Ionospheric Density Irregularities Research Articles

Assessment of Polar Ionospheric Observations by VIPIR/Dynasonde at Jang Bogo Station, Antarctica: 2. Ionospheric Ion Drift Velocity

AbstractSince the installation at the Antarctic Jang Bogo Station (JBS) in 2017, Korea Polar Research Institute (KOPRI) has been operating the Vertical Incidence Pulsed Ionospheric Radar (VIPIR) equipped with Dynasonde analysis (JVD). The two‐dimensional ion velocity is one of the key ionospheric parameters obtained from the JVD. The ionospheric ion velocities are compared with simultaneous, but independent, measurements of the Doppler velocity obtained from SuperDARN East radar at Dome C. The JVD ion velocity vector is projected to the line‐of‐sight direction of the SuperDARN observation over the JBS to be directly compared with each other. The result of comparison shows a reasonable agreement with the correlation coefficient of 0.72. The linear regression coefficient of about 0.5 represents that the JVD ion velocity is generally smaller than the SuperDARN observations by the regression coefficient, which may result from the different height ranges of the measurements. It is also found that the correlation coefficient increases with increasing magnetic activity (Kp), which suggests that the small‐scale ionospheric density irregularities tend to move with large‐scale plasma motion that is driven by enhanced plasma convection with increasing Kp.

Space Diversity Mitigation Effects on Ionospheric Amplitude Scintillation With Basis on the Analysis of GNSS Experimental Data

Ionospheric density irregularities embedded in Equatorial Plasma Bubbles, with scale sizes varying from several hundred kilometers to several tens of meters, may cause amplitude and phase scintillation of transionospheric radio waves, degrading the performance and availability of space-based communication and navigation systems. A recent computer simulation study, based on ionospheric irregularities detected by the Planar Langmuir Probe onboard the Communication/ Navigation Outage Forecasting System satellite, analyzed the mitigation effects from space diversity on amplitude scintillation of transionospheric signals received on the ground. The present work, based on experimental data, will confirm and extend the previous results, indicating, in statistically quantitative terms, how space diversity, effective on uplink and downlink ground-satellite paths, particularly in the strong and saturated scintillation regimes, depends on Ionospheric Pierce Point dip-latitude and distance intervals, as well as on a well-known amplitude scintillation index.

Fading of Wideband Signals Due to Propagation Through Equatorial Plasma Bubbles and Diversity Mitigation Effects

Equatorial Plasma Bubbles (EPBs) and their ionospheric density irregularities exhibit structures with scale sizes that can vary from several hundred kilometers to tens of meters. These random irregularities degrade both the performance and the availability of space-based communication and navigation systems, in different ways. As examples of effects to be analyzed, wideband UHF (360 MHz – 380 MHz) transmissions received by Ascension Island (7.93°S, 14.25°W) monitors during an observational campaign in October 2016 displayed flat-fading and frequency-selective fading conditions. In this report, the ionospheric structures that are responsible for the harmful observed behavior (flat and frequency-selective fading) will initially be identified, through the application of a discrete version of the single phase-screen formulation to low-and high-resolution in-situ ion density data recorded by the Planar Langmuir Probe (PLP) onboard the Communications/ Navigation Outage Forecasting System (C/NOFS) satellite. Next, the resulting hypothesis will be confirmed, by the application of the continuous version of the same formulation to single-scale irregularities with different scale sizes. Finally, mitigation effects from space and frequency diversity on the observed behavior will be discussed, with basis on the application of the initial formulation to the high-resolution C/NOFS PLP data.

L‐Band Synthetic Aperture Radar Observation of Ionospheric Density Irregularities at Equatorial Plasma Depletion Region

AbstractPlasma density irregularities in the equatorial ionosphere are thought to cause the distortions of L‐band Synthetic Aperture Radar (SAR) images, which have been observed in recent years, but the origin of the image distortion has not yet been clearly identified experimentally. We report on the first simultaneous observation of equatorial plasma bubbles (EPBs) by the ALOS‐2/PALSAR‐2 satellite and ground 630‐nm airglow imager in northern Brazil. We observe stripe‐like distortions of SAR signal power that are aligned in the direction of local magnetic field lines. The stripe‐like patterns are observed in the vicinity of airglow depletion. The result shows that the observed L‐band SAR stripes are caused by ionospheric scintillation due to plasma irregularities with the scale size of hundreds of meters associated with EPBs. We show that the SAR scintillation stripes are predominantly found at the location of sharp density gradients in the two‐dimensional form.

Bistatic Observations With SuperDARN HF Radars: First Results

AbstractSuper Dual Auroral Radar Network (SuperDARN) radars operate in a coordinated but monostatic configuration whereby high‐frequency (HF) signals scattered from ionospheric density irregularities or from the surface of the Earth return to the transmitting radar where Doppler parameters are then acquired. A bistatic arrangement has been developed for SuperDARN radars in which HF signals transmitted from one radar are received and analyzed by another radar that is separated by a large distance (>1,000 km). This new capability was developed and tested on radars located in Oregon and Kansas. Numerous 3‐day bistatic campaigns have been conducted over a period extending from September 2019 through March 2020. During these campaigns three distinct bistatic propagation modes have been identified including a direct mode in which signals are transmitted and received through the radar side lobes. Direct mode signals propagate along the great‐circle arc connecting the two bistatic radar sites, reflecting from the ionosphere at both E region and F region altitudes. Two additional modes are observed in which HF signals transmitted from one radar scatter from either ionospheric density irregularities or from the surface of the Earth before propagating to the bistatic receiving radar. Ray tracing simulations performed for examples of each mode show good agreement with observations and confirm our understanding of these three bistatic propagation modes. Bistatic campaigns continue to be scheduled in order to improve technical aspects of this new capability, to further explore the physical processes involved in the propagation and scattering of HF bistatic signals and to expand the coverage of ionospheric effects that is possible with SuperDARN.

Ionospheric Density Irregularities, Turbulence, and Wave Disturbances During the Total Solar Eclipse Over North America on 21 August 2017

AbstractData from ionosonde and global positioning system (GPS) total electron content (TEC) observations reveal a number of ionospheric phenomena during the total solar eclipse on 21 August 2017 over North America. The eclipse started over the West Coast at ∼16:00 UTC (∼07:53 LT) and ended over the East Coast at ∼20:00 UTC (∼14:46 LT). We identify a growth of plasma density irregularities and turbulence on the bottomside ionosphere during the eclipse totality (at ∼10:03 LT over Idaho Falls), signified by the distinct appearance of spread‐F echoes in the ionosonde data. In addition, data from the ionosonde observations also show some characteristic signatures of traveling ionospheric disturbances at ∼300‐km altitude during the eclipse. Finally, large reductions in TEC and ionospheric plasma densities (by 33–45%) due to the eclipse were observed in both the GPS TEC and ionosonde data.

Low‐Latitude Ionospheric Density Irregularities and Associated Scintillations Investigated by Combining COSMIC RO and Ground‐Based Global Positioning System Observations Over a Solar Active Period

AbstractThis study for the first time presents a locally integrated analysis of occurrences of ionospheric E and F region irregularities/scintillations in southeast China, by employing radio occultation (RO) profile data retrieved from Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites and observations from a ground‐based Global Navigation Satellite System receiver over a solar active period from 2014 to 2015. Their occurrences in both nighttime and daytime were examined by using both amplitude scintillation index (S4) and the rate of change of total electron content index. It is found that (1) F region irregularities occurred predominantly during 20–03 local time (LT) and exhibited maximum (minimum) during equinoxes (solstices) and equinoctial (solstice) asymmetry. Their geographic mapping reveals the maximum occurrence in the westward tilted structure of equatorial plasma bubbles. In addition, the altitude‐time variations indicate that their occurrences at higher altitudes were prevailing at 20–22 LT. (2) The E region irregularities were found prominently during 15–00 LT at altitudes of 90–110 km with an even geographic distribution. Their occurrences with maximum in summer (May–August) were distinctly detected by RO observations but insignificantly by ground‐based observations. (3) By examining simultaneous observations of E and F region irregularities, it is found that they appeared absent during 21–00 LT and predominant after midnight. This could be related to the weakening/disruption of sporadic E (Es) layers during the development of equatorial plasma bubbles. A sign of coupling of E and F regions during nighttime is likely revealed from RO profile data.

Dynamics of density cavities generated by frictional heating: Formation, distortion, and instability

AbstractA simulation study of the generation and evolution of mesoscale density cavities in the polar ionosphere is conducted using a time‐dependent, nonlinear, quasi‐electrostatic model. The model demonstrates that density cavities, generated by frictional heating, can form in as little as 90 s due to strong electric fields of ∼120 mV/m, which are sometimes observed near auroral zone and polar cap arcs. Asymmetric density cavity features and strong plasma density gradients perpendicular to the geomagnetic field are naturally generated as a consequence of the strong convection and finite extent of the auroral feature. The walls of the auroral density cavities are shown to be susceptible to large‐scale distortion and gradient‐drift instability, hence indicating that arc‐related regions of frictional heating may be a source of polar ionospheric density irregularities.

Intensity of low-latitude nighttime F-region ionospheric density irregularities observed by ROCSAT and ground-based GPS receivers in solar maximum

This study examines the global correlation between the instantaneous total amplitude of ion density fluctuations observed by ROCSAT and the phase fluctuation of the total electron content (TEC) recorded by worldwide ground-based Global Positioning System (GPS) receivers during the high solar activity period of March 1999–December 2002 for Kp<3. The Hilbert–Huang transform (HHT) is applied to compute the instantaneous total amplitude of ROCSAT ion densities. The event-based and climatological comparisons of the total amplitude and occurrence probability of irregularities observed by ROCSAT show that the total amplitude can reveal both the occurrence probability of irregularities and the associated intensity. The noise level of the total amplitude is about 103.5 (near 3000) ions/cm3. The high correlation (correlation coefficient=0.81) between the GPS TEC phase fluctuation index FP and instantaneous total amplitude of ROCSAT electron densities suggests that the total amplitude can be used to globally monitor the intensity of irregularities at equatorial and within the latitude belt of ±15°. The relationship between the ionospheric background ionization and the irregularity intensity is further investigated.

On the spatial distribution of decameter‒scale subauroral ionospheric irregularities observed by SuperDARN radars

AbstractThe midlatitude Super Dual Auroral Radar Network (SuperDARN) radars regularly observe nighttime low‒velocity Sub‒Auroral Ionospheric Scatter (SAIS) from decameter‒scale ionospheric density irregularities during quiet geomagnetic conditions. To establish the origin of the density irregularities responsible for low‒velocity SAIS, it is necessary to distinguish between the effects of high frequency (HF) propagation and irregularity occurrence itself on the observed backscatter distribution. We compare range, azimuth, and elevation data from the Blackstone SuperDARN radar with modeling results from ray tracing coupled with the International Reference Ionosphere assuming a uniform irregularity distribution. The observed and modeled distributions are shown to be very similar. The spatial distribution of backscattering is consistent with the requirement that HF rays propagate nearly perpendicular to the geomagnetic field lines (aspect angle ≤1°). For the first time, the irregularities responsible for low‒velocity SAIS are determined to extend between 200 and 300 km altitude, validating previous assumptions that low‒velocity SAIS is an F‒region phenomenon. We find that the limited spatial extent of this category of ionospheric backscatter within SuperDARN radars' fields‒of‒view is a consequence of HF propagation effects and the finite vertical extent of the scattering irregularities. We conclude that the density irregularities responsible for low‒velocity SAIS are widely distributed horizontally within the midlatitude ionosphere but are confined to the bottom‒side F‒region.

Generation and evolution of density irregularities due to self‐focusing in ionospheric modifications

Simulations of the generation and evolution of ionospheric density irregularities due to thermal self‐focusing are presented. A nonlinear, self‐consistent development of the self‐focusing instabilities (SFI) is studied with a two‐dimensional model of HF radio wave propagation in inhomogeneous magnetized plasmas. The full‐wave model is coupled with the electron temperature and plasma density equations. In these studies, the pump beam is incident vertically or tilted to the magnetic zenith direction. Our results demonstrate that irregularities generated and developed due to a local heating of the anisotropic ionosphere plasma are density depletions/striations with a strong enhancement of electron temperature inside depletions elongated along the magnetic field lines. For the ordinary mode (O mode) heating the reflection point is shifted upward with decreasing electron concentration. During the nonlinear evolution of the SFI instability the bunching of striations is observed. A number of the bunch‐scale irregularities increase when the beam is tilted to the magnetic zenith direction. The presented results are consistent with the experiments carried out at the midlatitude SURA HF heating facility.

Application of generalized singular value decomposition to ionospheric tomography

Abstract. The electron density distribution of the low- and mid-latitude ionosphere has been investigated by the computerized tomography technique using a Generalized Singular Value Decomposition (GSVD) based algorithm. Model ionospheric total electron content (TEC) data obtained from the International Reference Ionosphere 2001 and slant relative TEC data measured at a chain of three stations receiving transit satellite transmissions in Alaska, USA are used in this analysis. The issue of optimum efficiency of the GSVD algorithm in the reconstruction of ionospheric structures is being addressed through simulation of the equatorial ionization anomaly (EIA), in addition to its application to investigate complicated ionospheric density irregularities. Results show that the Generalized Cross Validation approach to find the regularization parameter and the corresponding solution gives a very good reconstructed image of the low-latitude ionosphere and the EIA within it. Provided that some minimum norm is fulfilled, the GSVD solution is found to be least affected by considerations, such as pixel size and number of ray paths. The method has also been used to investigate the behaviour of the mid-latitude ionosphere under magnetically quiet and disturbed conditions.

Electromagnetic wave structures within subauroral polarization streams

We report on oscillations in electric (δEY) and magnetic (δBZ) fields and plasma density (δNi) observed by Defense Meteorological Satellite Program (DMSP) satellites within fast subauroral convection streams in the evening sector during the magnetic storm of 6 November 2001. There are two types of wave phenomena. The first and more common is characterized by electromagnetic and plasma density variations that have the same frequency range of ∼0.15 Hz in the spacecraft frame of reference. The second is characterized by large‐amplitude plasma and field oscillations over a broader range of frequencies ∼0.1 to 0.3 Hz. In this case the perturbation densities and fields appear to have different frequency responses. In this and other magnetic storms, strong waves are associated with the precipitation of ∼30 keV ions. Ratios of δEY/δBZ indicate encounters with mixtures of electromagnetic (in part Alfvénic) and electrostatic modes. Poynting vectors associated with the oscillations can be directed either into or out of the ionosphere. The density perturbations appear to be extended east‐west corrugations in the plasma flow streams with north‐south wavelengths of ∼50 km. The δEY and δNi variations were anticorrelated, as required for current conservation. Our analysis shows that Alfvénic perturbations are consistent with expected effects of irregular potential distribution around ionospheric density irregularities mapped to the magnetosphere. Inertial currents act to generate mesoscale field‐aligned currents carried by Alfvén waves, as was previously discussed with regards to auroral arcs formation. We suggest that δNi irregularities observed by DMSP satellites in the evening sector began as striated plasma patches in the polar cap that convected to subauroral latitudes.

Analysis of electromagnetic wave scattering by ionospheric irregularities

A quasi‐particle approach is used to analyze the effect of bottomside sinusoidal irregularities on the propagation of beacon satellite signals through the ionosphere. In this work, the radio wave is treated as a distribution of quasi‐particles described by a Wigner distribution function, and the irregularities are modeled as a two‐dimensional density modulation on an uniform background plasma. The collision of quasi‐particles with the irregularities modifies the quasi‐particle distribution and gives rise to the wave‐scattering phenomenon. The multiple scattering process is generally considered in this deterministic analysis of radio wave scattering off the ionospheric density irregularities. The analysis shows that this two‐dimensional density grating effectively modulates the intensity of the beacon satellite signals. This spatial modulation of the wave intensity is converted into time modulation due to the drift of the ionospheric irregularities, which then results in the scintillation of the beacon satellite signals. The modulation is due mainly to the transverse (to the wave propagation) grating. The results of the analysis agree well with the existing experimental data on the scintillation of Atmosphere Explorer satellites signal (Basu et al., 1986). The model is further based on to predict the scintillation of radio signals for their long distance propagation. For a fixed scintillation index (S4) the predicted propagation distance (d) versus the wave frequency (ƒ) follows a power law, namely, d∝ƒK, where k ≈ 2.

Spectral classification of medium‐scale high‐latitude F region plasma density irregularities

Plasma diagnostics aboard the S3‐4 satellite have provided direct measurements of high‐latitude F region ionospheric density irregularities in the wavelength domain of tens of meters to hundreds of kilometers. The irregularities are found to be ubiquitous, generally populating the auroral oval, the polar cap, and the dayside oval/polar cusp area with varying degrees of intensity and scale size distributions. To investigate the causal mechanisms for various irregularity distributions, including the primary input term from precipitating particles and possible plasma instability processes, we have spectrally analyzed 20 orbits of data during geomagnetically quiet and moderately disturbed conditions (Kp ≤ 3). In the lower F region (Z ∼ 200 km), where molecular ions dominate, power spectra of medium‐scale plasma density structures (1–50 km) can be characterized by a weak most probable spectral index of 2.8 for the nightside‐dayside ovals and the polar cap and an index of 1.4 for the dusk oval. At higher altitudes (Z ≥ 260 km), where O+ is the dominant ion, the analyses show a well‐defined most probable spectral index of 1.9 for all the high‐latitude regions. Our spectral analyses suggest that at the molecular‐ion‐dominant altitudes, fast chemical recombination inhibits plasma convection/diffusion and that the observed irregularities are primarily the consequence of locally structured particle precipitation patterns. On the other hand, in the O+‐dominant F region, particle‐produced irregularities are long lived. Such irregularities convect into and across the polar cap and can be a long‐lived input term for a number of plasma instability processes. As a result, the molecular‐dominant polar cap F region (Z ≤ 200 km) is generally free of plasma disturbances, while its atomic‐dominant counterpart (Z ≥ 260 km) is mostly structured with a broad range of intense plasma density irregularities.

Earth's magnetic field perturbations as the possible environmental impact of the conceptualized solar power satellite

It is predicted that the earth's magnetic field can be significantly perturbed locally by the microwave beam transmitted from the conceptualized solar power satellite (SPS) at a frequency of 2.45 GHz with incident power density of 230 W/m² at the center of the beam. The simultaneous excitation of earth's magnetic field fluctuations and ionospheric density irregularities is caused by the thermal filamentation instability of microwaves with scale lengths greater than a few hundred meters. Earth's magnetic field perturbations with magnitudes (∼ a few tens of gammas) comparable to those in magnetospheric substorms can be expected. Particle precipitation and airglow enhancement are the possible, concomitant ionospheric effects associated with the microwave‐induced geomagnetic field fluctuations. Our present work adds earth's magnetic field perturbations as an additional effect to those such as ionospheric density irregularities, plasma heating, etc., that should be assessed as the possible environmental impacts of the conceptualized solar power satellite program.