmF2, the height hmF2 at which it is reached, the thickness parameter B₀ and the shape parameter B₁, is carried out. Further, a comparison of bottom side profiles and F-layer parameters with the corresponding outputs of IRI-2012 and NeQuick2 models is also reported. The variations of NmF2 at both stations reveal the absence of semi-annual anomaly for low solar activity (LSA), evidencing the anomalous activity of the last solar minimum, while those related to hmF2 show an uplift of the ionosphere for high solar activity (HSA). As expected, the EIA is particularly visible at both stations during equinox for HSA, when its strength is at maximum in the South American sector. Despite the similar latitude of the two stations upon the southern crest of the EIA, the anomaly effect is more pronounced at Tucumán than at São José dos Campos. The differences encountered between these very close stations suggest that in this sector relevant longitudinal-dependent variations could occur, with the longitudinal gradient of the Equatorial Electrojet that plays a key role to explain such differences together with the 5.8° separation in dip latitude between the two ionosondes. Furthermore at Tucumán, the daily peak value of NmF2 around 21:00 LT during equinox for HSA is in temporal coincidence with an impulsive enhancement of hmF2, showing a kind of “elastic rebound” under the action of the EIA. IRI-2012 and NeQuick2 bottom side profiles show significant deviations from ionosonde observations. In particular, both models provide a clear underestimation of the EIA strength at both stations, with more pronounced differences for Tucumán. Large discrepancies are obtained for the parameter hmF2 for HSA during daytime at São José dos Campos, where clear underestimations made by both models are observed. The shape parameter B₀ is quite well described by the IRI-2012 model, with very good agreement in particular during equinox for both stations for both LSA and HSA. On the contrary, the two models show poor agreements with ionosonde data concerning the shape parameter B₁.     

A topside investigation over a mid-latitude digisonde station in Cyprus

We examine the systematic differences between topside electron density measurements recorded by different techniques over the low-middle latitude operating European station in Nicosia, Cyprus (geographical coordinates: 35.14^oN, 33.2^oE), (magnetic coordinates 31.86^oN, 111.83 ^oE). These techniques include space-based in-situ data by Langmuir probes on board.European Space Agency (ESA) Swarm satellites, radio occultation measurements on board low Earth orbit (LEO) satellites from the COSMIC/FORMOSAT-3 mission and ground-based extrapolated topside electron density profiles from manually scaled ionograms. The measurements are also compared with International Reference Ionosphere Model (IRI-2016) topside estimations and IRI-corrected NeQuick topside formulation (method proposed by Pezzopane and Pignalberi (2019)). The comparison of Swarm and COSMIC observations with digisonde and IRI estimations verifies that in the majority of cases digisonde underestimates while IRI overestimates Swarm observations but in general, IRI provides a better topside representation than the digisonde. For COSMIC and digisonde profiles matched at the F layer peak the digisonde systematically underestimates topside COSMIC electron density values and the relative difference between COSMIC and digisonde increases with altitude (above hmF2), while IRI overestimates the topside COSMIC electron density but after a certain altitude (~150 km above hmF2) this overestimation starts to decrease with altitude. The IRI-corrected NeQuick underestimates the majority of topside COSMIC electron density profiles and relative difference is lower up to approximately 100 km (above the hmF2) and then it increases. The overall performance of IRI-corrected NeQuick improves with respect to IRI and digisonde.     

The impact of dust grains on fast fly-by spacecraft: Momentum multiplication,measurements and theory

Energy partitioning during the very high impact speed encountered in a cometary fly-by mission causes a target mass expulsion which leads to a momentum impulse on the target exceeding that of the incident momentum. Theoretical and computational studies are required to provide a basis for predictions of the response at Halley encounter, since experimental data from acceleration of microspheres extends currently only to some 10 kms^?1. Such data obtained from the 2 MV Canterbury microparticle accelerator is presented: this demonstrates a target momentum enhancement E which can be approximated by a form E = 1+(V/V_o)^β. Over the range 1 to 8 kms^?1 the relationship is satisfied by V_o = 2 kms^?1 and β = 2. Theoretical considerations of energy partitioning lead to constraints on the extrapolation of this functional dependence to very high velocities and the transition to β ≤ 1 is shown to apply. Results are examined and their significance to impact sensing and spacecraft deceleration discussed. An enhancement of momentum nearer to 12±3 at 69 kms^?1 is anticipated for non-penetrating particles, from the ballistic pendulum data, but the ES data indicates a figure considerably higher.     

Wave emissions at half electron gyroharmonics in the equatorial plasmasphere region: CLUSTER observations and statistics

Intense (n + 1/2) f_ce emissions are a common phenomenon observed in the terrestrial inner magnetosphere. One of their interests is their possible effect in the pitch angle scattering of plasmasheet keV-electron, leading to diffuse auroras. In this paper, we present CLUSTER’s point of view about this topic, in the equatorial region of the plasmasphere, via a statistical study using 3 years of data. Spectral characteristics of these waves, which represent an important clue concerning their generation mechanism, are obtained using WHISPER data near perigee. Details on the wave spectral signature are shown in an event study, in particular their splitting in fine frequency bands. The orbit configuration of the four spacecraft offers a complete sampling on all MLT sectors. A higher occurrence rate of the emissions in the dawn sector and their confinement to the geomagnetic equator, pointed out in previous studies, are confirmed and described with additional details. The proximity of emission sites, both to the plasmapause layer and to the geomagnetic equator surface, seems to be of great importance in the behaviour of the (n + 1/2) f_ce wave characteristics. Our study indicates for the first time, that both the intensity of (n + 1/2) f_ce emissions, and the number of harmonic bands they cover, are increasing as the observation point is located further away outside from the plasmapause layer. Moreover, a study of the wave intensity in the first harmonic band (near 3/2 f_ce) shows higher amplitude for these emissions than previous published values, these emissions can play a role in the scattering of hot electrons. Finally, geomagnetic activity influence, studied via time series of the D_st index preceding observations, indicates that (n + 1/2) f_ce emission events are observed at CLUSTER position under moderate geomagnetic activity conditions, no specific D_st time variation being required.     

The investigation of powerful solar flares characteristics by analysis of excited states of C and various neutrons capture lines

Accelerated energetic particles in solar flares produced nuclear γ-lines in interactions with ambient solar atmosphere. Analysis of intensity of ratios between various γ-lines allows us to make estimations of abundance of elements, parameters of surrounding media and other solar characteristics. In this article we discuss the flux ratio between two lines from excited states of ¹²C (f_15.11/f_4.44) and our results of preliminary calculation of intensity ratio between two neutron capture lines at ³He and ¹H (f_20.58/f_2.223). In particular we consider the opportunity to obtain n(³He)/n(¹H) ratio during solar flares and using high-energy gamma-emission studying, based on the satellite data. Possible interpretation of spectral features observed during the January 20, 2005 solar flare is discussed. Preliminary analysis of energy spectrum in the band of 2–21 MeV gives n(³He)/n(¹H) ∼ 8 × 10⁻⁴ for January 20, 2005 solar flare.