Ionospheric Trough Research Articles

Plasma and Field Observations in the Magnetospheric Source Region of a Stable Auroral Red (SAR) Arc by the Arase Satellite on 28 March 2017

AbstractA stable auroral red (SAR) arc is an aurora with a dominant 630 nm emission at subauroral latitudes. SAR arcs have been considered to occur due to the spatial overlap between the plasmasphere and the ring‐current ions. In the overlap region, plasmaspheric electrons are heated by ring‐current ions or plasma waves, and their energy is then transferred down to the ionosphere where it causes oxygen red emission. However, there have been no study conducted so far that quantitatively examined plasma and electromagnetic fields in the magnetosphere associated with SAR arc. In this paper, we report the first quantitative evaluation of conjugate measurements of a SAR arc observed at 2204 UT on 28 March 2017 and investigate its source region using an all‐sky imager at Nyrölä (magnetic latitude: 59.4°N), Finland, and the Arase satellite. The Arase observation shows that the SAR arc appeared in the overlap region between a plasmaspheric plume and the ring‐current ions and that electromagnetic ion cyclotron waves and kinetic Alfven waves were not observed above the SAR arc. The SAR arc was located at the ionospheric trough minimum identified from a total electron content map obtained by the GNSS receiver network. The Swarm satellite flying in the ionosphere also passed the SAR arc at ~2320 UT and observed a decrease in electron density and an increase in electron temperature during the SAR‐arc crossing. These observations suggest that the heating of plasmaspheric electrons via Coulomb collision with ring‐current ions is the most plausible mechanism for the SAR‐arc generation.

An Ephemeral Red Arc Appeared at 68° MLat at a Pseudo Breakup During Geomagnetically Quiet Conditions

AbstractVarious subauroral optical features have been studied by analyzing data collected during periods of geomagnetic disturbances. Most events have been typically found at geomagnetic latitudes of 45–60°. In this study, however, we present a red arc event found at geomagnetic 68° north (L ≈ 7.1) in the Scandinavian sector during a period of geomagnetically quiet conditions within a short intermission between two high‐speed solar wind events. The red arc appeared to coincide with a pseudo breakup at geomagnetic 71–72°N and a rapid equatorward expansion of the polar cap. However, the red arc disappeared in approximately 7 min. Simultaneous measurements with the Swarm A/C satellites indicated the appearance of the red arc at the ionospheric trough minimum and a conspicuous enhancement of the electron temperature, suggesting the generation of the arc by heat flux. Since there are meaningful differences in the red arc features from already‐known subauroral optical features such as the stable auroral red (SAR) arc, we considered that the red arc is a new phenomenon. We suggest that the ephemeral red arc may represent the moment of SAR arc birth associated with substorm particle injection, which is generally masked by bright dynamic aurorae.

Universal time control of the parameters of the electron temperature enhancement zone in the winter subauroral ionosphere

Comparison of the numerical simulation results with experimental data obtained aboard the CHAMP spacecraft has shown that in the eastern longitudes during the local winter period the parameters of electron temperature (Te) enhancement zone in the subauroral ionosphere undergo significant changes depending on the universal time (UT control) as a result of the formation of the shadow region, almost devoid of direct sources of ionization due to the displacement of the geographic and geomagnetic poles. Thus, in the time interval of 04–07 UT in the northern hemisphere and 16–19 UT in the southern hemisphere, a zone of increase of Te in the region of main ionospheric trough can have the greatest length and square, and take a ring-shaped configuration; in the time interval of 16–19 UT in the northern hemisphere and 04–07 UT in the southern hemisphere the length and square of Te enhancement zone decrease, and its configuration becomes crescent. The study has been carried out under the assumption that the main reason for the Te enhancement is the ring current that causes a downward heat flow from the plasmasphere.

Comparison of Electron Density and Temperature From the CSES Satellite With Other Space‐Borne and Ground‐Based Observations

AbstractIn this paper we provide a comprehensive comparison of in situ electron density (Ne) and temperature (Te) measured by Langmuir probe (LAP) on board the China Seismo‐Electromagnetic Satellite (CSES), with nearly simultaneous measurements from the Swarm B satellite, incoherent scatter radar (ISR) at Millstone Hill, as well as predictions from empirical models including the incoherent scatter radar ionospheric model (ISRIM) for Millstone Hill and International Reference Ionosphere model (IRI‐2016). Results reveal that the global distributions and their relative variations of Ne/Te from CSES and Swarm are quite consistent during conjunction periods of the two satellites, although the absolute values of Swarm Ne are proportionally larger than that of CSES. The large‐scale ionospheric structures, such as the equatorial ionization anomaly (EIA), the longitudinal wave number (WN3/4), the Weddell Sea anomaly (WSA), the northern midlatitude summer nighttime anomaly (MSNA), and the midlatitude ionospheric trough (MIT), are well represented by the CSES measurements. For the temporal variation over Millstone Hill station, CSES Ne at nightside shows some different characteristics from the predictions of IRI and ISRIM, possibly due to the influences of MIT and midlatitude arc (MLA) that are often observed at latitudes of Millstone Hill. Our results suggest that the CSES in situ plasma parameters are reliable with a high scientific potential for investigation of geophysics and space physics.

Morphology Evolution of the Midlatitude Ionospheric Trough in Nighttime Under Geomagnetic Quiet Conditions

AbstractThe dynamics and climatology of midlatitude trough minimum have been widely investigated. However, the detailed magnetic local time (MLT) evolution of the trough walls (both equatorward and poleward) has not been well addressed. In this study, we used nearly 10‐year Planar Langmuir Probe (PLP) data from the CHAMP satellite to investigate how the location and shape of midlatitude trough evolve during night hours. We find that (1) the trough equatorward wall experiences an obvious equatorward extension in the premidnight sector, with steepness rapidly decreasing during the same local time sector; (2) the trough equatorward wall is as steep as poleward wall in the dusk sector; and (3) the trough poleward wall shows no remarkable change during the whole nighttime, except for a weakly reduced width and a slightly enhanced steepness near midnight. We suggest that the rapid extension of the equatorward wall in the premidnight sector is closely related to the plasma equatorward movement caused by the neutral winds. The rapid decrease of the steepness of the equatorward wall in the premidnight sector is not only influenced by its own width expansion but possibly also related to the upward plasma drift caused by neutral winds. The trough minimum and equatorward/poleward boundaries are found located at slightly higher magnetic latitude during equinoxes than that during solstice seasons.

Detection of high-latitude ionospheric structures using GNSS

For detection of rather large ionospheric structures (such as high-latitude ionospheric troughs, main ionospheric trough, etc.) using Global Navigation Satellite Systems (GNSS) we suggest employing the analysis of anomalous deviations in behavior of the absolute slant total electron content (TEC) on particular “GNSS receiver - GNSS satellite” lines of sight (LOSs). Using data of four high-latitude GNSS-stations, the proposed method allowed us to detect the 5 May 2013 high-latitude ionospheric trough (HLIT), determine its location and size, and follow the dynamics. The HLIT was located on the dayside within 25-185°E. Initially (04:40–06:00 UT), the HLIT equatorial wall and bottom were within 68-78°N and 70-80°N, respectively. The trough northern wall was not registered at this time, most likely due to insufficient number of LOSs. During the next hour (06:00–07:00 UT), the trough has moved 2-4° southward. Its bottom lay near 68-76°N; equatorial and polar walls were within 67-74°N and 77-78°N, respectively. The trough width was about 4°. At 07:00–08:00 UT, the HLIT structure and location remained almost the same. To interpret the observation results, we used the model for ionosphere-plasmasphere coupling (developed at the Institute of Solar-Terrestrial Physics SB RAS) to simulate the high-latitude ionosphere conditions on 4–6 May 2013. This simulation showed that on 5 May 2013, a high-latitude trough was formed on the dayside at 40-200°E and 78-85°N. The most likely reason for the formation of HLIT both in model calculations and in reality was a change in the magnetospheric convection in the high-latitude ionosphere caused by the moderate geomagnetic disturbances (substorms) on 4–5 May 2013. The trough location determined from the analysis of anomalous TEC deviations on particular LOSs was in good agreement with location of the high-latitude ionospheric trough obtained from numerical simulation. Thus, the analysis of anomalous deviations in TEC behavior on particular LOSs expands the GNSS potential in detecting large-scale irregularities in the ionosphere.

Simultaneous Observations of SAR Arc and Its Ionospheric Response at Subauroral Conjugate Points (L ≃ 2.5) During the St. Patrick's Day Storm in 2015

AbstractDuring the St. Patrick's Day storm on 17 March 2015, an all‐sky imager at King Sejong Station (KSS; geo: 62.2°S, 58.8°W; mag: 50.2°S) captured diffuse aurora and a stable auroral red (SAR) arc. Ground‐based Global Positioning System observations were also simultaneously conducted at KSS, but there was no significant increase in the total electron content's rate‐of‐change index or the ionospheric scintillation indices around the SAR arc region. Auroral activities including a SAR arc were also detected by the all‐sky imager at Millstone Hill Station (geo: 42.6°N, 71.4°W; mag: 52.5°N), which is magnetically conjugate to KSS. The total electron content's rate‐of‐change index map around Millstone Hill Station, too, indicates that the ionospheric irregularities occurred only near the diffuse aurora and not in the SAR arc. The northern SAR arc is broader than the southern one and also shows a multiplicity pattern, which may be due to latitude structure within the seasonally dependent midlatitude ionospheric trough. These conjugate observations, despite their hemispheric differences, validate the classical SAR arc mechanism of emission driven by heat conduction from the inner magnetosphere that does not generate small‐scale ionospheric irregularities that can affect Global Positioning System radio signals. Previous studies of SAR arcs and very high frequency radiowave scintillations did show a positive correlation.

Statistical Analysis of the Main Ionospheric Trough Using Swarm in Situ Measurements

AbstractA statistical analysis of the topside main ionospheric trough is implemented by using the Swarm constellation in situ plasma density measurements from December 2013 to November 2019. The key features of the main trough, such as the occurrence rate, minimum position, width, and depth, are characterized and quantified. The distribution patterns of these parameters are investigated with respect to magnetic local time, season, longitude, solar activity, and geomagnetic activity levels, respectively. The main results are as follows: (1) The diurnal variation of the trough occurrence rate usually exhibits a primary peak in the early morning, a subsidiary peak in the late evening, and a slight reduction around midnight especially in the Northern Hemisphere. (2) The seasonal variation of the nighttime trough has maximum occurrence rates around equinoxes, higher than those in local winter. (3) The trough distribution has an evident hemispherical asymmetry. It is more pronounced in the Northern Hemisphere during the winter and equinoctial seasons, with its average nighttime occurrence rate being 20–30% higher than that in the Southern Hemisphere. The trough minimum position and the trough width also exhibit more significant fluctuation in the Northern Hemisphere. (4) The longitudinal pattern of the trough shows clear east‐west preferences, which has a higher occurrence rate in eastern (western) longitudes around the December (June) solstice. (5) Conditions for the trough occurrence are more favored in low solar activity and high geomagnetic activity periods.

Initial nighttime ionospheric observations with advanced ionospheric probe onboard FORMOSAT-5

FORMOSAT-5 satellite was launched into a sun-synchronous orbit at 720 km altitude with 98.28° inclination on 25 August 2017. The onboard scientific payload, Advanced Ionospheric Probe (AIP) is capable of measuring topside ionospheric ion density, cross-track flow velocities, ion composition and temperature, and electron temperature. Initial observations of nighttime midlatitude ionospheric density and vertical flow velocity variations at 2230 LT sector during a few quiet magnetic days in December 2017 are studied here. Longitudinal density variations in the equatorward edge of midlatitude ionospheric trough (MIT) region are noticed. Accompanied with this density variation, the vertical flow velocities also behave differently. Although the density difference has been stated due to zonal wind effect related to the declination of the geomagnetic field lines, the vertical flow velocity variation seems to play the opposite role. All these density and vertical flow observations in the northern winter hemisphere can only be explained by the longitudinal differences in the diffusion velocity coming down from the protonsphere (plasmasphere). In addition, the hemispheric asymmetry in the vertical flow velocity can also be explained by the interaction between the topside ionosphere and the protonsphere. The observed vertical flow variations near MIT at different longitudes should present a new potential tool for the study of MIT formation.

When Plasma Streams Tie up Equatorial Plasma Irregularities with Auroral Ones

AbstractWe present a new pattern of storm‐induced ionospheric irregularities behavior at midlatitudes—poleward‐streaming plasma density depletions. Under disturbed conditions, they appear at North America low latitudes as a part of extended postsunset equatorial plasma bubbles, and further, they are streaming from low latitudes in a northwestward, poleward direction toward the main ionospheric trough and auroral irregularities zone. The poleward‐streaming plasma depletions represent a new phenomenon with the similar northwestward transportation path across the continental United States as storm‐enhanced density (SED) plumes. The channels of poleward‐streaming plasma depletions were stretched from low‐latitude base toward higher latitudes—they are found to occur for geomagnetic storms under specific combination of steady southward interplanetary magnetic field, subauroral polarization streams (SAPS) electric fields, and enhanced westward drifts at midlatitudes, resulting in northwestward plasma transportation equatorward of the SAPS region. The poleward‐streaming plasma depletions form an illusion of traveling ionospheric disturbances (TIDs) moving in a poleward, northwestward direction—this propagation direction is opposite to typical equatorward propagation of storm‐induced large‐scale TIDs generated in the auroral zone and propagated toward the equator. This phenomenon is accompanied by strong ionospheric irregularities that occurred over both edges of plasma depletion channel at midlatitudes. For two comparable geomagnetic storms, these poleward‐streaming plasma depletions persisted for several hours, posing a localized threat for GPS‐based positioning applications. Even moderate‐to‐intense storms (Dst minimum−145 nT) can promote such effects at midlatitudes.

Additional Horizontal Traces of Ground Reflections on Ionograms from the Interkosmos-19 Satellite in the Night Trough

Additional horizontal traces of ground reflections were observed in a studied series of high-latitude, Kp sounding ionograms from the Interkosmos-19 satellite. The ionograms were recorded on May 11, 1979, in the nighttime (0000–0100 LT), weakly perturbed (Kp = 3+) ionosphere of the Southern hemisphere in the region of the main ionospheric trough. The trough had a flat bottom, as well as a steep and high polar wall formed by the precipitation of particles on the equatorial edge of the auroral oval. An additional ground trace was removed from the main trace from 100 to 800 km when the satellite moved from the polar wall to midlatitudes. Ray tracing showed that additional ground traces are formed by multiple reflections from the polar wall and the Earth’s surface. At the same time, it was possible to achieve almost complete agreement between the model ionogram and the experimental one.

The northern and southern mid-latitude ionospheric trough using global IGS vTEC maps

The aim of this work is the analysis of the mid-latitude ionospheric trough (MIT) using the Global Ionospheric Maps from IGS (GIMs) during the solar minimum, year 2008. This study was performed for different local times, 22, 00, 02 and 04 LT on the Northern and Southern hemisphere simultaneously. In the two hemispheres the MIT show asymmetric pattern. The high-latitude troughs are clearly distinguished in autumn and winter. Another feature is the longitudinal development towards the west of the geomagnetic pole covering a wider area in the Northern Hemisphere. Five empirical reference models were tested and compared with the MIT minimum position obtained from GIMs at different local times for both hemisphere. The results show a better agreement with the observations for the Northern Hemisphere specially with the Köehnlein & Raitt model. Fluctuations of 9 days and 27 days of the MIT minimum position are found, which could be related with the solar wind oscillations, especially for 00 and 02 LT in both hemisphere, suggesting a link between them.

Variations in the Winter Troughs' Position With Local Time, Longitude, and Solar Activity in the Northern and Southern Hemispheres

AbstractComprehensive pattern of ionospheric troughs' location in winter for all local times, longitudes, high (HSA), and low (LSA) solar activity, in both hemispheres, is at first investigated. Statistical analysis based on a large dataset of Interkosmos‐19, CHAMP, and Kosmos‐900 satellites was performed for quiet geomagnetic conditions Kp = 1–3. Three troughs were considered: high‐latitude trough (HLT) located inside the auroral oval, main ionospheric trough (MIT) located equatorward from the auroral oval, that is, at subauroral latitudes, and mid‐latitude ring ionospheric trough (RIT). The main purpose was to study the formation of the troughs' diurnal pattern in different conditions. The main problem was to distinguish MIT from RIT and MIT from HLT. For this purpose, early morning hours (04–06 LT), late morning hours (07–10 LT), day, evening, and night conditions were examined. In the early morning sector, RIT was separated from MIT and eliminated from the dataset. In the late morning sector, MIT and HLT were first clearly divided, although only for HSA. During the day, in the Northern Hemisphere under all conditions, HLT is mainly observed, in the Southern Hemisphere at poorly lit longitudes only the daytime MIT, and at well‐lit longitudes only the HLT is observed. The division of MIT and HLT was carried out according to the (statistical) position of the equatorial boundary of the auroral oval precipitation. At night, longitudinal variations in the MIT position determine the asymmetry of the hemispheres. Thus, the occurrence and position of MIT and HLT depend on the hemisphere, longitude, and solar activity.

Global Dynamic Model of Critical Frequency of the Ionospheric F2 Layer

A global dynamic model of the F2 layer of the ionosphere GDMF2 is designed to calculate foF2 in both quiet and geomagnetically disturbed conditions. The term “dynamic model” implies that the change in foF2 at middle, subauroral, and auroral latitudes depends on the current level of geomagnetic activity with consideration of the prehistory of its development. A distinctive feature of this model is the empirical approach used to create the median (background) model of foF2 for quiet geomagnetic conditions and a number of aeronomic corrections to it related to the formation of the main ionospheric trough and the auroral peak of electron density, as well as changes in the temperature and composition of the thermosphere. All of these corrections depend on the changes in the heliogeophysical conditions. The global dynamic foF2 model demonstrates considerable improvement as compared to IRI-2016 (with the option ‘STORM = ON’) by on average of ~16% at high and middle and ~7% at equatorial latitudes.

Model of the Ionospheric Trough for Daytime Winter Conditions Based on Data from Interkosmos-19 and Champ Satellites

The principles and algorithm for the construction of an empirical model of the ionospheric trough for quiet (Kр = 2) daytime (06−18 LT) winter conditions in the Northern and Southern Hemispheres are described. The model consists of models of the trough position and shape. The model of the trough position describes longitudinal variations of the trough position in terms of the geographic latitude for fixed moments of local time. The model of the trough shape describes the latitudinal–longitudinal variations of foF2 within the interval of geographic latitudes 40°−85° in both hemispheres. The model of the ionospheric trough is probabilistic, since the trough is not always observed under midday conditions. In other words, the problem consists of the construction of a model of foF2 in the daytime, high-latitude ionosphere that would describe the trough structure. Data from the Interkosmos-19 and CHAMP satellites were used to solve this problem. The IRI international model of the ionosphere was also widely used; however, according to the tests, the constructed model more adequately describes the diurnal, longitudinal, and latitudinal variations of foF2 than IRI-2016. This model, together with the previously constructed model of the nighttime trough, makes it possible to speak of the creation of a complete model of foF2 in the winter ionosphere. Soon, the complete foF2 model will be accessible at the IZMIRAN site http://www.izmiran.ru/ionosphere/sm-mit/ . The model make it possible to calculate the longitudinal variations of the trough position and the latitudinal–longitudinal variations of foF2 for any local time and any level of solar activity within the range F10.7 = 70−200 for all winter months.

A New Artificial Neural Network‐Based Global Three‐Dimensional Ionospheric Model (ANNIM‐3D) Using Long‐Term Ionospheric Observations: Preliminary Results

AbstractIn this paper, we present the preliminary results of a new global three‐dimensional (3‐D) ionospheric model developed using artificial neural networks (ANNs) by assimilating long‐term ionospheric observations from nearly two decades of ground‐based Digisonde, satellite‐based topside sounders, and global positioning system‐radio occultation measurements. The present 3‐D model is named ANN‐based global 3‐D ionospheric model (ANNIM‐3D), which is the extension of previous work on the ANN‐based two‐dimensional ionospheric model by Sai Gowtam and Tulasi Ram (2017a, https://doi.org/10.1002/2017JA024795) and Tulasi Ram et al. (2018, https://doi.org/10.1029/2018JA025559). The vertical electron density profiles derived from ANNIM‐3D model are found to be consistent with the ground‐based incoherent scatter radar observations at Jicamarca and Millstone Hill. The model results have been thoroughly validated and found in good agreement with the ground‐based Digisonde and satellite in situ observations at different altitudes. This model successfully reproduces the large‐scale ionospheric phenomena like diurnal and seasonal variations of equatorial ionization anomaly and its hemispheric asymmetries, ionospheric annual anomaly, and the main ionospheric trough. Also, the present model has predicted the ionospheric response that is consistent with the neutral composition changes and meridional wind circulations during disturbed geomagnetic activity periods. Finally, the merits and limitations of this model and the scope for the potential improvements have been discussed.

Исследование сезонных особенностей формирования областей повышения электронной температуры в субавроральной ионосфере

The electron temperature enhancement is known to occur in the main ionospheric trough during geomagnetic disturbances. In this paper, we study fea-tures of the formation of the electron temperature (Te) enhancement in the subauroral ionosphere by comparing results of the numerical simulation with measurements of Te onboard the CHAMP satellite under moderate geomagnetic activity conditions. It is shown that depending on the terminator position and universal time (UT), the location of the enhanced Te regions in the subauroral ionosphere varies in different seasons. So, in winter ring-shaped and sickle-shaped regions can be formed, whereas during the equinox and summer periods sickle-shaped regions of different lengths and clarity are generally observed.

Investigating seasonal features of electron temperature enhancement regions in the subauroral ionosphere

The electron temperature enhancement is known to occur in the main ionospheric trough during geomagnetic disturbances. In this paper, we study fea-tures of the formation of the electron temperature (Te) enhancement in the subauroral ionosphere by comparing results of the numerical simulation with measurements of Te onboard the CHAMP satellite under moderate geomagnetic activity conditions. It is shown that depending on the terminator position and universal time (UT), the location of the enhanced Te regions in the subauroral ionosphere varies in different seasons. So, in winter ring-shaped and sickle-shaped regions can be formed, whereas during the equinox and summer periods sickle-shaped regions of different lengths and clarity are generally observed.

Ionospheric Plasma Irregularities Characterized by the Swarm Satellites: Statistics at High Latitudes

AbstractThe polar ionosphere is often characterized by irregularities and fluctuations in the plasma density. We present a statistical study of ionospheric plasma irregularities based on the observations from the European Space Agency's Swarm mission. The in situ electron density obtained with the Langmuir probe and the total electron content from the onboard global positioning system receiver are used to detect ionospheric plasma irregularities. We derive the irregularity parameters from the electron density in terms of the rate of change of density index and electron density gradients. We also use the rate of change of total electron content index as the irregularity parameter based on the global positioning system data. The background electron density and plasma irregularities are closely controlled by the Earth's magnetic field, with averaged enhancements close to the magnetic poles. The climatological maps in magnetic latitude/magnetic local time coordinates show predominant plasma irregularities near the dayside cusp, polar cap, and nightside auroral oval. These irregularities may be associated with large‐scale plasma structures such as polar cap patches, auroral blobs, auroral particle precipitation, and the equatorward wall of the ionospheric trough. The spatial distributions of irregularities depend on the interplanetary magnetic field (IMF). By filtering the irregularity parameters according to IMF By, we find a clear asymmetry of the spatial distribution in the cusp and polar cap between the Northern (NH) and Southern Hemispheres (SH). For negative IMF By, irregularities are stronger in the dusk (dawn) sector in the NH (SH) and vice versa. This feature is in agreement with the high‐latitude ionospheric convection pattern that is regulated by the IMF By component. The plasma irregularities are also controlled by the solar activity within the current declining solar cycle. The irregularities in the SH polar cap show a seasonal variation with higher values from September to April, while the seasonal variation in the NH is only obvious around solar maximum during 2014–2015.

Ionospheric Regions Producing Anomalous GNSS Radio Occultation Results

Anomalous GPS radio occultation (RO) events are characterised as those with L1 bending angle greater than their corresponding L2 bending angle. An investigation by EUMETSAT and the United Kingdom Meteorological Office revealed there are regions in the earth’s atmosphere where at times up to 60% of Global Navigation Satellite System Receiver for Atmospheric Sounding RO events, at the Tangent Point height of 50 km, exhibited anomalous bending angle results. The exact source of these anomalous RO events has been unclear to the RO data user community, i.e., data processing artifact or atmospheric phenomenon. In this paper, the regions of increased occurrence of anomalous RO have been identified to be the mid-latitude ionospheric trough, ionospheric polar hole, and poleward edges of the equatorial anomaly. They are more frequent at nighttime and in the southern hemisphere winter months. This is when the plasma density in these regions is depleted. However, within these regions, there are ionospheric features of increased electron density gradients such as at the walls of the mid-latitude ionospheric trough. 3-D numerical ray tracing simulations of GPS RO are presented, showing that these increased electron density features in a weakly ionized ionosphere can produce the anomalous bending angle results.