Zonal Electric Field Research Articles

A Simulation Study on the Relationship Between Field‐Aligned and Field‐Perpendicular Plasma Velocities in the Ionospheric F Region

AbstractThis study addresses a long‐standing scientific puzzle regarding ionospheric F2 region dynamics. Incoherent scatter radar observations of F2 region plasma drifts showed a strong anticorrelation between temporal variations of field‐aligned upward plasma velocity (Vi‖) and field‐perpendicular poleward plasma drift (Vi ⊥ N) over time scales from a few hours to a day at middle latitudes. The underlying physical processes remain a highly controversial issue, despite a number of speculations and qualitative inspections. Previous studies lacked especially quantitative analysis that could lead to decisive conclusions. In this study, we provide a comprehensive modeling study to explore the physical processes relating Vi‖ with Vi ⊥ N variations using a self‐consistent Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model. It is found that the anticorrelation between Vi‖ and Vi ⊥ N has strong altitudinal and latitudinal dependences. The anticorrelation between the diurnal variations of Vi‖ and Vi ⊥ N is associated with the neutral wind dynamo. Poleward meridional winds result in downward Vi‖ and poleward Vi ⊥ N, and vice versa. The anticorrelation between short‐term temporal disturbances of Vi‖ and Vi ⊥ N is mainly caused by ion drag, in response to high‐latitude convection electric field forcing. This forcing penetrates to lower latitudes and affects poleward plasma drifts Vi ⊥ N, which drags poleward meridional winds and modulates downward Vi‖. As the enhanced convection electric fields subside, the anticorrelation is mainly associated with disturbance meridional wind dynamo. The storm time high‐latitude energy and momentum inputs change global meridional winds which modify zonal electric fields to induce Vi ⊥ N changes. Furthermore, ambipolar diffusion plays a significant role in modulating the relationship between Vi‖ and Vi ⊥ N.

Characteristics of equatorial nighttime spread F – An analysis on season-longitude, solar activity and triggering causes

To understand global variability and triggering mechanisms of ionospheric nighttime equatorial spread F (ESF), we analyzed measurements from satellite and a ground-based GPS station for the years between 2010 and 2017. In this study we present seasonal-longitudinal as well as monthly variability of ESF occurrence for solar minimum and yearly variations of ESF occurrence for solar maximum and minimum periods. One of the long standing open questions in the study of ESF is what exactly initiates the Rayleigh-Taylor (RT) plasma instability growth. This question is the focus of the present work. Zonal background eastward electric field and E × B upward plasma drift speed patterns are found to be critically important in understanding plasma irregularity formation. In addition to particular patterns observed on these parameters, the background plasma density in the local evening hours just before the onset of ESF occurrence is very important. Stronger plasma densities just before the onset of irregularities resulted in stronger plasma irregularities, while relatively less dense plasma just before the onset of irregularities resulted in relatively lower plasma irregularities. Seasonal variations in ESF activity between March and September equinox seasons with comparable plasma densities can be defined in terms of the rate of change of solar flux F10.7 (dF10.7/day) index. Strongest ESF occurrence and strongest dF10.7/day are measured in the same month out of all other months in 2016 and 2017. Longitudinal variations of ESF activity in our measurements are related to longitudinal variations of plasma densities. We also found that ESF occurrence is better correlated with rate of change of F10.7 index for months in equinox seasons than for months in solstice seasons for the years between 2013 and 2016.

On developing a new ionospheric plasma index for Brazilian equatorial F region irregularities

Abstract. F region vertical drifts (Vz) are the result of the interaction between ionospheric plasma with the zonal electric field and the Earth's magnetic field. Abrupt variations in Vz are strongly associated with the occurrence of plasma irregularities (spread F) during the nighttime periods. These irregularities are manifestations of space weather in the ionosphere's environment without necessarily requiring a solar burst. In this context, the Brazilian Space Weather Study and Monitoring Program (Embrace) of the National Institute for Space Research (INPE) has been developing different indexes to analyze these ionospheric irregularities in the Brazilian sector. Therefore, the main purpose of this work is to produce a new ionospheric scale based on the analysis of the ionospheric plasma drift velocity, named AV. It is based on the maximum value of Vz (Vzp), which in turn is calculated through its relationship with the virtual height parameter, h′F, measured by the Digisonde Portable Sounder (DPS-4D) installed in São Luís (2∘ S, 44∘ W; dip: −2.3∘). This index quantifies the time relationship between the Vz peak and the irregularity observed in the ionograms. Thus, in this study, we analyzed 7 years of data, between 2009 and 2015, divided by season in order to construct a standardized scale. The results show there is a delay of at least 15 min between the Vzp observation and the irregularity occurrence. Finally, we believe that this proposed index allows for evaluating the impacts of ionospheric phenomena in the space weather environment.

Variation of the Equatorial Height Anomaly During the Main Phase of 2015 St. Patrick's Day Geomagnetic Storm Using ANNIM and TIEGCM

AbstractA double‐peak structure in the peak height of ionospheric F2 layer around ±10o geomagnetic latitudes similar to the equatorial ionization anomaly was recently reported. This unique feature was referred as the equatorial height anomaly (EHA). In the present paper, a simulation study is carried out using the data‐driven artificial neural network‐based two‐dimensional ionospheric model (ANNIM‐2D) and the physics‐based thermosphere‐ionosphere‐electrodynamics general circulation model (TIEGCM) to understand the local time and latitudinal variation of EHA during the main phase of St. Patrick's Day geomagnetic storm. Both the ANNIM‐2D and TIEGCM consistently show pronounced EHA during the main phase of the geomagnetic storm. Further, the local time of EHA development on the storm day is much earlier (nearly 2 hr) than the quiet time over Brazilian sector (90°W). The TIEGCM simulation revealed that the storm time enhancement of the equatorial fountain associated with the enhanced equatorial zonal electric field is the main controlling factor for the pronounced EHA during the main phase. The storm time meridional neutral winds positively contribute to the development of EHA. This study revealed the direct manifestation of the storm time‐enhanced plasma fountain on the EHA.

Response of ionosphere over Korea and adjacent areas to 17 March 2015 geomagnetic storm

This study analyze and discuss the response of the ionosphere over Korea and adjacent areas (20–60°N in latitude, 110–160°E in longitude, and 80–1000 km in altitude) during geomagnetic quiet days and geomagnetic storm days (17–18 March 2015). Ionospheric Data Assimilation Four-Dimension (IDA4D) technique is used to assimilate measured data into IRI (International Reference Ionosphere) model. Assimilation 3D electron density was generated every 15 min, with slant total electron content (STEC) data from GPS, and NmF2 data from ionosonde as the ingested data into International Reference Ionosphere (IRI) model. Results show a pre-SSC (sudden storm commencement) and post-SSC enhancements of foF2 and equatorial ionization anomaly (EIA) vertical total electron content (VTEC) on 16th and 17th March, respectively. Furthermore, on 18 March, result shows a negative ionosphere response. Pre-SSC enhancement is observed in the nighttime and is caused by enhanced zonal electric field. During nighttime of 17 March the post-SSC enhancement of electron density is caused by LSTIDs. On daytime of 18 March westward DDEF and reduction of O/N2 ratio cause depletion of foF2 and VTEC over the region. During nighttime of this day the negative storm is due to eastward disturbance dynamo electric field (DDEF).

A Simulation Study on the Latitudinal Variations of Ionospheric Zonal Electric Fields Under Geomagnetically Quiet Conditions

AbstractThe influence of zonal electric fields as well as E × B plasma drifts on the ionosphere has been widely investigated, but the latitudinal variations of zonal electric fields have not been well understood. In this study, we investigate the driving mechanisms responsible for the latitudinal variations of zonal electric fields under geomagnetically quiet conditions using the Thermosphere‐Ionosphere Electrodynamics General Circulation Model (TIEGCM). A series of case‐controlled TIEGCM simulations were conducted to explore the effects of neutral wind and ionospheric conductivity on the latitudinal variations of zonal electric fields. The major results are given as follows: (1) The eastward electric field at noon increases with latitude, which is mainly associated with latitudinal changes of poleward neutral winds. (2) The prominent latitudinal variations of zonal electric fields at sunrise and sunset are attributed to the strong longitudinal gradients of zonal winds, that is, the curl‐free mechanism. (3) The changes of the zonal electric fields at a given location are affected by the dynamo effect globally, which also produces the latitudinal structure of zonal electric fields.

Identification of the mechanisms responsible for anomalies in the tropical lower thermosphere/ionosphere caused by the January 2009 sudden stratospheric warming

We apply the Entire Atmosphere GLobal (EAGLE) model to investigate the upper atmosphere response to the January 2009 sudden stratospheric warming (SSW) event. The model successfully reproduces neutral temperature and total electron content (TEC) observations. Using both model and observational data, we identify a cooling in the tropical lower thermosphere caused by the SSW. This cooling affects the zonal electric field close to the equator, leading to an enhanced vertical plasma drift. We demonstrate that along with a SSW-related wind disturbance, which is the main source to form a dynamo electric field in the ionosphere, perturbations of the ionospheric conductivity also make a significant contribution to the formation of the electric field response to SSW. The post-sunset TEC enhancement and pre-sunrise electron content reduction are revealed as a response to the 2009 SSW. We show that at post-sunset hours the SSW affects low-latitude TEC via a disturbance of the meridional electric field. We also show that the phase change of the semidiurnal migrating solar tide (SW2) in the neutral wind caused by the 2009 SSW at the altitude of the dynamo electric field generation has a crucial importance for the SW2 phase change in the zonal electric field. Such changes lead to the appearance of anomalous diurnal variability of the equatorial electromagnetic plasma drift and subsequent low-latitudinal TEC disturbances in agreement with available observations.Plain Language Summary– Entire Atmosphere GLobal model (EAGLE) interactively calculates the troposphere, stratosphere, mesosphere, thermosphere, and plasmasphere–ionosphere system states and their response to various natural and anthropogenic forcing. In this paper, we study the upper atmosphere response to the major sudden stratospheric warming that occurred in January 2009. Our results agree well with the observed evolution of the neutral temperature in the upper atmosphere and with low-latitude ionospheric disturbances over America. For the first time, we identify an SSW-related cooling in the tropical lower thermosphere that, in turn, could provide additional information for understanding the mechanisms for the generation of electric field disturbances observed at low latitudes. We show that the SSW-related vertical electromagnetic drift due to electric field disturbances is a key mechanism for interpretation of an observed anomalous diurnal development of the equatorial ionization anomaly during the 2009 SSW event. We demonstrate that the link between thermospheric winds and the ionospheric dynamo electric field during the SSW is attained through the modulation of the semidiurnal migrating solar tide.

Signatures of Sudden Storm Commencement on the equatorial thermospheric dayglow

It has been observed that the OI 630.0 nm dayglow emission over a dip equatorial station, Trivandrum (8.5° N, 77° E, dip 0.5° N), India registered an abrupt increase of ~ 2000 R during the compression phase of the magnetosphere as dictated by a sudden increase in solar wind ram pressure. Furthermore, an unusual depletion of these emissions has been observed during the eastward interplanetary electric field (IEF), concomitant with southward excursion of IMF Bz. The ionosonde and magnetometer observations confirmed the effects of prompt penetration electric field (PPEF). Associated with the eastward PPEF, formation of F3 layers were also noticed. These unique results, which emphasize the effect of Sudden Storm Commencement/IEF on these equatorial daytime airglow emissions are discussed in context of changes in the equatorial zonal electric field and F region height variations associated with polar/auroral activities due to the magnetosphere-ionosphere coupling.

The Improved Two‐Dimensional Artificial Neural Network‐Based Ionospheric Model (ANNIM)

AbstractAn artificial neural network‐based two‐dimensional ionospheric model (ANNIM) that can predict the ionospheric F2‐layer peak density (NmF2) and altitude (hmF2) had recently been developed using long‐term data of Formosat‐3/COSMIC GPS radio occultation (RO) observations (Sai Gowtam & Tulasi Ram, 2017a, https://doi.org/10.1002/2017JA024795). In this current paper, we present an improved version of ANNIM that was developed by assimilating additional ionospheric data from CHAMP, GRACE RO, worldwide ground‐based Digisonde observations, and by using a modified spatial gridding approach based on the magnetic dip latitudes. The improved ANNIM better reproduces the spatial and temporal variations of NmF2 and hmF2, including the postsunset enhancement in equatorial hmF2 associated with the prereversal enhancement in the zonal electric field. The ANNIM‐predicted NmF2 and hmF2 exhibit excellent correlations with ground‐based Digisonde observations over different solar activity periods. The ANNIM simulations under enhanced geomagnetic activity predict the depletion of NmF2 at auroral‐high latitudes, and enhancement over low latitude to midlatitude with respect to quiet conditions, which is consistent with the storm time meridional wind circulation and the associated neutral composition changes. The improved ANNIM also predicts a significant enhancement in hmF2 around auroral latitudes due to increased plasma scale height associated with particle and Joule heating during storm periods. Further, the ANNIM successfully reproduces the coherent oscillations in NmF2 and hmF2 with recurrent cororating interaction region‐driven geomagnetic activity during the extreme solar minimum year 2008 and can distinguish the roles of recurrent geomagnetic activity and solar irradiance through controlled simulations.

Effects of Electric Field and Neutral Wind on the Asymmetry of Equatorial Ionization Anomaly

AbstractThe zonal electric field and the meridional neutral wind are the principal drivers that define the geometry and characteristics of the equatorial ionization anomaly (EIA). Here we present the response of the EIA to the variability of the zonal electric field based on measurements of the equatorial electrojet (EEJ) currents and trans‐equatorial neutral winds for the generation and control of the asymmetries of the EIA crests of total electron content (TEC) in the western side of the South American continent. The EEJ strengths are determined using a pair of magnetometers. The 24‐hr trans‐equatorial neutral wind profile is measured using the Second‐Generation, Optimized, Fabry‐Perot Doppler Imager (SOFDI) located near the geomagnetic equator. The EIA is evaluated using TEC data measured by Global Positioning System (GPS) receivers from the Low‐Latitude Ionospheric Sensor Network and several other networks in South America. A physics‐based numerical model, Low‐Latitude Ionospheric Sector, and SOFDI data are used to study the effects of daytime meridional neutral winds on the consequent evolution of an asymmetry in equatorial TEC anomalies during the afternoon and onward for the first time. We find that the configuration parameters such as strength, shape, amplitude, and latitudinal width of the EIAs are affected by the eastward electric field associated with the EEJ under undisturbed conditions. The asymmetries of EIA crests are observed more frequently during solstices and the September equinox than in the March equinox season. Importantly, this study indicates that the meridional neutral wind plays a very significant role in the development of the EIA asymmetry by transporting the plasma up the field lines. This result suggests that a precise observation of the latitudinal TEC profile at low latitudes can be used to derive the meridional wind.

Interannual Variability of the Daytime Equatorial Ionospheric Electric Field

AbstractUnderstanding the variability of the ionosphere is important for the prediction of space weather and climate. Recent studies have shown that forcing from the lower atmosphere plays a significant role for the short‐term (day‐to‐day) variability of the low‐latitude ionosphere. The present study aims to assess the importance of atmospheric forcing for the variability of the daytime equatorial ionospheric electric field on the interannual (year‐to‐year) time scale. Magnetic field measurements from Huancayo (12.05°S, 75.33°W) are used to augment the equatorial vertical plasma drift velocity (VZ) measurements from the Jicamarca Unattended Long‐term Investigations of the Ionosphere and Atmosphere radar during 2001–2016. VZ can be regarded as a measure of the zonal electric field. After removing the seasonal variation of ∼10 m/s, midday values of VZ show an interannual variation of ∼2 m/s with an oscillation period of 2–3 years. No evidence of solar cycle influence is found. The Ground‐to‐topside Atmosphere‐Ionosphere model for Aeronomy, which takes into account realistic atmospheric variability below 30 km, reproduces the pattern of the observed interannual variation without having to include variable forcing from the magnetosphere. The results indicate that lower atmospheric forcing plays a dominant role for the observed interannual variability of VZ at 1200 local time.

Impact of disturbance electric fields in the evening on prereversal vertical drift and spread F developments in the equatorial ionosphere

Abstract. Equatorial plasma bubble/spread F irregularity occurrence can present large variability depending upon the intensity of the evening prereversal enhancement in the zonal electric field (PRE), that is, the F region vertical plasma drift, which basically drives the post-sunset irregularity development. Forcing from magnetospheric disturbances is an important source of modification and variability in the PRE vertical drift and of the associated bubble development. Although the roles of magnetospheric disturbance time penetration electric fields in the bubble irregularity development have been studied in the literature, many details regarding the nature of the interaction between the penetration electric fields and the PRE vertical drift still lack our understanding. In this paper we have analyzed data on F layer heights and vertical drifts obtained from digisondes operated in Brazil to investigate the connection between magnetic disturbances occurring during and preceding sunset and the consequent variabilities in the PRE vertical drift and associated equatorial spread F (ESF) development. The impact of the prompt penetration under-shielding eastward electric field and that of the over-shielding, and disturbance dynamo, westward electric field on the evolution of the evening PRE vertical drift and thereby on the ESF development are briefly examined. Keywords. Ionosphere (ionospheric irregularities)

Occurrence characteristics of ionospheric irregularities over Indian low latitude region Varanasi during ascending phase of solar cycle 24

Ionospheric irregularities degrade the performance of radio technological system by producing fluctuations in amplitude and phase of signal passing through them, a phenomenon which is known as scintillations. This study presents diurnal and seasonal variations of ionospheric irregularities during ascending phase of solar activity from 2009 to 2014 by using the amplitude scintillation index S 4 computed from a dual frequency GPS receiver installed at the low-latitude station of Varanasi (Lat. 25.3176° N, Long. 82.9739° E). Scintillation occurrences are found to be higher during nighttime hours (1930-0130 LT) and characterized by an equinoctial maximum throughout the years 2009-2014, except for the peculiar solar minimum year 2009. Gravity wave seed perturbation from lower atmosphere and pre-reversal enhancement (PRE) in zonal electric field have been considered to explain the observed seasonal occurrences, which have been also compared with the previous results obtained from observations and model. Influence of solar activity on scintillation occurrence has also been studied, and it was found that there is linear dependence between the solar activity and scintillation occurrence, which is seasonally variable.

Ionospheric climatology at Africa EIA trough stations during descending phase of sunspot cycle 22

The African equatorial ionospheric climatology during the descending phase of sunspot-cycle 22 (spanning 1992–1996) was investigated using 3 ionosondes located at Dakar (14.70 N, 342.60 E), Ouagadougou (12.420 N, 358.60 E), and Korhogo (9.510 N, 354.40 E). The variations in the virtual height of the F-layer (h’F), maximum electron density (NmF2), vertical plasma drift (Vp) and zonal electric field (Ey) were presented. Significant decrease in the NmF2 amplitude compared to h’F in all of the stations during the descending period is obvious. While NmF2 magnitude maximizes/minimizes during the E-seasons/J-season, h’F attained highest/lowest altitude in J-season/D-season for all stations. D-season anomaly was evident in NmF2 at all stations. For any season, the intensity (Ibt) of NmF2 noon-bite-out is highest at Dakar owning to fountain effect and maximizes in March-E season. Stations across the EIA trough show nearly coherence ionospheric climatology characteristics whose difference is of latitudinal origin. Hemispheric dependence in NmF2 is obvious, with difference more significant during high-solar activity and closes with decreasing solar activity. The variability in the plasma drift during the entire phase is suggested to emanate from solar flux variations, and additionally from enhanced leakage of electric fields from high-to low-latitudes. Existing African regional model of evening/nightttime pre-reversal plasma drift/sunspot number (PREpeak/R) relationship compares well with experimental observations at all stations with slight over-estimation. The correlation/root-mean-square-deviation (RMSdev) pair between the model and observed Vp during the descending phase recorded 94.9%/0.756, 92.4%/1.526, and 79.1%/3.612 at Korhogo, Ouagadougou and Dakar respectively. The Ey/h’F and Ey/NmF2 relationships suggest that zonal electric field is more active in the lifting of h’F and suppression of NmF2 during high- and moderate-solar activities when compared with low-solar activity. This is the first work to show higher bite-out at the equatorial northern-station (Dakar) than southern-station (Korhogo) using ionosonde data.

Coupled nature of evening-time ionospheric electrodynamics

The F region evening electrodynamics in the equatorial region is characterized by a pre-reversal enhancement (PRE) in the zonal eastward electric field. Although the theoretical mechanisms for PRE are known, its variability, particularly day-to-day variability is not fully resolved. PRE is a large scale phenomenon driven by the F region dynamo after the sunset hours. This paper investigates whether the variability of the E region conductivity (particularly the one associated with the sporadic E, Es) has any influence on the F region dynamo and hence on the PRE of zonal electric field. Interestingly, ionosonde observations have indicated a higher occurrence of the blanketing type Es (Esb) over the low latitude on days with highly suppressed PRE of zonal electric field in comparison with the days with significantly larger PRE. Observational evidences presented in this paper suggests that the formation of the Esb in the evening hours is a sovereign process, not always controlled by the sheared F region vertical electric field of equatorial origin, mapping along the magnetic field line on to the low latitude E region. Model computations of the PRE suppression based on the measured Es densities have further substantiated the observational findings presented in this paper. These results clearly indicate that the low latitude Es has the potential to suppress the PRE of zonal electric field and possibly can play a vital role in explaining the PRE variability, particularly the day-to-day variability. Results have been discussed in light of earlier reports on PRE mechanisms and E-F region coupling processes.

Study of the Equatorial and Low-Latitude Electrodynamic and Ionospheric Disturbances During the 22-23 June 2015 Geomagnetic Storm Using Ground-Based and Spaceborne Techniques.

We use a set of ground‐based instruments (Global Positioning System receivers, ionosondes, magnetometers) along with data of multiple satellite missions (Swarm, C/NOFS, DMSP, GUVI) to analyze the equatorial and low‐latitude electrodynamic and ionospheric disturbances caused by the geomagnetic storm of 22–23 June 2015, which is the second largest storm in the current solar cycle. Our results show that at the beginning of the storm, the equatorial electrojet (EEJ) and the equatorial zonal electric fields were largely impacted by the prompt penetration electric fields (PPEF). The PPEF were first directed eastward and caused significant ionospheric uplift and positive ionospheric storm on the dayside, and downward drift on the nightside. Furthermore, about 45 min after the storm commencement, the interplanetary magnetic field (IMF) Bz component turned northward, leading to the EEJ changing sign to westward, and to overall decrease of the vertical total electron content (VTEC) and electron density on the dayside. At the end of the main phase of the storm, and with the second long‐term IMF Bz southward turn, we observed several oscillations of the EEJ, which led us to conclude that at this stage of the storm, the disturbance dynamo effect was already in effect, competing with the PPEF and reducing it. Our analysis showed no significant upward or downward plasma motion during this period of time; however, the electron density and the VTEC drastically increased on the dayside (over the Asian region). We show that this second positive storm was largely influenced by the disturbed thermospheric conditions.

Latitude-dependent delay in the responses of the equatorial electrojet and <i>S</i><sub><i>q</i></sub> currents to X-class solar flares

Abstract. We have analyzed low-latitude ionospheric current responses to two intense (X-class) solar flares that occurred on 13 May 2013 and 11 March 2015. Sudden intensifications, in response to solar flare radiation impulses, in the Sq and equatorial electrojet (EEJ) currents, as detected by magnetometers over equatorial and low-latitude sites in South America, are studied. In particular we show for the first time that a 5 to 8 min time delay is present in the peak effect in the EEJ, with respect that of Sq current outside the magnetic equator, in response to the flare radiation enhancement. The Sq current intensification peaks close to the flare X-ray peak, while the EEJ peak occurs 5 to 8 min later. We have used the Sheffield University Plasmasphere-Ionosphere Model at National Institute for Space Research (SUPIM-INPE) to simulate the E-region conductivity enhancement as caused by the flare enhanced solar extreme ultraviolet (EUV) and soft X-rays flux. We propose that the flare-induced enhancement in neutral wind occurring with a time delay (with respect to the flare radiation) could be responsible for a delayed zonal electric field disturbance driving the EEJ, in which the Cowling conductivity offers enhanced sensitivity to the driving zonal electric field. Keywords. Ionosphere (equatorial ionosphere)

Equatorial plasma bubble generation/inhibition during 2015 St. Patrick's Day storm

AbstractAll‐sky camera observations carried out over Taiwan showed intense equatorial plasma bubbles (EPBs) in 630.0 nm airglow images on consecutive nights of 13–16 March 2015 but were absent in the following night of 17 March when St. Patrick's Day magnetic storm occurred. Rate of total electron content (TEC) index by using Global Positioning System (GPS) network data also confirmed the absence of irregularities on the night 17 March. The results, however, revealed strong irregularities over Indian sector on the same night. Flux tube integrated Rayleigh‐Taylor instability growth rates computed using the prior (forecast) state of Thermosphere‐Ionosphere Electrodynamics General Circulation Model output after assimilating the GPS‐TEC measurements also agree with the observations, showing smaller values over Taiwan and larger values over India on the night of 17 March. The ionospheric response to the storm over Taiwan that resulted in the apparent inhibition of EPB is investigated in this study by using the data assimilation output. Results indicate that on the night of the magnetic storm, prereversal enhancement of zonal electric field over Taiwan was weaker when compared to that over India. Further analysis suggests that the absence of enhancement in the zonal electric field could be due to westward penetration electric field in response to rapid northward turning of interplanetary magnetic field that occurred during the dusk period over Taiwan.

A Simulation Study of the Equatorial Ionospheric Response to the October 2013 Geomagnetic Storm

AbstractThe ionospheric observation from ionosonde at Sao Luis (2.5°S, 44.2°W; 6.68°S dip latitude) around the magnetic equator showed that the nighttime ionospheric F2 layer was uplifted by more than 150 km during the October 2013 geomagnetic storm. The changes of the F2 peak height (hmF2) at the magnetic equator were generally attributed to the variations of vertical drift associated with zonal electric fields. In this paper, the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) simulation results are utilized to explore the possible physical mechanisms responsible for the observed increase of hmF2 at Sao Luis. The TIEGCM generally reproduced the changes of F2 peak electron density (NmF2) and its height (hmF2) during the main and recovery phases of the October 2013 storm. A series of controlled simulations revealed that storm time hmF2 changes at the magnetic equator are not purely associated with the changes of electric fields; horizontal plasma transport due to meridional winds and thermospheric expansion also contributed significantly to the profound increase of nighttime hmF2 observed at Sao Luis on 2 October. Moreover, the changes of meridional winds and neutral temperature in the equatorial region are associated with storm time traveling atmospheric disturbances originating from high latitudes.

Spread F modeling over Brazil

Based on a numerical model developed by Carrasco et al. (2014), the dynamics and morphology of ionospheric plasma bubbles for both magnetic hemispheres over Brazil are examined. Observational results show that the post sunset maximum velocity of the pre-reversal vertical drift velocity, VP, has different values at the conjugate points. This information on the asymmetry of the vertical drift or zonal electric field is used in the simulation of bubbles over Brazilian sector. The simulations were conducted considering two different cases: (a) simulation of trans-equatorial bubble neglecting the meridional winds, and (b) considering the effect of meridional winds. In case (a) the bubble development is symmetric in relation to the magnetic equator, while in case (b) we raise the conjecture of a possible asymmetry. The simulation shows that the trans-equatorial bubbles may develop between ±30° of latitude and present internal structures of different sizes and shapes. In addition, three spatial perspectives of the bubbles are displayed, where each perspective reveals interesting aspects of the bubble development.