Development Of Equatorial Plasma Bubbles Research Articles

Impact of meridional winds on the development of equatorial plasma bubbles: a review

Impact of meridional winds on the development of equatorial plasma bubbles: a review

Identifying the Onset Location of Equatorial Plasma Bubbles (EPBs) and Its Relationship With the Background Ionospheric Conditions

AbstractUsing radars and C/NOFS satellite observations we studied the spatio‐temporal evolution of Equatorial Plasma Bubbles (EPBs) and estimated its onset location across a wide longitudinal sector over Indian and Southeast Asian longitudes. The vertical E × B drift velocity measurements obtained from the Ion Velocity Meter (IVM) on board the C/NOFS satellite and collocated ionosonde observations were used to examine the background ionospheric conditions. Our study shows that the periodic EPBs were present in those longitudes where periodic wave structure in the E × B drift and elevated F layer were observed. In this case study, the comprehensive analysis using the observations from radars and satellite data provides a better understanding on the longitudinal preference of the EPB occurrence and its responsible background mechanisms. This understanding of the onset location and background conditions of EPBs over a large longitudinal area for an extended period can contribute to the development of accurate EPB forecasting models, which are essential to mitigate the detrimental effects of EPBs on communication and navigation systems.

Investigation of the Physical Mechanisms of the Formation and Evolution of Equatorial Plasma Bubbles During a Moderate Storm on 17 September 2021

AbstractWe investigate in detail the occurrence and evolution of ionospheric equatorial plasma bubbles (EPBs) during a moderate storm on 17 September 2021, using Global‐scale Observations of the Limb and Disk (GOLD) observations and Whole Atmosphere Community Climate Model‐eXtended (WACCM‐X) simulations. GOLD observations show that there were no EPBs on 16 September before the storm but EPBs occurred after the storm commencement on 17 September. The EPBs extended to ∼30° magnetic latitude. A diagnostic analysis of WACCM‐X simulations reveals that the rapid enhancement of prompt penetration electric fields (PPEFs) after the sudden storm commencement is the main reason that triggered the occurrence of the EPBs. Further quantitative analysis shows that vertical plasma drifts, which are enhanced by the PPEF, played a dominant role in strengthening the Rayleigh‐Taylor instability, leading to the occurrence of the EPBs and the large latitudinal extension of the EPBs to ∼30° magnetic latitude during the night of 17 September.

Numerical simulation of the equatorial plasma bubble: the effect of seeding by the vertical winds and random background noise perturbations

A wide variety of small-amplitude waves widely exist in the ionosphere and have significant effects on the evolution of equatorial plasma bubbles. In this paper, we simulated equatorial plasma bubbles (EPB) seeded by vertical neutral wind perturbations with wavelengths of 125 km and 250 km, and compared the morphology characteristics of plasma bubble structures with those under random noise perturbations in the background density. The numerical results showed that both vertical winds and random background noise perturbations can contribute to the growth of plasma bubbles, and the perturbations under additional random background noise can promote the growth of the plasma bubble structures faster. Additionally, several processes of the nonlinear behavior of bifurcated EPB structures, including bifurcation, pinching, and small-scale turbulent structures, were successfully obtained. Our simulation captured supersonic flows within the low-density plasma structures characterized by vertical velocities of about 1.5 km/s, which is consistent with experimental studies found in the literature.

Towards mitigating the effect of plasma bubbles on GPS positioning accuracy through wavelet transformation over Southeast Asian region

Ionospheric irregularities remain as a challenging concern among space scientists and engineers as it can pose severe threats to satellite-based services by introducing amplitude and phase scintillations in the transionospheric radio signals. The scintillations may cause degradation of the positioning, navigation, and timing (PNT) services or even complete system failure under extreme conditions. The present work focuses on understanding the ionospheric irregularities during the space weather event on May 12, 2021, witnessing the first strong geomagnetic storm in the 25th solar cycle. This particular event commenced around 13.00 UT, apparently altering the regular development of equatorial plasma bubbles around the local post-sunset terminator. We selected three global positioning system (GPS) stations, one at the near-magnetic equator (CUSV) and two at far low-latitude locations with a longitudinal separation (THKK and HWKS) over the Southeast Asian longitudes to investigate the characteristics of ionospheric plasma irregularities on the storm day. The results showed that the amplitude of ROTI was highest at the low-latitude CUSV, followed by the THKK and HKWS stations. The increased ROTI at CUSV is due to its latitudinal location close to the geomagnetic equator in the equatorial ionization anomaly (EIA) region which often witnesses more fluctuations than the inter-mediate latitudes. The strong TEC fluctuation occurrences were comparatively minimal at HWKS station on the storm day (May 12, 2021), whereas both intermediate and relatively higher TEC fluctuation occurrences were found at THKK and CUSV stations, respectively. Moreover, the occurrence rate of moderate irregularities was the highest at CUSV, followed by THKK and HKWS, while that of weak irregularities was maximum at HKWS, followed by THKK and CUSV. This happens because the systematic longitudinal time delay observed at the onset of ionospheric scintillations corresponds to the rise of PBs velocity at the magnetic equator that strongly depended upon the F-region dynamo electric fields in the east–west direction. Furthermore, the three-dimensional (3-D) positioning errors were estimated at a 30-sec interval to understand the impact of storm-induced irregularity on the positioning solution. An excellent agreement is realized between the ROTI pattern and positioning errors at all stations, irrespective of latitudinal extent from the magnetic equator. In order to model the amplitude of the scintillation index against the SNR measurements, the linear, quadratic, and cubic regression curve fittings are utilized that indicate the cubic polynomial manifesting the best fitted SNR measurements with the weak as well as moderate scintillation index. The continuous wavelet transformation (CWT) method was implemented to mitigate the ionospheric scintillation effects on positioning accuracy. The CWT estimation reveals the hidden information in positioning error due to the scintillation index, which makes it easy to understand the ionospheric scintillation effects on positioning. The attempt in this work is aligned with the efforts towards reliable PNT operations over equatorial and low latitudes.

Multi-instrumental analysis of the day-to-day variability of equatorial plasma bubbles

This paper presents a multi-instrument observational analysis of the equatorial plasma bubbles (EPBs) variation over the American sector during a geomagnetically quiet time period of 07–10 December 2019. The day-to-day variability of EPBs and their underlying drivers are investigated through coordinately utilizing the Global-scale Observations of Limb and Disk (GOLD) ultraviolet images, the Ionospheric Connection Explorer (ICON) in-situ and remote sensing data, the global navigation satellite system (GNSS) total electron content (TEC) observations, as well as ionosonde measurements. The main results are as follows: 1) The postsunset EPBs’ intensity exhibited a large day-to-day variation in the same UT intervals, which was fairly noticeable in the evening of December 07, yet considerably suppressed on December 08 and 09, and then dramatically revived and enhanced on December 10. 2) The postsunset linear Rayleigh-Taylor instability growth rate exhibited a different variation pattern. It had a relatively modest peak value on December 07 and 08, yet a larger peak value on December 09 and 10. There was a 2-h time lag of the growth rate peak time in the evening of December 09 from other nights. This analysis did not show an exact one-to-one relationship between the peak growth rate and the observed EPBs intensity. 3) The EPBs’ day-to-day variation has a better agreement with that of traveling ionospheric disturbances and atmospheric gravity waves signatures, which exhibited relatively strong wavelike perturbations preceding/accompanying the observed EPBs on December 07 and 10 yet relatively weak fluctuations on December 08 and 09. These coordinate observations indicate that the initial wavelike seeding perturbations associated with AGWs, together with the catalyzing factor of the instability growth rate, collectively played important roles to modulate the day-to-day variation of EPBs. A strong seeding perturbation could effectively compensate for a moderate strength of Rayleigh-Taylor instability growth rate and therefore their combined effect could facilitate EPB development. Lacking proper seeding perturbations would make it a more inefficient process for the development of EPBs, especially with a delayed peak value of Rayleigh-Taylor instability growth rate.

Asymmetric Development of Equatorial Plasma Bubbles Observed at Geomagnetically Conjugate Points Over the Brazilian Sector

AbstractGround‐based global navigation satellite systems (GNSS) receivers have been used to monitor the meridional (north‐south) development of equatorial plasma bubbles (EPBs) at geomagnetically conjugate points over the Brazilian sector. EPBs were studied using detrended total electron content plots obtained from Boa Vista (MLat: 9.6°N), Itacoatiara (MLat: 3.3°N), Colíder (MLat: 5.0°S), and Cuiabá (MLat: 8.8°S) GNSS receivers. All GNSS receivers are located approximately under the same magnetic meridian. 655 nights with EPBs occurrence were analyzed using data from January 2012 to February 2016. In 459 nights (∼70%) the EPBs presented a symmetric development with respect to the geomagnetic equator. However, in 196 nights (∼30%) the EPBs presented an apparent asymmetry. The asymmetries are characterized as a displacement of the EPBs to north (or south) of the geomagnetic equator. The highest north (south) asymmetry occurrence was observed during December to January (March to April and September to October), and lowest during March to April and August to September (December to January). To investigate these asymmetries, we analyzed meridional wind data and used a numerical model to simulate the EPBs evolution. Both meridional wind data and numerical simulation results suggested that a trans‐equatorial meridional wind blowing to north (south) would be able to cause a displacement of the EPBs to north (south) of the geomagnetic equator.

Signatures of Equatorial Plasma Bubbles and Ionospheric Scintillations from Magnetometer and GNSS Observations in the Indian Longitudes during the Space Weather Events of Early September 2017

Scintillation due to ionospheric plasma irregularities remains a challenging task for the space science community as it can severely threaten the dynamic systems relying on space-based navigation services. In the present paper, we probe the ionospheric current and plasma irregularity characteristics from a latitudinal arrangement of magnetometers and Global Navigation Satellite System (GNSS) stations from the equator to the far low latitude location over the Indian longitudes, during the severe space weather events of 6–10 September 2017 that are associated with the strongest and consecutive solar flares in the 24th solar cycle. The night-time influence of partial ring current signatures in ASYH and the daytime influence of the disturbances in the ionospheric E region electric currents (Diono) are highlighted during the event. The total electron content (TEC) from the latitudinal GNSS observables indicate a perturbed equatorial ionization anomaly (EIA) condition on 7 September, due to a sequence of M-class solar flares and associated prompt penetration electric fields (PPEFs), whereas the suppressed EIA on 8 September with an inverted equatorial electrojet (EEJ) suggests the driving disturbance dynamo electric current (Ddyn) corresponding to disturbance dynamo electric fields (DDEFs) penetration in the E region and additional contributions from the plausible storm-time compositional changes (O/N2) in the F-region. The concurrent analysis of the Diono and EEJ strengths help in identifying the pre-reversal effect (PRE) condition to seed the development of equatorial plasma bubbles (EPBs) during the local evening sector on the storm day. The severity of ionospheric irregularities at different latitudes is revealed from the occurrence rate of the rate of change of TEC index (ROTI) variations. Further, the investigations of the hourly maximum absolute error (MAE) and root mean square error (RMSE) of ROTI from the reference quiet days’ levels and the timestamps of ROTI peak magnitudes substantiate the severity, latitudinal time lag in the peak of irregularity, and poleward expansion of EPBs and associated scintillations. The key findings from this study strengthen the understanding of evolution and the drifting characteristics of plasma irregularities over the Indian low latitudes.

Occurrence characteristics of equatorial plasma bubbles over Kisumu, Kenya during Solar maximum of Solar Cycle 24

Equatorial Plasma Bubbles (EPBs) are irregular plasma density depletions in the ambient electron density in the equatorial F-region ionosphere generated after sunset. EPBs are known to bring disruptions to telecommunication and navigation systems. This paper investigates the occurrence of EPBs over Kisumu, Kenya (Geomagnetic coordinates: 9.64o S, 108.59o E; Geographic coordinates: 0.02o S, 34.6o E) for a few selected quiet and storm days between 1st January 2013 and 31st December 2014 which was a high Solar activity period for Solar Cycle 24. The study brings out EPB occurrence pattern over Kisumu, Kenya for the selected quiet and storm days of 2013 and 2014. The Receiver Independent Exchange (RINEX) data was retrieved from the Kisumu high data-rate NovAtel GSV4004B SCINDA-GPS receiver. The data was unzipped and processed to obtain Vertical Total Electron Content (VTEC), amplitude scintillation (S4) and Universal Time (UT) which were then fed into MATLAB to generate VTEC and S4 plots against UT for each selected quiet and storm day within the years 2013 and 2014. The Total Electron Content (TEC) depletion depths and S4 index values between 16:00 and 20:00 UT for each selected quiet and storm day were extracted from the VTEC and S4 plots and used to plot TEC depletion depths and S4 plots. The Rate of Change of TEC (ROT) and Rate of Change of TEC Index (ROTI) between 16:00 and 20:00 UT were generated from VTEC and used to plot ROT and the corresponding ROTI plots against UT. TEC depletion depths and ROTI values for each selected quiet and storm day between 16:00 and 20:00 UT were extracted and used to plot TEC depletion depths and ROTI plots and S4 index and ROTI plots. In this study, the enhancement of S4 index corresponded well with TEC depletions, increased fluctuation of ROT and higher ROTI values between 16:00UT and 20:00UT for most days. This correspondence was used in inferring the occurrence of EPBs during the selected quiet and storm days of the years 2013 and 2014. The obtained results showed that the highest EPB occurrence was during March equinox with 33.33% occurrence in the year 2013 and 30.76% occurrence in the year 2014, followed by the September equinox which had 20.38% occurrence in 2013 and 17.26% occurrence in 2014. The seasonal variation of EPB occurrence was attributed to the variation in the daytime E x B drift velocities. Larger E x B drift velocities resulted in increased EPB occurrence in the equinoctial period (March, April, August and September) and November solstice period (November and December) while lower E x B drift velocities resulted in reduced EPB occurrence in the June solstice period (June and July). The percentage EPB occurrence in the year 2013 was 6.49% while in the year 2014 was 4.32%. The storm period had percentage EPB occurrence of 21.42% in the year 2013 and 21.88% in the year 2014 while the quiet period had percentage EPB occurrence of 18.75% in the year 2013 and 7.89% in the year 2014. These results clearly showed that the percentage EPB occurrence was higher during the storm period than in the quiet period. Hence the development of EPBs was enhanced by geomagnetic activity through several competing dynamics such as Prompt Penetration Electric Field (PPEF), Disturbance Dynamo Electric Field (DDEF) and reduction in electron density due to increased recombination rates. Â

Occurrence characteristics of branching structures in equatorial plasma bubbles: a statistical study based on all-sky imagers in China

Branching structure (BS) is a very important phenomenon in the evolution of equatorial plasma bubbles (EPBs), the mechanism of which is widely studied from observation and from simulation. However, occurrence characteristics of branching structure of equatorial plasma bubbles (BSEPBs) have not been well addressed. In this work, we used seven-years (2012−2018) of observations from two all-sky imagers to study occurrence of BSEPBs in detail. These data reveal a high incidence of BS in EPB cases; in particular, most EPBs occurring on days with geomagnetic disturbances exhibited BS. Periods when all EPBs exhibited BS increased significantly in the 2014 solar maximum. Occurrence times of BSEPBs varied with local time; most of the BSEPBs began to appear between 21:00 and 22:00 LT. During the solar maximum, some BSEPBs were observed after midnight. The data also reveal that BSEPBs are characterized primarily by two branches or three branches. Multi-branching appeared only in the solar maximum. EPB events with different coexisting branching structures increased from 2012 to 2014 and decreased from 2014 to 2018. These results strongly suggest that BSEPB occurrence is related to solar activity and geomagnetic activity, and thus provide a new perspective for future studies of EPBs as well as enriching our understanding of ionospheric irregularity.

Observation and Simulation of the Development of Equatorial Plasma Bubbles: Post‐Sunset Rise or Upwelling Growth?

AbstractThis study investigates the underlying physics of equatorial plasma bubbles (EPBs) on 11 December 2019, under solar minimum conditions. The Global‐scale Observations of the Limb and Disk (GOLD) ultraviolet nightglow images exhibit a periodic distribution of reduced emissions related to EPBs. Remarkably, FORMOSAT‐7/COSMIC‐2 (F7/C2) observes a significant altitudinal difference of ~45 km in the bottomside ionosphere between two nearly collocated electron density profiles before the onset of EPBs, indicating the presence of an upwelling. Distinct ionospheric perturbations are also observed in F7/C2 and ground‐based Global Positioning System observations, suggesting that gravity waves may contribute to the upwelling. Simulations with SAMI3/ESF are further carried out to evaluate the upwelling growth and pre‐reversal enhancement (PRE) effect on EPB development. Simulations reveal that the crests of upwellings show a localized uplift of ~50 km, and EPBs only develop from the crest of upwellings. The uplift altitude of upwellings is comparable to the F7/C2 observations and the post‐sunset rise in moderate solar conditions. The polarization electric field (Ep) developed within the upwellings can drive vertical Ep × B drifts of ~32–35 m/s, which are comparable to the PRE vertical E × B drifts. We find that the PRE alone cannot drive EPBs without upwelling growth, but it can facilitate the upwelling growth. Observations and simulations allow us to conclude that upwelling growth could play a vital role in the formation of EPBs.

Challenges to Equatorial Plasma Bubble and Ionospheric Scintillation Short-Term Forecasting and Future Aspects in East and Southeast Asia

Equatorial plasma bubbles (EPBs) can cause rapid fluctuations in amplitude and phase of radio signals traversing the ionosphere and in turn produce serious ionospheric scintillations and disrupt satellite-based communication links. Whereas numerous studies on the generation and evolution of EPBs have been performed, the prediction of EPB and ionospheric scintillation occurrences still remains unresolved. The generalized Rayleigh–Taylor (R–T) instability has been widely accepted as the physical mechanism responsible for the generation of EPBs. But how the factors, which seed the development of R–T instability and control the dynamics of EPBs and resultant ionospheric scintillations, change on a short-term basis are not clear. In the East and Southeast Asia, there exist significant differences in the generation rates of EPBs at closely located stations, for example, Kototabang (0.2°S, 100.3°E) and Sanya (18.3°N, 109.6°E), indicating that the decorrelation distance of EPB generation is small (hundreds of kilometers) in longitude. In contrast, after the initial generation of EPBs at one longitude, they can drift zonally more than 2000 km and extend from the magnetic equator to middle latitudes of 40° or higher under some conditions. These features make it difficult to identify the possible seeding sources for the EPBs and to accurately predict their occurrence, especially when the onset locations of EPBs are far outside the observation sector. This paper presents a review on the current knowledge of EPBs and ionospheric scintillations in the East and Southeast Asia, including their generation mechanism and occurrence morphology, and discusses some unresolved issues related to their short-term forecasting, including (1) what factors control the generation of EPBs, its day-to-day variability and storm-time behavior, (2) what factors control the evolution and lifetime of EPBs, and (3) how to accurately determine ionospheric scintillation from EPB measurements. Special focus is given to the whole process of the EPB generation, development and disruption. The current observing capabilities, future new facilities and campaign observations in the East and Southeast Asia in helping to better understand the short-term variability of EPBs and ionospheric scintillations are outlined.

Observation and simulation of the development of equatorial plasma bubbles: post-sunset rise or upwelling growth?

Observation and simulation of the development of equatorial plasma bubbles: post-sunset rise or upwelling growth?

Modeling the Impact of Metallic Ion Layers on Equatorial Spread With SAMI3/ESF

AbstractThe impact of region metal ion layers on the development of equatorial plasma bubbles is investigated using the SAMI3/ESF model. We find that metal ion layers reduce the growth rate of the generalized Rayleigh‐Taylor instability (GRTI) and act to suppress the development of equatorial plasma bubbles. This is consistent with theoretical expectations and observations and is attributed to the increase in both the Pedersen and Hall conductances. Additionally, inhomogeneities in the region metal ion layer can map into the layer and alter the morphology of equatorial spread (ESF) bubble evolution. Lastly, we find that if EFS bubbles develop in the presence of a metal ion layer, then the electric fields generated by the instability can lift the metal ions into the region. This is consistent with observations of Fe in the region during equatorial spread .

Equatorial plasma bubbles developing around sunrise observed by an all-sky imager and global navigation satellite system network during storm time

Abstract. A large number of studies have shown that equatorial plasma bubbles (EPBs) occur mainly after sunset, and they usually drift eastward. However, in this paper, an unusual EPB event was simultaneously observed by an all-sky imager and the global navigation satellite system (GNSS) network in southern China, during the recovery phase of a geomagnetic storm that happened on 6–8 November 2015. Observations from both techniques show that the EPBs appeared near dawn. Interestingly, the observational results show that the EPBs continued to develop after sunrise, and they disappeared about 1 h after sunrise. The development stage of EPBs lasted for at least about 3 h. To our knowledge, this is the first time that the evolution of EPBs developing around sunrise was observed by an all-sky imager and the GNSS network. Our observation showed that the EPBs drifted westward, which was different from the usual eastward drifts of post-sunset EPBs. The simulation from the Thermosphere–Ionosphere–Electrodynamics General Circulation Model (TIE-GCM) suggest that the westward drift of EPBs should be related to the enhanced westward winds at storm time. Besides this, bifurcation and merging processes of EPBs were observed by the all-sky imager in the event. Associated with the development of EPBs, an increase in the peak height of the ionospheric F region was also observed near sunrise, and we suggest the enhanced upward vertical plasma drift during the geomagnetic storm plays a major role in triggering the EPBs near sunrise.

Daytime Evolution of Equatorial Plasma Bubbles Observed by the First Republic of China Satellite

AbstractPlasma bubbles in the equatorial F region are thought of as nighttime phenomena because they develop at night and are assumed to vanish after sunrise. However, bubbles occasionally persist throughout the night and into the day. This study investigates the origin of daytime irregularities and their evolution using data from the first Republic of China satellite. Our results show that daytime irregularities occur in the longitudes where bubbles have developed on previous nights. A newly reported feature is the observation of the temporal variation of the locations of daytime irregularities; daytime irregularities are concentrated near the magnetic equator early in the morning, but the location gradually shifts to higher latitudes with time. This phenomenon is explained in terms of the latitudinal redistribution of fossil bubbles by the ionospheric fountain effect.

Numerical simulation of recombination rate effect on development of equatorial plasma bubbles

Plasma bubbles in the ionosphere have a significant effect on radio wave communication and navigation. Under the worst condition, it will fail to function the systems relying on the ionosphere. On the other hand, the physical mechanisms of evolution and diurnal variations of the plasma bubbles in the ionosphere are still unclear. Therefore, it is still worthy to simulate the plasma bubbles in the ionosphere for radio wave propagation and science. In this study, the equatorial plasma bubbles induced by one- and two- dimensional disturbance in the ionosphere are simulated, where traveling ionospheric disturbance (TID) is used to produce the two-dimensional disturbance in the ionosphere. The dispersion relationship of gravity wave is used to represent the corresponding wavelength of TID in this study. More importantly, we investigate the effect of recombination rate on the development of plasma bubbles by numerical simulation. The simulation results show that the recombination rate of the plasma in the ionosphere has a significant effect on the development of plasma bubbles. The greater the recombination rate, the more time it takes to produce the plasma bubbles. For one-dimensional disturbance in the ionosphere, there is no significant effect in the structure of plasma bubbles for the recombination rate of the plasma. However, the recombination rate plays a significant effect on the structure of plasma bubbles induced by TID. When the recombination rate is less in numerical simulation, the complex structure including bifurcation, plume-like structures, and pinching of plasma bubbles can occur in the development of bubbles. In contrast, the structure of plasma bubbles is simpler when the recombination rate is greater in simulation. As a result, the recombination rate of the plasma is a significant factor for simulating the plasma bubbles in the ionosphere. The greater recombination rate can result in the slowing down of equatorial plasma bubbles and the simplifying the structure of the bubbles as well. In addition, it is found that not all of plasma bubbles at the bottom of the ionosphere can grow to the top of the ionosphere when many bubbles occur on the bottom side. The direction of the polarization electric field near the bubbles can be changed in the non-linear development of the bubbles. Therefore, only the bubbles where the polarization electric field is always eastward can develop to the topside.

Observation of equatorial plasma bubbles by the airglow imager on ISS-IMAP

Using 630 nm airglow data observed by an airglow imager on the International Space Station (ISS), the occurrence of equatorial plasma bubbles (EPBs) is studied. In order to examine the physical mechanisms in the boundary region between the Earth and the outer space, an ionosphere, mesosphere, upper atmosphere, and plasmasphere mapping (IMAP) mission had been conducted onboard the ISS since October 2012. The visible light and infrared spectrum imager (VISI) is utilized in the ISS-IMAP mission for nadir-looking observation of the earth’s atmospheric airglow. In this study, we automatically select EPBs according to the criterion for extracting the tilted dark lines from VISI data. Using the selected events, the dependence of the occurrence rate of EPBs is examined. There is no other report of the occurrence rate of EPBs using downward-looking visible airglow data (630 nm). In this result, the occurrence rate is high at all longitudes in the equinoctial seasons. In the solstice seasons, in contrast, the occurrence rate is very small especially in the Pacific and American sectors. This result is basically consistent with previous studies, e.g., those determined by plasma density data on DMSP satellites.During the June solstice in 2013, EPBs were observed in association with geomagnetic storms that occurred due to a southward turning of the IMF Bz. Using these events, we examined the storm-time features of the occurrence of EPBs in the Pacific-American sectors during the June solstice. In these sectors, where the occurrence rate of EPBs is very small during solstice seasons, some EPBs were observed in the peak and recovery phases of the storms. This result shows that the prompt penetration of electric fields causes the development of EPBs, in the data we analyzed, the geomagnetic storms did not inhibit the generation of EPB in the Pacific–American sectors.

Storm‐Enhanced Development of Postsunset Equatorial Plasma Bubbles Around the Meridian 120°E/60°W on 7–8 September 2017

AbstractStorm time development of equatorial plasma bubbles (EPBs) around the meridian 120°E/60°W during early September 2017, when the Bz component of interplanetary magnetic field (IMF) experienced two large southward excursions, producing a strong geomagnetic storm that included two main phase decreases, was investigated. The observations from networks of Global Navigation Satellite Systems total electron content receivers, very high frequency radars, and ionosondes operated around the meridian reveal that in the American and Asian sectors, intense EPB irregularities developed and extended to dip latitudes of ~30°N and 46°N, respectively, following rapid sunset F layer height rises during two episodes of strong southward IMF Bz excursions. The storm‐enhanced EPB irregularities, however, were not observed following the sunset terminator in the Pacific sector, where the sunset rise of F layer was not detected. More interestingly, the EPBs in the Asian sector were observed to drift toward the west, with velocity increasing from ~30 m/s at low latitude to ~95 m/s at middle latitude. The poleward increasing westward drifts drove the formation of west‐titled structure of irregularities. For the EPBs in the American sector, no apparent west‐tilted structure was detected. The results indicate that the prompt penetration undershielding electric fields (PPEF) of eastward polarity resulting from the two IMF Bz southward excursions dominated the generation of postsunset EPBs in the American and Asian sectors, respectively. The westward drifts of PPEF‐induced EPBs in the Asian sector could be attributed dominantly to disturbance westward wind, with a possible contribution to it arising from the PPEF.

Ionospheric response to major storm of 17th March 2015 using multi-instrument data over low latitude station Kolhapur (16.8°N, 74.2°E, 10.6°dip. Lat.)

The optical observations of ionospheric and mesospheric OI 630.0 nm, OI 557.7 nm along with OH emission carried out from low latitude Indian station, Kolhapur (16.8°N, 74.2°E) using CCD based all-sky camera system. The features of night airglow variations observed during the period of a strong geomagnetic storm, which commenced on March 17, 2015, at ∼04:30 UT (10:00 IST (Indian Standard Time = UT + 5.5 h)). Dst of ∼−222 nT was seen in this storm suggest that this is among the strongest. The OI 630.0 nm images on 16, 17 and 18 March show the development of Equatorial Plasma Bubbles (EPBs) and bright intensity regions in OI 630.0 nm emission. Generally, EPBs move from west to east direction but it moved in reverse direction on the strong magnetically disturbed night. The EPBs drift velocity was less by ∼100 m/s than the velocity measured on magnetically quite night 16–17 March 2015. The bright intensity regions are also observed in OI 557.7 nm airglow, but there is no intensity enhancement seen in OH emission. It is also observed that the OI 630.0 nm intensity variation well matches with the GPS VTEC variation for PRN-2. The reversal in EPBs drift velocity and the nightglow intensity variations due to the strong magnetic storm are discussed.