Equatorial Electron Density Research Articles

Equatorial vertical plasma drift velocities and electron densities inferred from ground-based ionosonde measurements during low solar activity

Average values of ionosonde hmF2 data acquired from an African equatorial station have been used to determine vertical plasma drift (Vz) measurements during period of low solar activity. Pre-noon peak was around 1000h LT for all seasons. The peak daytime F2 drift is higher during the equinoctial months with an average of 18.1m/s than the solsticial months (14.7m/s). At nighttime, Vz is characterized first by upward enhancement around 1900h LT with a range of 0.3–8.0m/s, then by a downward reversal. The highest enhancement was recorded in December solstice and start earliest during the March equinox. The peak reversal values are 13.3, 10.7, 9.0 and 4.2m/s for December Solstice, September Equinox, March Equinox and June Solstice respectively. The observed simultaneous post-sunset rise in hmF2 and in vertical E×B drift together with a sharp drop in NmF2 at all season infer that electrons moving away from the equator are at a region of low recombination loss rate. The abrupt faster drift of the plasma away from the equator as indicated by the pre-reversal enhancement (PRE) in upward plasma drift is responsible for the sharp drop in NmF2 immediately after sunset. Some past results were also confirmed in this work.

Modeling the plasmasphere with SAMI3

The study of the plasmasphere is extremely important to understanding space weather phenomena; for example, it plays a critical role in the regulation of radiation belt dynamics. In this paper, the first 3D simulation of the plasmasphere based on the first‐principles physics model SAMI3 is presented. We include the co‐rotation potential, the neutral wind dynamo potential, and a time‐dependent Volland‐Stern‐Maynard‐Chen potential to model the response of the convection potential to an idealized magnetic storm. We find that prior to the storm the plasmasphere is largely toroidal and symmetric in magnetic local time with He+/H+ ~ 5–10%. After the storm, the plasmasphere substantially contracts because plasma is convected away from the outer plasmasphere by the enhanced convection velocity. Moreover, a plume‐like structure forms in the mid‐afternoon sector because of the modified convection pattern associated with the storm. Additionally good agreement is found between the simulation results and data for the L‐shell dependence of the equatorial electron density as well as the electron density along the field line at a given L‐shell under quiet geomagnetic conditions.

Estimation of Magnetospheric Plasma Parameters from Whistlers Observed at Low Latitudes

Whistler observations during nighttimes made at low latitude Indian ground stations Jammu (geomag. lat., 29°26'N; L = 1.17), Nainital (geomag. lat., 19°1'N; L = 1.16) and Varanasi (geomag. lat., 14°55'N; L = 1.11) are used to deduce electron temperatures and electric field in the vicinity of the magnetospheric equator. The accurate curve fitting and parameter estimation technique are used to compute nose frequency and equatorial electron densities from the dispersion measurements of short whistlers recorded at Jammu, Nainital and Varanasi. In this paper, our aim is to estimate the Magnetospheric electron temperatures and electric field from the dispersion analysis of short whistlers observed at low latitudes by using different methods. The results obtained are in good agreement with the results reported by other workers.

The plasmasphere during a space weather event: first results from the PLASMON project

The results of the first 18 months of the PLASMON project are presented. We have extended our three, existing ground-based measuring networks, AWDANet (VLF/whistlers), EMMA/SANSA (ULF/FLRs), and AARDDVARK (VLF/perturbations on transmitters’ signal), by three, eight, and four new stations, respectively. The extended networks will allow us to achieve the four major scientific goals, the automatic retrieval of equatorial electron densities and density profiles of the plasmasphere by whistler inversion, the retrieval of equatorial plasma mass densities by EMMA and SANSA from FLRs, developing a new, data assimilative model of plasmasphere and validating the model predictions through comparison of modeled REP losses with measured data by AARDDVARK network. The first results on each of the four objectives are presented through a case study on a space weather event, a dual storm sudden commencement which occurred on August 3 and 4, 2010.

Does the Equatorial Ionospheric Peak Electron Density Really Record the Lowest During the Recent Deep Solar Minimum?

AbstractThe solar activity in 2008–2009, located at the minimum phase of solar cycle 23/24, was unusually low, which attracts ionospheric physics scientists to explore the plasma behaviors in the ionosphere during the deep solar minimum. Some investigators reported a reduction in ionospheric electron density during this period, while others found a marginal and minor change. In this study we collected the critical frequency of the F2‐layer ionosphere (foF2) data retrieved from ionogram records observed by an ionosonde at Jicamarca (12°S, 76.9°W), an American station located near the dip equator, to examine the changes in electron density in equatorial regions and clarify what caused the inconsistence between the differences of solar minimum foF2 in published work. We determined the foF2 differences in moving yearly, seasonal and monthly medians and the Fourier series analysis. The picture of ionospheric changes is found to be related with the data analysis methods used. The seasonal median and moving yearly values of foF2 were smaller in 2008–2009 than in 1996–1997, both in the daytime and nighttime. In contrast, the monthly median foF2 is found to be of varying solar minimum‐to‐minimum differences. Although greater values in the last minimum are prevailing in the monthly median case at most local times, opposite changes are also found at some time intervals. Further analysis reveals that the reduction in foF2 during the current solar cycle minimum is certainly presented in the longer time scale variations of foF2. Therefore, the inconsistent changes in the published investigations reflect the impact of foF2 variability on the solar minimum‐to‐minimum difference over different time scales.

The annual and longitudinal variations in plasmaspheric ion density

This paper shows that at solar maximum, equatorial ion densities at L = 2.5 are substantially higher at American longitudes in the December months than in the June months. This arises because the configuration of the geomagnetic field causes a longitude‐dependent asymmetry in ionospheric solar illumination at conjugate points that is greatest at American longitudes. For example, at −60°E geographic longitude the L = 2.5 field line has its foot point near 65° geographic latitude in the Southern Hemisphere but near 42° latitude in the Northern Hemisphere. We investigated the consequent effects on equatorial electron and ion densities by comparing ground‐based observations of ULF field line eigenoscillations with in situ measurements of electron densities (from the CRRES and IMAGE spacecraft) and He+ densities (IMAGE) for L = 2.5 at solar maximum. Near −60°E longitude the electron and ion mass densities are about 1.5 and 2.2 times larger, respectively, in the December months than in the June months. Over the Asia‐Pacific region there is little difference between summer and winter densities. Plasmaspheric empirical density models should be modified accordingly. By comparing the electron, helium, and mass densities, we estimate the annual variation in H+, He+, and O+ concentrations near −3°E longitude and −74°E longitude. In each case the He+ concentration is about 5% by number, but O+ concentrations are substantially higher at −3°E longitude compared with −74° E. We speculate that this may be related to enhanced ionospheric temperatures associated with the South Atlantic anomaly.

Magnetospheric electron density long‐term (>1 day) refilling rates inferred from passive radio emissions measured by IMAGE RPI during geomagnetically quiet times

Using measurements of the electron density ne found from passive radio wave observations by the IMAGE spacecraft RPI instrument on consecutive passes through the magnetosphere, we calculate the long‐term (>1 day) refilling rate of equatorial electron density dne,eq/dt from L = 2 to 9. Our events did not exhibit saturation, probably because our data set did not include a deep solar minimum and because saturation is an unusual occurrence, especially outside of solar minimum. The median rate in cm−3/day can be modeled with log10(dne,eq/dt) = 2.22 − 0.006L − 0.0347L2, while the third quartile rate can be modeled with log10(dne,eq/dt) = 3.39 − 0.353L, and the mean rate can be modeled as log10(dne,eq/dt) = 2.74 − 0.269L. These statistical values are found from the ensemble of all observed rates at each L value, including negative rates (decreases in density due to azimuthal structure or radial motion or for other reasons), in order to characterize the typical behavior. The first quartile rates are usually negative for L < 4.7 and close to zero for larger L values. Our rates are roughly consistent with previous observations of ion refilling at geostationary orbit. Most previous studies of refilling found larger refilling rates, but many of these examined a single event which may have exhibited unusually rapid refilling. Comparing refilling rates at solar maximum to those at solar minimum, we found that the refilling rate is larger at solar maximum for small L < 4, about the same at solar maximum and solar minimum for L = 4.2 to 5.8, and is larger at solar minimum for large L > 5.8 such as at geostationary orbit (L ∼ 6.8) (at least to L of about 8). These results agree with previous results for ion refilling at geostationary orbit, may agree with previous results at lower L, and are consistent with some trends for ionospheric density.

Variability of the ionospheric electron density at fixed heights and validation of IRI-2007 profile’s prediction at Ilorin

A study on the variability of the equatorial ionospheric electron density was carried out at fixed heights below the F2 peak using one month data for each of high and low solar activity periods. The data used for this study were obtained from ionograms recorded at Ilorin, Nigeria, and the study covers height range from 100 km to the peak of the F2 layer for the daytime hours and height range from 200 km to the peak of the F2 layer for the nighttime hours. The results showed that the deviation of the electron density variation from simple Chapman variation begins from an altitude of about 200 km for the two months investigated. Daytime minimum variability of between 2.7% and 9.0% was observed at the height range of about 160 and 200 km during low solar activity (January 2006) and between 3.7% and 7.8% at the height range of 210 and 260 km during high solar activity (January 2002). The nighttime maximum variability was observed at the height range of 210 and 240 km at low solar activity and at the height range of 200 and 240 km at high solar activity. A validation of IRI-2007 model electron density profile’s prediction was also carried out. The results showed that B0 option gives a better prediction around the noontime.

Reply to comment by Lei et al. on “A new aspect of ionospheric E region electron density morphology”

[1] Since the FORMOSAT‐3/COSMIC satellites were launched in April 2006, ionospheric electron density profiles have been retrieved from the excess phase of the GPS signal by using the radio occultation technique and can be accessed from the Web site http://www.cosmic.ucar.edu/. On the basis of these electron density profiles and the use of data quality control criteria developed by Yang et al. [2009], Chu et al. [2009] investigated E region electron density morphology and showed that the general properties of the COSMIC‐ retrieved E region electron density are in good agreement with the predictions of the Chapman layer theory that was developed in accordance with photochemical process and controlled by solar zenith angle. Nevertheless, Chu et al. [2009] found existences of salient enhancements in the noontime E region electron density not only at the geomagnetic equator but also in the geomagnetic latitude regions ±15°−35°, which cannot be explained by the Chapman layer theory. In addition, they also provided compelling evidence to show the presence of longitudinal wave number 3 and 4 structures of the equatorial electron density in a height range of 100–200 km, which is in excellent agreement with longitudinal structures of equatorial electrojet intensity derived from equatorial magnetic field data obtained by the Orsted, CHAMP, and SAC‐C satellites during the years 1999–2006. [2] On the basis of the simulation result obtained by Yue et al. [2010], Lei et al. [2010] question the validity of the E region electron density retrieved by the GPS radio occultation technique. They argue that because of the presence of the ionospheric electron density gradient in the horizontal direction that violates the spherical symmetry assumption of the Abel transform for inverting ionospheric electron density profile from calibrated total electron content along the GPS raypath, the COSMIC‐measured E region electron density enhancements in midlatitude regions were caused by the retrieval error of the GPS radio occultation process. From Figure 1 of Lei et al. [2010] (identical to Figure 2 of Yue et al. [2010]), the simulation‐retrieved E region electron densities around 100 km in equinox season are approximately 50– 100% and 150–200% greater than the “true” model values at the geomagnetic equator and in geomagnetic latitude regions ±30°−50°, respectively. In addition, the former are about 150–200% smaller than the latter in geomagnetic latitude regions ±10°−30°. If the simulation‐retrieved results obtained by Yue et al. [2010] were true and able to be representative of the general GPS occultation‐retrieved results, the morphologies of the COSMIC‐measured E region electron density should be in accord with those of the simulation results. Namely, the E region electron density retrieved by COSMIC satellites should be much greater (smaller) than true measurement made by the ground‐based ionosonde in geomagnetic latitude regions ±30°–50° (±10°–30°). In order to validate the simulation‐retrieved results, we compare peak values of E layer electron density NmE between COSMIC retrieval and global ionosonde measurement in the different latitudinal regions for July 2006 to July 2009. The COSMIC data were selected for comparison if the separation between COSMIC occultation point and ionosonde station is 10 min in time and 2.5° in space. Figure 1 presents an example of latitudinal variation in histograms of percent errors of NmE between COSMIC retrieval and ionosonde measurement for spring (March–May) 2007–2009, in which the percent error (PE) is defined by

Longitudinal behaviors of the IRI-B parameters of the equatorial electron density profiles retrieved from FORMOSAT-3/COSMIC radio occultation measurements

The electron density profiles in the bottomside F 2-layer ionosphere are described by the thickness parameter B0 and the shape parameter B1 in the International Reference Ionosphere (IRI) model. We collected the ionospheric electron density ( N e) profiles from the FORMOSAT-3/COSMIC (F3/C) radio occultation measurements from DoY (day number of year) 194, 2006 to DoY 293, 2008 to investigate the daytime behaviors of IRI-B parameters ( B0 and B1) in the equatorial regions. Our fittings confirm that the IRI bottomside profile function can well describe the averaged profiles in the bottomside ionosphere. Analysis of the equatorial electron density profile datasets provides unprecedented detail of the behaviors of B0 and B1 parameters in equatorial regions at low solar activity. The longitudinal averaged B1 has values comparable with IRI-2007 while it shows little seasonal variation. In contrast, the observed B0 presents semiannual variation with maxima in solstice months and minima in equinox months, which is not reproduced by IRI-2007. Moreover, there are complicated longitudinal variations of B0 with patterns varying with seasons. Peaks are distinct in the wave-like longitudinal structure of B0 in equinox months. An outstanding feature is that a stable peak appears around 100°E in four seasons. The significant longitudinal variation of B0 provides challenges for further improving the presentations of the bottomside ionosphere in IRI.

Error analysis of Abel retrieved electron density profiles from radio occultation measurements

Abstract. This letter reports for the first time the simulated error distribution of radio occultation (RO) electron density profiles (EDPs) from the Abel inversion in a systematic way. Occultation events observed by the COSMIC satellites are simulated during the spring equinox of 2008 by calculating the integrated total electron content (TEC) along the COSMIC occultation paths with the "true" electron density from an empirical model. The retrieval errors are computed by comparing the retrieved EDPs with the "true" EDPs. The results show that the retrieved NmF2 and hmF2 are generally in good agreement with the true values, but the reliability of the retrieved electron density degrades in low latitude regions and at low altitudes. Specifically, the Abel retrieval method overestimates electron density to the north and south of the crests of the equatorial ionization anomaly (EIA), and introduces artificial plasma caves underneath the EIA crests. At lower altitudes (E- and F1-regions), it results in three pseudo peaks in daytime electron densities along the magnetic latitude and a pseudo trough in nighttime equatorial electron densities.

A new whistler inversion method

A new whistler inversion method has been developed to obtain plasmaspheric electron densities and propagation paths deduced from measured whistler data. It is based on the exact Appleton‐Hartree dispersion relation and recent experimental density distribution models, comprising the following components: a longitudinal whistler wave propagation model; an empirical electron density distributions model along the field lines based on Polar spacecraft data; and dipole and International Geomagnetic Reference Field approximation of the Earth's magnetic field. The new method predicts electron densities and propagation paths different from the earlier methods. The validation of the method is difficult owing to lack of in situ measurements. A multiple‐path whistler group model was introduced in addition to the whistler inversion method assuming a simplified (logarithmic) dependence of equatorial electron density. A new, time‐frequency domain transformation of multiple path groups led us to validate the components of the whistler inversion method, and it adds a major extension to the inversion method in the case of multiple‐path whistler groups. Beside that, the method is capable of providing electron density profiles for the plasmasphere by the analysis of multiple‐path propagation whistlers.

Theoretical study of new plasma structures in the low‐latitude ionosphere during a major magnetic storm

Theoretical model simulations for an intense magnetic storm show the creation of a low‐latitude electron density arch aligned along the geomagnetic field created by strong uplift of the F2 layer that is driven by the penetration electric field. When the arch forms during the day, a new F2 layer is created at the original altitude by photoionization, and a density hole can be created between this new F2 layer and the arch. When the arch forms during the night, the F2 layer is not recreated and no hole forms. In a vertical profile of electron density, the daytime elevated ionospheric layer can appear distinctly from the recreated F2 layer, in which case the elevated layer is called the F3 layer. A latitude cut through the night‐side arch shows the characteristics of an equatorial electron density trough, bounded to the north and south by enhanced densities associated with plasma that has diffused down along the geomagnetic field from the elevated layer. It is pointed out that the signature of the night‐side arch has been seen in low Earth orbit spacecraft data, but has not been previously associated with an arch structure.

Geminal interaction and “anomeric effect”

Quantum-chemical calculations by the methods of RHF/6-31G(d) and MP2/6-31+G(d) show equal ratio of the lengths of axial and equatorial bonds and electron density on the atoms forming them in cyclohexane and its mono derivatives and in six-membered heterocyclic molecules as well. This feature is due to the interaction of atoms in the chair form of these molecules regardless of the presence in them of a heteroatom. Introducing heteroatom to a cyclohexane ring leads only to increase in the difference between axial and equatorial bonds. This equation excludes the possibility to ascribe the elongation of axial bonds and increase in the electron density on atoms forming them in the heterocyclic molecules to the p,σ*-conjugation of the lone electron pair of the ring heteroatom with the antibonding orbital of the axial bond. Features of molecular geometry defined by the mutual influence of atoms in them, including inductive and non-inductive interaction of geminal atoms in triatomic groups Y-Z-M, result in energetic preference of gauche conformations of these molecules and “anomeric effect” in them.

Solar activity dependence of the electron density in the equatorial anomaly regions observed by CHAMP

We have investigated the solar activity dependence of the electron density at equatorial and low latitudes using 6 years (a) of measurements between 1 August 2000 and 1 August 2006 from CHAMP and compared it with the international reference ionosphere (IRI) model. The solar activity dependence observed by CHAMP at 400 km altitude exhibits significant variation with latitude, season, and local time. First, the electron density in the crest regions of the equatorial ionization anomaly (EIA) grows roughly linearly from solar minimum to solar maximum, with a higher growth rate than that in the EIA trough region. Second, the solar activity dependence in the EIA crest regions varies strongly with season. The growth rate of the electron density with increasing solar activity around equinoxes is about 1.5 to 2 times greater than that near solstices. Third, the solar activity dependence of the EIA structure varies significantly with local time. In the noon sector the crest‐to‐trough ratio (CTR) obtained at 400 km altitude varies within only a small range between 1.14 and 1.43 from solar minimum to solar maximum. In the postsunset local time sector, however, the CTR grows remarkably with solar activity level, reaching values of above 3.9 at solar maximum. These differences are attributed to the different solar activity dependence of the vertical plasma drift in corresponding local time sectors. The IRI model was found to reproduce the equatorial electron density well near 400 km in the noon sector at all solar activity levels. However, it significantly overestimates it in the postsunset to premidnight sector at high solar activity levels. The major cause for this overestimation has been found to be the IRI's inadequate representation of the F2 layer maximum height (hmF2) in this sector, while the IRI's lack of equatorial spread F seems to play only a minor role.

Longitudinal and seasonal variations in plasmaspheric electron density: Implications for electron precipitation

The tilt and offset of the Earth's magnetic field can significantly affect the longitudinal and seasonal distribution of electron density in the plasmasphere. Here we show that for the solar maximum conditions of 1990–1991, the largest annual variation determined from CRRES measurements of plasmaspheric equatorial electron density in the range L = 2.5–5.0 occurs at American longitudes (−60°E), while no annual variation occurs at Asian longitudes (+100°E). Plasmaspheric electron density is larger in December than in June at most longitudes, from −180°E eastward to +20°E. At all other longitudes the density ratio from December to June is very close to 1.0. The largest December/June density ratio is at L = 3.0 at American longitudes (−60°E). At L = 4.5 and above, the annual variation disappears. The lowest electron density values for a given L‐shell occur at American longitudes, in June. Ion densities also show significant annual variations, with similar longitudinal and seasonal characteristics in the case of IMAGE EUV He+ measurements. Atomic mass density measurements calculated using the magnetometer cross‐phase technique show significant seasonal variations but also imply composition changes with longitude. Using the quasilinear PADIE code we calculate the bounce‐averaged diffusion rate of electrons by plasmaspheric hiss with a fixed wave intensity. December to June variations in plasmaspheric density, particularly at American longitudes, drive changes in the wave‐particle interactions, increasing diffusion into the loss cone by a factor of ∼3 at 1 MeV at L = 3.0, thus hardening the electron precipitation spectrum during the southern hemisphere winter (in June).

A study of Sq(H) variations over equatorial electrojet regions

The newly established geomagnetic field observations in Japan, have enabled us to analyse the 1998 data of Huancayo, Kiritimati (Christmas Island) and Pohnpei where the geomagnetic Sq(H) variations of equatorial electrojet have been studied. The diurnal variation of the monthly means of Sq(H) on the five international quiet days showed mostly expected diurnal variation of equatorial electrojet regions as well as some abnormal variations. Seasonal variation with equinoctial maxima and solstial minima was also observed from the analysis. The presence of equinoctial maxima is likely to be due to enhanced equatorial electron density and the electric field at equinox. The day to day variability may be attributed to electric field, a dawn to dusk phenomenon. Keywords: Equatorial electrojet, seasonal variation, amplitude, abnormal variation and solar quiet day variation Nigerian Journal of Physics Vol. 18 (1) 2006: pp. 45-52

Short‐term relationship between solar irradiances and equatorial peak electron densities

The short‐term relationship of the equatorial peak electron density and the solar short‐wavelength irradiance is examined using foF2 observations from Jicamarca, Peru and recent solar irradiance measurements from satellites. Solar soft X‐ray measurements from both the Student Nitric Oxide Explorer (SNOE) (1998–2000) and Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) (2002–2004) satellites as well as extreme ultraviolet (EUV) measurements from the TIMED satellite are used. Soft X‐rays show similar or higher correlation with foF2 at short timescales (27 days or less) than EUV does, although the EUV correlation is higher for longer periods. For the short‐term variations, both SNOE and TIMED observations have a higher correlation in the morning (∼0.46) than in the afternoon (∼0.1). In the afternoon, SNOE observations have a higher correlation (∼0.2) with foF2 than the TIMED observations (∼0.1 correlation), which may be due to differences in the solar cycle. At morning times, foF2 has a ∼27‐day variation, consistent with the solar rotation rate. After noon, but not in the morning, a ∼13.5‐day variation consistently appears in foF2. This ∼13.5‐day variation is attributed to geomagnetic influences.

A simple scale height model of the electron density in Saturn's plasma disk

A study of electron densities in Saturn's inner magnetosphere is presented using measurements of the upper hybrid resonance frequency obtained from the Radio and Plasma Wave Science (RPWS) instrument on the Cassini spacecraft. The study uses data from the first 16 months of operation in orbit around Saturn. The distribution of density data spans latitudes up to 20° and ranges from 3.6 ≤ L ≤ 8.6. The results are compared to a simple centrifugal potential model for the plasma density. The measurements for 5 ≤ L ≤ 8.6 yield a good fit to an equatorial electron density profile that varies as n0 = (51,880) L−4.1 cm−3 and a plasma scale height that varies as H = (0.047) L1.8 RS, where RS is the radius of Saturn. The measurements for L < 5 are more variable, most likely due to plasma injection effects by Saturn's moon Enceladus which is a known source of neutral gas. A contour map of the electron densities derived from the centrifugal potential model is presented over the entire range of L values and latitudes analyzed, using cubic polynomials to represent the radial profiles of the equatorial electron density and the plasma scale height.

Equatorial electron density measurements in Saturn's inner magnetosphere

Upper hybrid resonance emissions detected by the Radio and Plasma Wave Science (RPWS) instrument on the Cassini spacecraft are used to obtain electron densities on five equatorial orbits of Saturn at radial distances ranging from 3 to 9 saturnian radii (RS). The electron density profiles for these orbits show a highly repeatable radial dependence beyond 5 RS, decreasing with increasing radial distance approximately as (1/R)3.63. Inside 5 RS, the electron density profiles are highly variable. We show that these radial variations are consistent with a centrifugally‐driven outward transport of plasma from a source inside 5 RS.