F-region Peak Electron Density Research Articles

On the quiet-time occurrence rates, severity and origin of L-band ionospheric scintillations observed from low-to-mid latitude sites located in Puerto Rico

In December 2021 three Global Navigation Satellite System (GNSS) ionospheric scintillation and Total Electron Content (TEC) monitors were installed in Puerto Rico (∼24.5° N dip latitude). The installation is part of an effort to better understand the occurrence of ionospheric irregularities and scintillation at mid-latitudes. Puerto Rico (PR) is commonly referred to as being located at mid-latitudes or at the boundary between low and mid-latitudes. Previous reports already presented observations of ionospheric irregularities and scintillation detected from PR. These observations, however, were limited in number. Additionally, the reports related the observed scintillation and irregularities events with different types of mid-latitude ionospheric disturbances. Here we present results from the analyses of the first three months of observations, December 21st, 2021 to March 20th, 2022. The analyses focus on the occurrence rate, severity and origin of scintillation detected on the signals tracked by the PR monitors during geomagnetically quiet times. We show that, out of the 43 geomagnetically quiet nights, 34 (∼80%) showed the occurrence of L-band scintillation. The results also indicate the recurrent existence of severe scintillation (S4 ∼ 1.0) accompanied by large TEC depletions. The temporal evolution of the observed scintillation showed that scintillation-causing plasma irregularities first occurred near the magnetic equator and propagated towards PR (reaching up to ∼22o dip latitude). Simultaneous space-based observations of the ionospheric F-region peak electron densities made by the GOLD mission confirmed that the quiet-time scintillation observed by the monitors in PR was associated with equatorial plasma bubbles. GOLD observations also show enhanced background F-region densities at the location of the Ionospheric Pierce Points of the affected GNSS signals, indicating conditions that favor the occurrence of intense scintillation.

Simulated east\u2013west differences in F-region peak electron density at Far East mid-latitude region

Using TIME3D-IGGCAS model, we simulated the east–west differences in F-region peak electron density (NmF2) in the Far East mid-latitude region near the longitudinal sectors with very clear zonal variations of geomagnetic declination, and mainly analyzed the influence of the geomagnetic field configuration on the east–west differences. We found that, after removing the longitudinal variations of neutral parameters, TIME3D-IGGCAS can better represent the observed relative east–west difference (Rew) features. Rew is mainly negative (West NmF2 > East NmF2) at noon and positive (East NmF2 > West NmF2) at evening–night. The magnitude of daytime negative Rew is weaker in winter and stronger in summer, and the daytime Rew shows two negative peaks around two equinoxes. With the increasing solar flux level, the magnitude of Rew mainly becomes larger, and the two daytime negative peaks slightly shift to June Solstice. With the decreasing geographical latitude, Rew mainly becomes positive, and the two daytime negative peaks slightly shift to June Solstice. Our simulation also suggested that the thermospheric zonal wind plays an important role in the formation of the ionospheric east–west differences in the Far East mid-latitude region. The observed solar activity dependency of the ionospheric EW differences may be driven primarily by corresponding zonal wind changes with solar activity, whereas the observed latitudinal dependency of the differences is associated with primarily zonal wind and secondarily meridional wind latitudinal variations.

Validation of International Reference Ionosphere model (IRI-2016) for F-region peak electron density height (hmF2): Comparison with Incoherent Scatter Radar (ISR) and ionosonde measurements at Millstone Hill

In this report, we have evaluated the performance of International Reference Ionosphere (IRI-2016) model in predicting the diurnal variation of F-region peak electron density height (hmF2) at Millstone Hill (MH, 42.6°N, 288.5°E) during the solar cycle 24 for both high and low-solar activity years. The new methodology is adopted to derive hmF2 values from the Incoherent Scatter Radar (ISR) measurements. The hmF2 derived from ISR along with ionosonde measurements operating at MH has been used to evaluate the performance of IRI-2016 model. The IRI-2016 model has three options to predict hmF2, namely, BSE-1979, AMTB-2013, and SHU-2015. The observed hmF2 from ISR and ionosonde are compared with all the three options provided by IRI-2016 to predict hmF2. Results show that hmF2 from IRI-2016, ISR and ionosonde exhibit similar diurnal variation. The deviations between measurements and model predictions were found to increase during nighttime. The SHU-2015 model emerged out to be the best in predicting the hmF2 values over MH as compared to other two options. The BSE-1979 option also provides reasonably better prediction as compared to the relatively latest AMTB-2013 options. Based on this statistical analyses, we recommend SHU-2015 option of IRI-2016 hmF2 model over MH area.

Formation mechanisms of the midlatitudinal NmF2 semiannual anomaly under daytime quiet geomagnetic conditions at low solar activity

The F-region peak electron densities NmF2 measured during daytime quiet geomagnetic conditions at low solar activity on January 22, 2008, April 8, 1997, July 12, 1986, and October 26, 1995, are compared. Ionospheric parameters are measured by the ionosonde and incoherent scatter radar at Millstone Hill and calculated with the use of a 1D nonstationary ionosphere–plasmasphere model of number densities and temperatures of electrons and ions at middle geomagnetic latitudes. The formation of the semiannual anomaly of the midlatitudinal NmF2 under daytime quiet geomagnetic conditions at low solar activity is studied. The study shows that the semiannual NmF2 anomaly occurs due to the total impact of three main causes: seasonal variations in the velocity of plasma drift along the geomagnetic field due to the corresponding variations in the components of the neutral wind velocity; seasonal variations in the composition and temperature of the neutral atmosphere; and the dependence of the solar zenith angle on a number of the day in the year at the same solar local time.

Ionospheric slab thickness at the equatorial anomaly region after the deep solar minimum of cycle 23/24

The ionospheric slab thickness is a measure of the shape of the ionospheric profile and can be obtained from the ratio of total electron content (TEC) to the F-region peak electron density (NmF2). In this study the ionospheric slab thickness was obtained during one year of extremely low solar activity (from March 2009 to February 2010). The period analyzed were separated in three seasonal groups: equinoxes, June solstice (winter) and December solstice (summer) months. The ionospheric slab thickness was studied at two stations located around the southern crest of the equatorial ionization anomaly (EIA). One station located in the inner edge of the anomaly region, named Palmas (10.12°S, 48.21°O, 7.73°S dip lat), and another station located under the southern crest of the anomaly, situated at São José dos Campos (23.07°S, 45.52°O, 19.61°S dip lat). The TEC data have been obtained from dual-frequency Global Navigation Satellite System (GNSS) receivers and the NmF2 data were calculated from the foF2 parameter scaled from ionograms recorded by simultaneous measurements of digital ionosondes. The diurnal and seasonal variations of the ionospheric slab thickness at the EIA region were then analyzed. The minimum values of TEC and NmF2 were observed in both stations during the early morning, and the maximum values during afternoon hours. During the summer and equinoctial periods the slab thickness reached their highest values. The nearly equatorial site of Palmas showed values of daytime slab thickness larger than those observed at the low latitude station of São José dos Campos, except during the June solstice months. At São José dos Campos, a pronounced pre-dusk increase in the equivalent slab thickness was observed during the winter solstice months. The atmospheric neutral temperature (Tn) was also estimated from the correlation analysis between the MSIS modeled Tn and the calculated ionospheric slab thickness over the two observations sites. Other relevant aspects of the ionospheric slab thickness behavior and its comparison with International Reference Ionosphere (IRI) model results will be presented and discussed.

HF radar observations of ionospheric backscatter during geomagnetically quiet periods

Abstract. The quiet-time coherent backscatter from the F-region observed by the Tasman International Geospace Environment Radar (TIGER) Bruny Island HF radar is analysed statistically in order to determine typical trends and controlling factors in the ionospheric echo occurrence. A comparison of the F-region peak density values from the IRI-2007 model and ionosonde measurements in the vicinity of the radar's footprint shows a very good agreement, particularly at subauroral and auroral latitudes, and model densities within the radar's footprint are used in the following analyses. The occurrence of F-region backscatter is shown to exhibit distinct diurnal, seasonal and solar cycle variations and these are compared with model trends in the F-region peak electron density and Pedersen conductance of the underlying ionosphere. The solar cycle effects in occurrence are demonstrated to be strong and more complex than a simple proportionality on a year-to-year basis. The diurnal and seasonal effects are strongly coupled to each other, with diurnal trends exhibiting a systematic gradual variation from month to month that can be explained when both electron density and conductance trends are considered. During the night, the echo occurrence is suggested to be controlled directly by the density conditions, with a direct proportionality observed between the occurrence and peak electron density. During the day, the echo occurrence appears to be controlled by both conductance and propagation conditions. It is shown that the range of echo occurrence values is smaller for larger conductances and that the electron density determines what value the echo occurrence takes in that range. These results suggest that the irregularity production rates are significantly reduced by the highly conducting E layer during the day while F-region density effects dominate during the night.

Monitoring the F-region peak electron density using HF backscatter interferometry

[1] High frequency (HF, 10–20 MHz) radars routinely utilize Bragg scatter from plasma fluctuations to monitor ionospheric turbulence. While the propagation of the probing HF radio waves is also strongly affected by the regular structure of the ionosphere, this information is rarely extracted from the data. In the present work we present and test a new technique for estimating the F-region peak electron densities, NmF2, using information about the vertical refraction of HF backscatter echoes that is readily available in the Super Dual Auroral Radar Network (SuperDARN) data set.

Deformation of the ionosphere structure during the space weather events on October–November 2003

We present the spatial maps of the ionosphere–plasmasphere slab thickness τ (ratio of the vertical total electron content, TEC, to the F-region peak electron density, NmF2) during the intense ionospheric storms of October–November 2003. The model-assisted technology for estimate of the upper boundary of the ionosphere, hup, from the slab thickness components in the bottomside and topside ionosphere – eliminating the plasmasphere contribution of τ – is applied at latitudes 35° to 70°N and longitudes −10° to 40°E, from the data of 20 observatories of GPS-TEC and ionosonde networks, for selected days and hours of October and November 2003. The daily–hourly values of NmF2, hmF2 and TECgps are used as the constrained parameters for the International Reference Ionosphere extended to the plasmasphere, IRI-Plas, during the ionospheric quiet days, positive and negative storm phases for estimate of τ and hup. Good correlation has been found between the slab thickness and the upper boundary of the ionosphere for the intense ionospheric storms at October–November 2003. During the negative phase of the ionospheric storm, when the ionospheric plasma density is exhausted, the nighttime upper boundary of the ionosphere is greatly uplifted towards the magnetosphere tail, while the daytime upper boundary of the ionosphere is reduced below 500km over the Earth.

Dependence of the F-region peak electron density (foF2) on solar activity observed in the equatorial ionospheric anomaly region in the Brazilian sector

The long-term (solar cycle) changes in the Sun and how it affects the ionospheric F-region observed at São José dos Campos (23.2° S, 45.9° W), Brazil, a location under the southern crest of the equatorial ionospheric anomaly, have been investigated in this paper. The dependence of the F-region peak electron density (foF2) on solar activity during the descending phase of the 23rd solar cycle for the periods of high, medium, and low solar activity has been studied. The ionospheric F-region peak electron densities observed during high and medium solar activity show seasonal variations with maxima close to the equinox periods, whereas during the low solar activity the maxima during the equinox periods is absent. However, during the low solar activity only change observed is a large decrease from summer to winter months. We have further investigated changes in the different ionospheric F-region parameters (minimum virtual height of the F-region (h′F), virtual height at 0.834foF2 (hpF2), and foF2) during summer to winter months in low solar activity periods, 2006–2007 and 2007–2008. Large changes in the two ionospheric parameters (hpF2 and foF2) are observed during summer to winter months in the two low solar activity periods investigated.

Night−day imprints of ionospheric slab thickness during geomagnetic storm

Spatial maps of the ionosphere–plasmasphere slab thickness ( τ) were generated as a ratio of the total electron content (TEC) to the F-region peak electron density (NmF2) at 1° spaced grid points from the instantaneous maps of TEC and foF2 at latitudes 35° to 70°N, and longitudes −10° to 40°E. Data of 23 observatories are used for the construction of TEC and foF2 maps with Kriging technique from independent networks of GPS–TEC and ionosonde observations at solar minimum (1995–1996) and maximum (2002) under quiet and disturbed magnetic conditions. The net-weight factor ( w) is introduced as a ratio of disturbance to quietness representing area mean TEC, foF2, and τ for a particular day and time normalized by relevant monthly median value. Analysis of w evolution for TEC, foF2 and τ maps have revealed that TEC and foF2 depletion is accompanied by positive increment of slab thickness for more than 48 hrs during the magnetic storm at solar maximum but τ enhancement is shorter and delayed by 12 to 24 hrs regarding the storm onset at solar minimum. The slab thickness positive increment at the main phase of geomagnetic storm has been associated with relevant increase of the real thickness of the topside ionosphere. To estimate the upper boundary of the ionosphere the International Reference Ionosphere expanded towards the plasmasphere (IRI*) is modified to assimilate the ionosonde F2 layer peak and the GPS–TEC observations. Slab thickness is decomposed in three parts (the bottomside and topside ionosphere, and the plasmasphere). Eliminating the plasmasphere part from the total slab thickness, we obtain the ratio of bottomside slab thickness to the real thickness below the F2 layer peak. Assuming that this ratio is also valid above the F2 layer peak, we obtain the topside boundary of the ionosphere varying from 500 km by day to 2300 km by night.

Climatology of ionospheric slab thickness

Abstract. The ionospheric slab thickness τ defined as a ratio of the total electron content (TEC) to the F-region peak electron density (NmF2) has been analysed during the solar maximum (1981) and minimum (1985) phases of an intense, the 21st, solar cycle. Hourly values of TEC and NmF2 collected at Hawaii (low-latitude), Boulder (mid-latitude) and Goosebay (high-latitude) are used in the study. Climatology of the slab thickness is described by the diurnal, seasonal, solar and magnetic activity variations of τ for the different latitude zones. It is found that, for magnetically quiet days of solar maximum, increased ionization of NmF2 and TEC during the daytime is accompanied by an increased thickness of the ionosphere compared to the night-time for non-auroral latitudes. However, the reverse is found to be true during the solar minimum compensating TEC against a weak night-time ionization of NmF2. For the high-latitude the night-time slab thickness is higher compared to the daytime for both the solar phases. Ratios of daily peak to minimum values of slab thickness vary from 1.3 to 3.75 with the peaks of τ often observed at pre-sunrise and post-sunset hours. The average night-to-day ratios of τ vary from 0.68 to 2.23. The day-to-day variability of τ, expressed in percentage standard deviation, varies from 10% by day (equinox, high-latitude) to 67% by night (summer, mid-latitude) during solar minimum and from 10% by day (winter and equinox, mid-latitude) to 56% by night (equinox, high-latitude) during solar maximum. A comprehensive review of slab thickness related literature is given in the paper. Key words. Ionospheric physics

Latitudinal variation of ionospheric slab thickness

The ionospheric slab thickness τ, defined as a ratio of the total electron content (TEC) to the F-region peak electron density N mF2, is a first order measure of the shape of the electron density profile. In the present study, we use GPS-derived TEC and vertical electron content estimates ITEC from ground-based ionosonde observations together with the corresponding f oF2 (F2 layer critical frequency) and h mF2 (F2 layer peak height) at 14 world-wide stations lying at different latitude regions from the geomagnetic equator to north pole. The period of study is during 2001–2002, which is close to the solar maximum phase of the 23rd solar cycle. Hourly values of TEC, ITEC, f oF2, h mF2 and τ during the period of study are used to compare τ values from the observation and model predictions using the International Reference Ionosphere extended towards the plasmapause with the plasmasphere option of the Russian standard model of the ionosphere, IRI*. For the three latitudinal zones (high, mid and low latitudes) the IRI* predictions of τ are compared with observed τ values for the bottomside ionosphere (below h mF2), topside ionosphere (between h mF2 and 1000 km), plasmasphere (from 1000 to 20,000 km), and the total height range through the ionosphere and plasmasphere. Significant overestimation of τ has been revealed when using IRI*, particularly for the topside ionosphere at high latitudes. Relation of the topside part of slab thickness with the topside half peak density height above the F2 layer peak provides new characteristic parameter for modeling of the topside shape of electron density profile.

630 nm nightglow observations from 17°N latitude

Regular measurements of [OI] 630 nm nightglow have been carried out at Kolhapur (16.8°N, 74.2°E, dip lat 10.6°N) in India with the help of tilting filter photometers during clear moonless nights. Several characteristic nightglow data sets are presented in the report for geomagnetically quiet and disturbed nights. A comparison has been made between the simultaneous nightglow and the ionospheric parameters of virtual F-layer height and critical frequency of the F-layer at Ahmedabad (23.02°N, 72.6°E). In general, a good correlation (c.c. = 0.8 to 0.9) is found between the measured airglow emission and F-region peak electron densities during quiet periods but for intense magnetic disturbances the airglow fluctuations are mainly controlled by sharp changes in height variations of the h′ F layer. The period of airglow variation generally corresponds to periods of F-region height variation during magnetic disturbances. Using a long series of data between December 1989 and April 1996, the mean seasonal nocturnal variations of 630 nm emission have been compared during high and low solar activity periods.

A comparative study of low-latitude ionospheric and OI 630nm nightglow observations with the SLIM and IRI models

Measurements of ionospheric F-region peak electron densities, peak heights and atomic oxygen 630 nm airglow emission column intensities, carried out at a low-latitude station located at Cachoeira Paulista (22.7° S, 45.0° W; dip latitude 11.9° S), in Brazil, are analyzed and compared with the ionospheric parameters predicted by the SLIM and by the IRI ionospheric models. The MSIS neutral atmosphere model is used with the IRI model to calculate the 630 nm nightglow emission intensities. This comparative analysis considers data representative of the summer, winter and equinox seasons, obtained during the high solar activity period around 1980. Reasonably good agreement is obtained between the local time variations of the ionospheric and airglow data, and the IRI model predicted parameter variations, but the SLIM model overestimates the F-region peak heights especially during the late afternoon and nighttime hours. These height discrepancies are explained in terms of the strong longitudinal dependence of the electromagnetic E × B vertical plasma drifts and thermospheric neutral winds, which greatly influences the dynamics of the low-latitude ionospheric plasma and airglow emissions. Due to the importance of the relative alignement condition between terminator and magnetic meridian, with its longitudinal and seasonal dependence, on the vertical plasma drifts around sunset, it is suggested that the SLIM ionospheric model should separately consider the longitudinal zones of positive, negative or zero magnetic declination.

Comparison of a low-latitude ionospheric model with observations of oi 630 nm emission and ionospheric parameters

A comparative study is carried out between a semi-empirical low-latitude ionospheric model (SLIM), including calculated OI 630 nm airglow emission intensities predicted by the SLIM model, with ionospheric and OI 630 nm airglow measurements carried out at Cachoeira Paulista (22.7°S, 45.0°W; geomag. 11.9°S), Brazil. A long series of regular measurements of ionospheric F-region peak electron densities and peak heights, and regular zenith measurements of the OI 630 nm airglow emission intensities, made during the period 1975–1982, are used for comparison with corresponding parameters predicted by the SLIM model. Large discrepancies between the observed and model-predicted OI 630 nm intensities have been found for high solar activity, mainly due to an overestimate of the F-region peak heights by the SLIM model. Use of SLIM profiles normalized by actual F-region peak height and density observations essentially removed the disagreements. The reasons for the height discrepancies are discussed in terms of the longitudinal dependence of the geomagnetic control on the E × B vertical plasma drifts and the thermospheric neutral wind effects on the F-region. The effects of the magnetic declination angle on both the E × B ionospheric plasma drifts and thermospheric neutral winds at low-latitude suggest that ionospheric models need to address separately the longitudinal zones of magnetic declination either positive, negative or zero.

Seasonal variations of tropical F-region nightglow emissions and correlative ionospheric studies

Simultaneous zenith measurements of the forbidden OI 630 nm and permitted 777.4 nm nightglow emissions have been carried out at Cachoeira Paulista (22.7°S, 45.0°W; geomag. 11.9° S), Brazil, during the period February 1983-May 1984, a period of medium solar activity. This first long series of simultaneous observations has been analysed to study the mean seasonal-nocturnal variations of these emissions in conjunction with simultaneous ionospheric data, obtained at the same location. Salient features of these observations are presented and discussed. The OI 630 nm emission mean seasonalnocturnal variations show the presence of pre- and post-midnight enhancements, with intensity levels slightly lower in the winter season. The OI 777.4 nm emission mean seasonal-nocturnal variations show a monotonie intensity decrease in time, with very low intensity levels during the winter season. A comparison has been made between the F-region peak electron densities, and heights determined from the optical and ionosonde remote sensing methods. In general, a good correlation is found between the measured and the nightglow inferred ionospheric parameters. The observed airglow intensity variations are also compared with those predicted by a semi-empirical low latitude ionospheric model.

Mapping of ionospheric F-region parameters from atomic oxygen airglow emissions

Simultaneous North-South scanning observations of the OI 7774and 6300nightlow emissions have been carried out at Cachoiera Paulista (geographic coordinates 22.7°S, 45.0°W; geomagnetic latitude 12.0°S), Brazil, to study the tropical F-region dynamics under spread- F and no spread- F conditions. From the observed emission intensities the ionospheric F-region peak electron densities and peak heights can be determined with a good coverage both in latitude and local time. The results are presented in the form of computer generated gray level shade maps showing the variations of the column intensities of the 7774emission ( J 7774) and of the 6300emission ( J 6300), as well as the variations of ( J 7774) 1/2 (related to F-region peak electron density) and of the ratio ( J 7774) 1/2/ J 6300 (related to F-region peak height), as a function of zenith distance (latitude) and local time. Significant features of these observations are discussed. This technique is useful to study dynamical processes and large scale plasma irregularities in the night-time tropical ionosphere.

Diurnal variations of annual and semiannual periodicities in F- region ionisation at Ibadan over a solar cycle

A harmonic analysis of the F-region peak electron density, NmF2 at Ibadan (geom. lat. 10.6°N, dip 6°S) for the years 1964 to 1974 has been performed. The Ibadan results conform with the global nature of the variations as other studies have shown. A pronounced solar cycle variation in the noon ‘bite-out’ at Ibadan is observed whereby the afternoon peak occurs earlier at higher solar activity. It is suggested that this may be attributed to the counter-eiectrojet and the E × B upward drift.

On the dispersal of artificially-injected gases in the night-time atmosphere

We have studied the extent to which various transport processes affect the dispersal of a gas artificially injected into the night-time atmosphere at F-region altitudes. In addition to diffusion, we have found that nonlinear acceleration, viscous stress, and thermospheric winds affect the dispersal of the injected gas. The magnitude of the effect depends on the atmospheric density, which is a function of solar activity. For an injected H 2 gas, non-linear acceleration and viscous stress rapidly become more important than diffusion above about 300 km for low solar activity ( T ∞ = 750K), 340 km for moderate solar activity ( T ∞ = 1000K), and 400 km for high solar activity ( T ∞ = 1500K). For an injected H 2O gas, the corresponding altitudes are 350, 400, and 470 km for low, moderate and high solar activity, respectively. The effect of nonlinear acceleration and viscous stress is to retard the expansion of the injected gas. Thermospheric winds of 150–400 m s −1 are important at altitudes near and below the F-region peak electron density. These winds act to transport the injected gas in the wind direction and this affects the shape and temporal development of the subsequent ionospheric hole. Because the H 2O diffusion coefficient is smaller than the H 2 diffusion coefficient, winds are more important for H 2O than for H 2.

Midlatitude F-region densities and temperatures at sunspot minimum

Abstract Results obtained for F-region densities and temperatures using the Millstone Hill Ionospheric Radar for the year 1964 are presented. The measurements were made during 30-hr periods which followed at intervals of about 2 weeks throughout the year. The data obtained in each month have been averaged to yield a mean electron density profile and mean ion and electron temperature curves for each hour in the day. These curves were in turn used to construct plots showing contours of constant temperature and density as functions of altitude and time for each month. In addition, the annual variation of the midday and midnight densities and temperatures was obtained. The seasonal anomaly in the F-region peak electron density is evident though less pronounced than at years of high sunspot number. The F-region layer thickness undergoes a smooth transition from winter to summer, being greatest in summer. It is shown that the daytime temperatures exhibit no marked seasonal dependence, and hence temperature effects (e.g. on reaction rates) cannot be invoked as the cause. The most striking nighttime phenomenon is the high electron temperature encountered in winter (Te/Ti ∼ 2.0) compared with summer. Evidence is presented for the existence of a flux of fast photoelectrons arriving from the conjugate ionosphere. The nighttime heating of the F-region is thought to be caused by heat conducted down from the protonosphere together, in winter months, with the flux fast photoelectrons from the conjugate hemisphere.