Region Electron Density Profiles Research Articles

A Simplified Method of True Height Analysis to Estimate the Real Height of Sporadic E Layers

AbstractThe sporadic E (Es) layer virtual height and the ordinary wave critical frequency are routinely measured ionogram parameters, used to characterize the Es altitude occurrence and intensity. It has become common practice to take the real height to be about equal to by assuming that signal propagation delays in the E region plasma below the layer are small and can be neglected. Although this applies for nighttime, during daytime it may overestimate significantly. The present paper relies on true height analysis theory to devise a simplified method and propose an algorithm that can estimate Es real heights reasonably well. The method relies on and ionosonde measurements and E region electron density profiles obtained from the International Reference Ionosphere model. The algorithm is applied to a typical set of Digisonde observations to compute and examine real height variations and functional dependencies. Whereas ≃ at nighttime, during daytime there are notable − differences taking values less than 10 km for most of the observed layers. During the early morning and early afternoon hours, however, when weak layers appear at upper heights, the virtual to real height differences become larger reaching 20–25 km. The method proposed here for the estimation of can be easily applied to improve the accuracy of the results of sporadic E layer studies.

Two-dimensional electron temperature and density profiles of Hall thruster plume plasmas using tomographically reconstructed optical emission spectroscopy

Two-dimensional electron temperature and density profiles in the plume region of 300 W-class Hall thruster Ar plasmas were obtained using tomographically reconstructed optical emission intensity profiles combined with a collisional-radiative (CR) model. A total of 1242 lines of sight were used by rotating the thruster to apply inverse Radon transform-based tomographic reconstructions and Abel inversion. The reconstruction accuracy of the developed diagnostic system was evaluated using a priori images derived from plasma pictures, and the reconstruction error was less than 1% in the region of interest, exhibiting higher accuracy than the Abel inversion. From the Ar Hall thruster plasma, more than 12 different two-dimensional profiles of Ar I emission intensity within a spectral range of 600–1000 nm were obtained 6 mm from the exit plane of the thruster. A CR model using 31 allowed transitions at 15 different states from the ground state to the 2p i states was incorporated with the tomographically reconstructed emission intensity sets. Consequently, two-dimensional electron temperature and density profiles in the range of 5–18 eV and 2.0 × 1016–4.7 × 1017 m−3 were obtained, respectively, exhibiting reasonable agreement with the double Langmuir probe measurements.

Assimilating VLF Transmitter Observations With an LETKF for Spatial Estimates of the ${D}$ -Region Ionosphere

The ${D}$ -region ionosphere plays an important role in coupling the neutral atmosphere to denser ionospheric plasma above. Practically, it impacts long-range radio communications between extremely low frequencies (ELFs), where communications are enabled by the layer, and high frequencies (HFs), which are attenuated by the layer. A combination of its altitude and extremely low electron density means that it is difficult to make measurements of the region using typical ionospheric remote sensing techniques, and our knowledge of the ${D}$ -region is limited as a result. This article presents the development of an ensemble Kalman filter method to spatially map ${D}$ -region electron density profiles over the continental United States using an array of very low frequency (VLF) radio transmitters and receivers. Data assimilation has previously been used to estimate higher altitude regions of the ionosphere, but its application to VLF radio measurements and the ${D}$ -region is new. The technique has several favorable features, including statistical confidence measures with every estimate, the ability to localize the influence of measurements, inclusion of physically realistic spatial correlations, relatively fast convergence, and the ability to add observations to the estimate as they become available. We describe the filter and present results for day, night, and terminator ionospheres using simulated data. We also present a study on the robustness of the filter when initialized with a prior that is far from the true ionosphere. The method shows promise for application to real data in the near future, especially for estimating the characteristics of large-scale features in the ${D}$ -region.

Ionospheric D Region Remote Sensing Using ELF Sferic Group Velocity

AbstractThe lowest region of the ionosphere, the D region, plays an important role in magnetosphere‐ionosphere coupling but is challenging to directly observe. The group velocity of the extremely low frequency (ELF; 3–300 Hz) portion of lightning induced electromagnetic radiation can be used to diagnose the D region electron density profile. Day‐night conditions can be assessed using ELF receivers and lightning detection networks. Analytical formulations and the Long Wave Propagation Capability software package show that ELF group velocity has particular sensitivity to the sharpness of the exponential electron density profile. Applying the technique to sudden ionospheric disturbances shows that the group velocity increases in response to incidence of solar X‐ray flux . A small number of ELF receivers can provide a large‐scale diagnostic of the D region. ELF remote sensing using lightning is complementary to very low frequency remote sensing and can be used to assess the Earth‐ionosphere propagation channel for very low frequency transmitters.

F region electron density profile inversion from backscatter ionogram based on international reference ionosphere

Ionospheric backscatter sounding transmits HF (3–30MHz) radio wave obliquely into ionosphere and receives echoes backscattered from remote ground. Due to the focusing effect, the echoes form leading edge on the swept frequency backscatter ionogram (BSI). This kind of backscatter ionogram contains plentiful ionospheric information, such as electron density, radio wave propagation modes and maximum usage frequency (MUF). By inversion algorithm, the backscatter ionogram can provide two-dimensional electron density profile (EDP) down range. In this paper, we propose an ionospheric F2 region EDP inversion algorithm. By utilizing the F2 bottomside electron density profile represented by the International Reference Ionosphere (IRI) model and ray tracing techniques, this approach inverts the leading edge of the backscatter ionogram to two dimensional F region EDP. Results of validation experiments demonstrate that the inverted ionospheric EDPs show good agreement with the results of vertical ionosonde and provide reliable information of ionosphere. Thus the proposed inversion algorithm provide an effective and accurate method for achieving large scale and remote ionospheric electron density structure.

High contrast 2D visualization of edge plasma instabilities by ECE imaging

High contrast high resolution 2D images of edge MHD instabilities have been obtained for the KSTAR H-mode plasmas in 2010 using an electron cyclotron emission (ECE) imaging system. A fast structural evolution of the edge instabilities has been identified where the validity of the observed structures, i.e., the local measurement is ensured by the high contrast. On the other hand, the exact interpretation of the ECE intensity (Trad) is not straightforward due to the marginal optical depth ( ∼ 1) in the plasma edge region. The effect of the electron temperature (Te) and density (ne) profiles in the edge region on the ECE localization and intensity have been evaluated for typical KSTAR H-mode discharges.

Daytime ionosphericDregion sharpness derived from VLF radio atmospherics

[1] We described and applied a technique to measure the local midlatitude daytime ionospheric D region electron density profile sharpness from the Earth-ionosphere waveguide mode interference pattern in the spectra of radio atmospherics (or sferics for short), which are the high-power, broadband, very low frequency (VLF, 3–30 kHz) signals launched by lightning discharges. VLF propagation simulations are used to show that the upper VLF frequency spectral minima of sferics on several hundred kilometers long propagation paths depend critically on the effective D region sharpness while depending only weakly on the effective D region height. This enables the straightforward extraction of the sharpness parameter from measured VLF spectra, which generally exhibit well-defined minima at upper VLF frequencies. By applying this technique, we calculated the profile sharpness during morning, noontime, and afternoon periods in 3 different days using sferics from ∼660–800 km away. The measured sharpness showed a weak dependence on the solar zenith angle, with values between 0.35 and 0.45 km−1 for angles from 20° to 75°. This is different from the previous narrowband measurement since the sharpness derived from narrowband VLF signals highly depends on the solar zenith angle. To better understand this discrepancy, we also used simulations to calculate the equivalent exponential profiles for International Reference Ionosphere (IRI) profiles and the empirical FIRI model profiles. The equivalent exponential profiles can best duplicate the sferic spectral characteristics for IRI and FIRI models. We find that both the magnitudes and solar zenith angle variations of the sharpness for our broadband measurements, previous narrowband measurements, and both models are completely different. This suggests the daytime ionosphere, particularly at larger solar zenith angles, may not be well described by a simple two-parameter exponential model.

Gyrokinetic modelling of electron and boron density profiles of H-mode plasmas in ASDEX Upgrade

Local gyrokinetic calculations of the logarithmic gradients at mid-radius of both electron and boron densities in ASDEX Upgrade H-mode plasmas are presented and compared with the experimental observations. The experimental results show that both the electron and the boron density profiles increase their peaking in response to the addition of central electron cyclotron heating over a background of neutral beam injection (NBI) heating. The boron density profiles are always less peaked than the electron density profiles in the confinement region, and are flat or even slightly hollow in the presence of NBI heating only. The experimental behaviours are well reproduced by the theoretical predictions. The agreement allows the identification, through theoretical modelling, of the transport mechanisms responsible for the observed dependences. In particular, the observed increase in the logarithmic electron density gradient with increasing central electron heating is explained by a concurrent reduction of the outward pure convection and an increase in the inward thermodiffusion. In addition, it is found that the plasma toroidal rotation velocity and its radial gradient play a non-negligible role in the turbulent boron transport, and allow the prediction of a decrease in boron peaking with increasing rotation velocity, which is consistent with the experimental observations.

Assessment of Ionospheric Models package for the Community Coordinated Modeling Center: Climatology

[1] Due to the paucity of ionospheric density observations, ionospheric models are important for constructing the day-to-day 3-D specifications of electron density. Quality specifications are required to mitigate ionospheric influences on modern GPS navigation and communications technologies. The Arecibo Radar Observatory Incoherent Scatter Radar has provided F region electron density profiles spanning 50 years and 4 solar cycles. We have collected, reduced, and cleaned the Arecibo radar electron density profiles to create a ground truth database for use in the metric assessment of the accuracy of empirical and physics-based ionospheric models. The metrics and skill assessments within this Assessment of Ionospheric Models package are described herein. The assessment package will be implemented at the Community Coordinated Modeling Center to provide a Web-based generator of metrics and skill scores based on several electron density profile parameters: peak density values, peak density altitudes, and the shape of the electron density profile with altitude. This Assessment of Ionospheric Models package will greatly assist in improving current models of the ionosphere.

Midlatitude daytime D region ionosphere variations measured from radio atmospherics

We measured the midlatitude daytime ionospheric D region electron density profile height variations in July and August 2005 near Duke University by using radio atmospherics (or sferics for short), which are the high‐power, broadband very low frequency (VLF) signals launched by lightning discharges. As expected, the measured daytime D region electron density profile heights showed temporal variations quantitatively correlated with solar zenith angle changes. In the midlatitude geographical regions near Duke University, the observed quiet time heights decreased from ∼80 km near sunrise to ∼71 km near noon when the solar zenith angle was minimum. The measured height quantitative dependence on the solar zenith angle was slightly different from the low‐latitude measurement given in a previous work. We also observed unexpected spatial variations not linked to the solar zenith angle on some days, with 15% of days exhibiting regional differences larger than 0.5 km. In these 2 months, 14 days had sudden height drops caused by solar flare X‐rays, with a minimum height of 63.4 km observed. The induced height change during a solar flare event was approximately proportional to the logarithm of the X‐ray flux. In the long waveband (wavelength, 1–8 Å), an increase in flux by a factor of 10 resulted in 6.3 km decrease of the height at the flux peak time, nearly a perfect agreement with the previous measurement. During the rising and decaying phases of the solar flare, the height changes correlated more consistently with the short, rather than the long, wavelength X‐ray flux changes.

Midlatitude nighttime D region ionosphere variability on hourly to monthly time scales

Significant temporal variability of the nighttime D region is well known, but its inaccessibility means that the time scales, magnitudes, and sources for that variability are not well understood. We probed the ionospheric D region by measuring the high‐power broadband very low frequency (VLF) signals launched by lightning and propagating in the Earth‐ionosphere waveguide. We analyzed broadband sferic data of July and August 2005 recorded by our sensors located near Duke University by comparing measured sferic spectra to model results and extracted the height of an assumed exponential electron density profile for each measurement. The measured nighttime D region electron density profile heights showed large temporal variations of several kilometers on some nights and relatively stable behaviors on others. The measured hourly average heights in 260 h ranged between 82.0 and 87.2 km, with a mean value of 84.9 km and a standard deviation of 1.1 km. The maximum variation in the 5 h period was around just above 4.0 km and the maximum variation in the 1 h period was around 1.3 km, with sharper gradients observed over shorter time periods. We also observed spatial variability as large as 2.0 km over 5° latitudes on some nights and no spatial variability on others. On some nights, the temporal variability exhibited a close correlation with the occurrence rate of lightning discharges under the probed region. This suggests that the direct energy coupling between lightning discharges and lower ionosphere can be a significant source of the D region variability. However, on other nights, the measured height temporal variability showed weak to no correlation with local lightning, displaced lightning (as would be expected for lightning‐induced electron precipitation), or geomagnetic activities. These measurements suggest that nighttime D region variability may be driven by many sources.

Pre-storm electron density enhancements at middle latitudes

Substantial increases of the F2 region peak electron density several hours to a day before the geomagnetic storm onset, the so-called pre-storm enhancements, belong to still not clear and hardly predictable features of the ionospheric disturbances. This paper presents analysis of the pre-storm enhancements observed at middle latitudes for 15 storms out of 65 strong-to-severe geomagnetic storms of the period 1995–2005. All 15 events were accompanied by significant (>20%) increases of foF2 before the storm onset over European area. We focus on the longitudinal extent and height profile of the pre-storm enhancements, particularly on their effects on the F1 and E regions of the ionosphere. Possible origin of such enhancements is also partly discussed. We observe no systematic effect of pre-storm enhancements of foF2 in electron density profiles in the F1 region. The E region (foE) appears to be insensitive to pre-storm enhancements. We find the pre-storm enhancements to be confined to the F2 region. The longitudinal extent of the pre-storm enhancements seems to be 120–240° based on comparison of simultaneous foF2 measurements in Europe, northern USA, and Eastern Asia.

Spatial distribution of ionization in the equatorial and low‐latitude ionosphere of the Indian sector and its effect on the pierce point altitude for GPS applications during low solar activity periods

The equatorial and low‐latitude ionosphere is characterized by dynamic changes in the spatial distribution of F region electron density. The in situ density data from Indian SROSS C2 satellite was used in conjunction with the international reference ionosphere (IRI) to demonstrate how the median models have limitations to represent the spatial gradients in the equatorial and subtropical latitudes in the Indian sector. The ground‐based ionosonde data was used to derive the bottomside electron density profiles which were matched with IRI to derive the complete F region electron density profile for different times to show the variability in the centroid of ionization mass distribution due to the equatorial plasma dynamics associated with Appleton ionization anomaly. The study brings out that a uniform ionospheric pierce point altitude cannot be assumed for GPS applications in this sector. However, during low sunspot activity periods, the GPS‐derived total electron content (TEC) using 2 × 2 grid point data indicates that spatial gradients in TEC are not too significant in meridian transects and that the assumption of homogeneous distribution is valid for the conversion of slant to vertical TEC even in the equatorial latitudes during solar minimum. Keeping this in view, a qualitative study has been carried out on the possible variability in ionospheric pierce point altitude at representative subtropical latitude in the Indian sector and the results are discussed in light of diurnal and seasonal features of the equatorial ionosphere.

Nighttime D region electron density profiles and variabilities inferred from broadband measurements using VLF radio emissions from lightning

Lightning discharges radiate most of their electromagnetic energy in the very low frequency (VLF, 3–30 kHz) and extremely low frequency (ELF, 3–3000 Hz) bands and are, consequently, an effective tool for remotely sensing the nighttime ionospheric D region electron density profile. Using broadband lightning‐generated VLF signals, we derived the night‐to‐night variations of the midlatitude ionospheric D region electron density profiles. For 16 nights, between 1 July and 4 August 2004, we examined measured VLF data from lightning occurring near the United States east coast (∼530–860 km away from Duke University) and received at Duke University from 0400 to 0600 UT. From these observed VLF radio atmospherics (sferics), we extracted the nighttime D region electron density profiles covering the range of electron densities from 100 to 103 cm−3, in the altitude range of approximately 70–95 km, using a two‐dimensional, laterally homogenous model of VLF propagation in the Earth‐ionosphere waveguide. The inferred electron density profile variabilities were in good agreement with those from past nighttime rocket experiments at similar latitudes. Using the rocket‐measured profiles in our propagation simulations, we determined that the two‐parameter exponential D region electron density profiles we inferred were the best exponential fit, in the electron density range of ∼3 to ∼500 cm−3, to the rocket‐measured D region electron density profiles. In an initial effort to determine the sources of the observed variabilities, we compared the SAMPEX precipitating electron measurements to the electron density profiles inferred during July 2000. The results indicate that high‐energy electron precipitation might account for at least part of the night‐to‐night variations of the D region electron densities at the midlatitudes.

Modeling polar ionospheric effects during the October–November 2003 solar proton events

At Ny Ålesund, Svalbard (78°54′N, 11°53′E, L ∼ 18), a narrowband VLF receiver was used to monitor the behavior of the amplitude of several high‐power transmitters located in the Northern Hemisphere under the influence of the solar proton events (SPE) of October/November 2003. We have used Sodankylä ion chemistry (SIC) atmospheric model profiles calculated at the midpoint location of the propagation paths in the northern wintertime polar region to investigate the radio propagation properties of several high‐latitude paths. Different paths showed different responses to the proton precipitation, but propagation modeling was broadly able to account for most of the positive and negative responses observed. Using the SIC‐based electron density profiles, we have been able to develop models of ionospheric effective height (h′) and sharpness (β) in order to describe the D region behavior as a function of proton flux, extending previous work which reported β and h′ values as functions of the X‐ray flux from solar flares. As a result of these models, our understanding of VLF propagation influenced by SPEs is such that VLF observations might be used to predict changes in the ionospheric D region electron density profiles during other particle precipitation events.

Electron density profiles in the equatorial E region ionosphere derived from a bistatic coherent scatter radar experiment in Perú

A new bistatic radar system has been developed at the Jicamarca Radio Observatory. The system is a permanent addition to the facility designed to monitor electron density profiles in the equatorial electrojet region using a coherent scatter radar technique that utilizes the Faraday rotation of the scattered signal. A series of radar experiments has been conducted at Jicamarca since March, 2004. In this report, representative E region electron density profiles of the equatorial ionosphere are presented. We compare our electron density estimates with: theoretical electron densities derived from the Chapman production function, electron densities measured by rocket experiments, the International Reference Ionosphere (IRI‐2001) model, and peak electron density estimates from the Jicamarca Digisonde. Overall, the radar measured density profiles have magnitudes and shapes comparable to the electron density estimators mentioned above. However, the measured and IRI‐2001 model density profiles disagree significantly below the E region peak.

Computer experiments on electromagnetic environment of plasma sheath at conducting surface

We focused on the electromagnetic environment of the sheath region created at the antenna surface by performing EM-PIC (Particle-In-Cell) simulations. In the previous theoretical studies electromagnetic wave modes called sheath waves are found, which propagate along the plasma–metal interface on the assumption that the sheath region is vacuum. To examine the property of the sheath waves, we adopted a two-dimensional model which includes a metal surface of antenna immersed in magnetized background plasma. Due to the difference of thermal velocity between electrons and ions in the isothermal plasma, an ion sheath where electron density is low is basically created at the antenna surface. In the simulations, we could basically confirm the sheath waves propagating along the metal–plasma interface at the frequencies lower than those of normal wave modes for uniform plasma with no external magnetic field. In magnetized plasma, we also could observe the sheath waves below the Upper Hybrid Resonance frequency. However, in the dispersion relation obtained by taking the Fourier transformation of electromagnetic components, the intensity of the sheath waves is rather weak and difficult to be recognized. The difference between the theoretical analysis and the present PIC simulations can be accounted for by the difference of electron density profiles in the sheath region. In the simulations, we found that the electron density gradually changes at the transition region from the background plasma to the sheath while, in the theory, a step function between a vacuum sheath and the background plasma is assumed. In the simulation of a large sheath case where low density region at the antenna surface can be assumed as a vacuum region, sheath waves are clearly observed as predicted in the linear dispersion relation. We could also confirm unidirectional waves propagating below the UHR frequency.

Ionospheric D region electron density profiles derived from the measured interference pattern of VLF waveguide modes

The altitude profile of electron density in the D region is determined using an array of GPS‐phase‐referenced VLF receivers, allowing for the simultaneous broadband (i.e., phase coherent) measurement of a 24.0 kHz signal at multiple sites ranging in propagation path length from 3060 to 3353 km. Measured data are compared with results of model calculations for four electron density profiles used in previous work, assuming a single density profile to be in effect along the entire signal path from transmitter to receiver. Interpolation between model profiles is used to find a new profile which best fits the data. Phase coherent measurements over a range of path lengths also allow for the identification of the parameters for individual waveguide modes. Measured mode parameters agree with model calculations, within the limitations imposed by the configuration of the present array. A methodology of analysis and the required set of measurements is described, which would allow the decomposition of a VLF signal into all of its constituent waveguide modes, and accurate measurement of the characteristic parameters of each mode, namely phase velocity, attenuation rate, amplitude, and phase.

Rocket and ground-based electron density soundings versus IRI representation

Monthly averages of F-region cut-off frequency foF2 and peak height hmF2 data obtained during the solar maximum years 1978 and 2000 through the use of ionosondes located at equatorial and low latitude regions in Brazil are compared with the IRI representations. The equatorial stations are Fortaleza (3°53'S, 38°25'W), and São Luís (2°20'S, 44°12'W), and the low latitude station was Cachoeira Paulista (22°41'S, 45°00'W). The ionosonde data analyzed here were taken at each 15 minutes, in the months of February, May, August and November, of 1978 and 2000. In a second part of this study, electron density profiles obtained by three rocket flights in the Brazilian equatorial region are also compared with the IRI representations. The rocket experiments were carried out on July 7, 1984, December 11, 1985 and October 14, 1994. The first and the second experiments were performed at the sub-equatorial station Natal (35°14'W, 5°55'S) and the third one at the equatorial station Alcântara (2°19'S, 44°22W). The results clearly show that IRI significantly underestimates the equatorial hmF2 during the pre-midnight period. On the other hand, the IRI representation for hmF2 over off-equatorial station Cachoeira Paulista is in rather better agreement with the experimental density profile. The IRI representation of the foF2 parameter is in reasonable agreement with the data for both the equatorial region and Cachoeira Paulista, although the model appears to systematically overestimate foF2 in August for both 1978 and 2000. The rocket electron density profiles show that the highly variable height distribution of the electron density during ionospheric plasma bubble events may always prevent the correct representation of electron density profiles in the equatorial region during the occurrence of those events.

Ohmic heating as evidence for strong field‐aligned currents in filamentary aurora

A large increase in electron temperature measured in filamentary aurora with the European incoherent scatter radar has been modeled with a one‐dimensional electron transport and ion chemistry code. To account for the observed changes in electron temperature, while also reproducing the measured E region electron density profiles, a source of electron heating is required in addition to local heating from energy degradation of the precipitating electrons. We show that ohmic heating in a strong field‐aligned current can account for the required heat source.