Penetration Of Electric Field Research Articles

Deep Submicron Normally Off AlGaN/GaN MOSFET on Silicon with VTH > 5V and On‐Current > 0.5 A mm−1

A submicron gate normally off AlGaN/GaN high‐electron‐mobility transistor (HEMT) with a high on‐current and high threshold voltage (VTH) is demonstrated. The high‐performance device is realized utilizing a gate recess with a length and depth of 200 and 124 nm, respectively. The recess‐etched region has a roughness of 0.7 nm. Various recess‐etch depths and dielectric annealing conditions are used to tune VTH. The optimized device exhibits an on‐current and VTH of 500 mA mm−1 and 5 V, respectively. The measured breakdown characteristics of the devices and their limitations are investigated using 2D‐technology computer‐aided design (TCAD) device simulation. The penetration of the residual electric field in most of the recess region can be the reason for the premature breakdown of deeply scaled recess‐gate e‐mode HEMTs.

The First Intense Geomagnetic Storm Event Recorded by the China Seismo‐Electromagnetic Satellite

AbstractOn 25 August 2018, the China Seismo‐Electromagnetic Satellite (CSES) encountered the first intense geomagnetic storm event since its launch on 2 February 2018. The main purpose of this work is to check in‐flight performance of the assembled payloads onboard CSES, as well as to investigate the ionosphere perturbations induced by this geomagnetic storm. The study shows that all investigated parameters simultaneously respond to the different phases of the geomagnetic storm, verifying the measuring capabilities of the assembled payloads onboard CSES. Specifically, the magnetic field obtained from the high‐precision magnetometer fits well with that obtained by the Swarm satellite, clearly demonstrating the development phase of the storm; joint analysis using the Langmuir probe, plasma analyzer, electric field detector, and GNSS occultation receiver data demonstrate that this is a positive storm event and that electric field penetration is the possible mechanism for the disturbance in the ionosphere. During this storm event, some significant ELF/VLF waves are also excited and there is enhancement of the energetic electron flux (of energy <1 MeV). These main features are consistent with results from previous works, indicating the excellent performance of the search coil magnetometer and high‐energy particle detector.

Анализ эмиссии электронов с одиночного кремниевого катода в квазивакуумную (воздушную) среду методом атомно-силовой микроскопии

Using atomic force microscopy (AFM), we experimentally examined the features of field-electron emission from a single point-type silicon cathode into a quasi-vacuum (air) medium. In the non-contact AFM operating mode, the current – voltage characteristics (CVCs) of a single cathode with a nanometer radius of curvature of the tip were measured at distances of 10 nm and 20 nm between the cathode tip and the top of the measuring probe. The electric field distribution was simulated both on the surface of the tip of a single cathode and on the surface of the tips of individual cathodes within the array, based on which a theoretical estimate of the field enhancement factor as a function of the cathode-probe distance was made. The field-enhancement factor calculated from the experimental CVCs in the Fowler-Nordheim coordinates is several orders of magnitude higher than its value obtained from theoretical calculations. Such a mismatch between the experimental data and the simulation results indicates the need to take into account additional quantum-size effects, which play an important role in the formation of the field-electron emission current in the nanoscale gap. In particular, deformation of the silicon emitter tip can occur at this scale due to the penetration of a strong electric field into its surface region, which, in turn, causes the distortion of the potential barrier at the interface with the quasi-vacuum medium.

Multi-scale ionosphere responses to the May 2017 magnetic storm over the Asian sector

We investigate multi-scale ionospheric responses to the May 27, 2017, geomagnetic storm over the Asian sector by using multi-instrumental observations, including ground-based global navigation satellite systems (GNSS) network, constellation observing system for meteorology, ionosphere and climate radio occultation, the FengYun-3C (FY-3C) GNSS occultation sounder electron density profiles and in situ plasma density observations provided by both Swarm and defense meteorological satellite program missions. This geomagnetic storm was an intense storm with the minimum symmetric horizontal component reaching − 150 nT and was caused by a coronal mass ejection released on May 23. The main observations are summarized below: (1) two ionospheric positive storm periods were observed. The first one was observed in the noon–afternoon sector during the main phase of the storm on May 28, with nearly 120% TEC enhancement. The second one was of a smaller scale and occurred on the nightside during the recovery phase of the storm on May 29. The first dayside positive storm was initiated by the interplanetary magnetic field (IMF) Bz southward turning and eastward penetration electric field, while the second nightside one was terminated by a later southward turning of the IMF Bz since the Asian sector was on the nightside and the penetration electric field changed westward. (2) A negative storm occurred from 00:00 to 12:00 UT on May 30 over the Asian sector, nearly 2 days after the main phase, which was due to the thermospheric composition change, i.e., decrease in the O/N2 ratio, as shown in the TIMED/GUVI measurements. (3) A band-like TEC enhancement was observed aligning in the northwest–southeast direction and propagated slowly southwestward from 15:00 to 20:00 UT (23:00–04:00 LT, near midnight) on May 28 during the recovery phase of the storm. In situ density observations from the Swarm B and DMSP F15&16 satellites confirmed the density enhancement at 510 km and 850 km, respectively, and revealed that this band-like TEC enhancement structure resembles the so-called plasma blob. The similarities of the observed plasma blob characteristics in terms of spatial structure, propagation trend and temporal evolution with the nighttime traveling ionospheric disturbance (TID) are consistent with the TID-blob theory.

Three Different Episodes of Prompt Equatorial Electric Field Perturbations Under Steady Southward IMF Bz During St. Patrick's Day Storm

AbstractThree different episodes of prompt penetration electric field (PPEF) disturbances are observed during the main phase of the St. Patrick's Day storm on 17 March 2015 under steady southward interplanetary magnetic field (IMF) Bz conditions unlike the conventional PPEF associated with southward or northward turnings of IMF Bz. These PPEF events took place during the period when strong disturbance dynamo fields are prevailing in the background. The first event is triggered by a solar wind dynamic pressure pulse that caused a sharp eastward PPEF and strong enhancement of equatorial electrojet current in Brazilian dayside. The second event caused another short but strong westward PPEF on dayside due to the reversal of IMF By from duskward to dawnward under steady IMF Bz. The third event caused a longer eastward PPEF in association with a solar wind dynamic pressure pulse followed by the onset of a substorm, which has led to strong enhancement of equatorial electrojet, quick rejuvenation and symmetric redistribution of equatorial ionization anomaly in the Brazilian sector. The signatures of the PPEF with opposite polarity and smaller magnitudes are also observed in the Asian sector on the nightside. The possible mechanisms for the observed PPEF events under steady IMF Bz are discussed in terms of changes in the high‐latitude field‐aligned currents and reconfiguration of high‐latitude convection fields using Active Magnetosphere and Planetary Electrodynamics Response Experiment and Super Dual Auroral Radar Network high‐frequency radar observations.

Influence of Geomagnetic Storms on Ionospheric F2-Layer at Low and Mid Latitudes in 300° E Meridian

The behaviour of the ionospheric F2-layer during geomagnetic storm is studied using the critical frequency parameter foF2. The magnetic storms of 2nd October 2013 and 22nd June 2015 are considered. The variations of foF2 at Millstone Hill (42.6° N, 288.5° E), Ramey (18.5° N, 292.9° E), Boa Vista (2.8° N, 299.3° E) and Port Stanley (51.6° S, 302.1° E) along the longitude of about 300° E are analysed. The main observations of this study are (1) strong and short term positive storm effects are observed at the beginning of recovery phase for the moderate storm at all the stations and during the main phase for the intense storm at Millstone Hill and Port Stanley, (2) long term negative phase is observed at the northern hemispheric stations for the intense storm and (3) the southern winter hemisphere station Port Stanley recorded positive storm effects during the main and recovery phases of the intense storm on 22-06-2015. The observed deviations in foF2 are discussed in terms of Prompt Penetration Electric Fields, Disturbance Dynamo Electric Fields, Disturbance neutral winds and Travelling Atmospheric Disturbance.

Features of Storm‐Induced Ionospheric Irregularities From Ground‐Based and Spaceborne GPS Observations During the 2015 St. Patrick's Day Storm

AbstractDuring geomagnetic disturbances, the solo or combined effect of prompt penetration electric fields (PPEFs) and disturbance dynamo electric fields (DDEFs) alters radically regular pattern of equatorial plasma bubbles (EPBs) occurrence. The severe 17–18 March 2015 storm is an excellent event to trace how the storm reshapes EPB generation at all longitudinal sectors. We present results of a comprehensive analysis of storm‐induced EPBs based on combination of in situ and remote sensing techniques including 6,100+ ground‐based GNSS stations, in situ plasma observations onboard Swarm, Communications/Navigation Outage Forecasting System (C/NOFS), and Defense Meteorological Satellite Program (DMSP) satellites, COSMIC Global Positioning System (GPS) scintillations, and up‐looking GPS observations onboard Swarm. During the main storm phase, the PPEF‐induced postsunset EPBs occurred over Australian and Indian sectors. Both events were characterized by plasma bite‐out near the magnetic equator and strong plasma depletions at low/midlatitudes that caused strong GPS signal scintillations in the topside ionosphere. Long‐lasting DDEFs led to suppression of postsunset EPBs and promoted postmidnight/predawn EPBs in African/American/Pacific sectors. The first DDEF‐induced EPBs were registered ~6 hr after storm commencement in the midnight‐dawn sector of the Pacific Ocean during short‐term recovery period (09–12 UT on 17 March). In a succession of postsunset EPB suppression under DDEFs, an exclusive event with a single postsunset EPB development was registered in South America at the recovery phase beginning (23–24 UT) due to an unknown PPEF‐like source. During recovery phase, several large‐scale EPBs with an estimated inter‐bubble distance of ~1,000 km occurred almost simultaneously in the predawn sector in South America and was well sustained till ~07–08 LT in the morning.

Climatology of intermediate descending layers (or 150 km echoes) over the equatorial and low-latitude regions of Brazil during the deep solar minimum of 2009

Abstract. In this work, we have performed a study for the first time on the climatology of the intermediate descending layers (ILs) over Brazilian equatorial and low-latitude regions during the extreme solar minimum period of 2009. The result of this study shows that the occurrence frequency of the ILs is very high, being > 60 % over São Luís (2∘ S, 44∘ W; inclination: −3.8∘) and > 90 % in Cachoeira Paulista (22.42∘ S, 45∘ W; inclination: −33.5∘). In most cases the ILs occur during the day at altitudes varying from 130 to 180 km and they may descend to lower altitudes (∼100 km) in a time interval of a few minutes to hours. The main driving force for the ILs at the low-latitude region, may be considered to be the diurnal tide (24 h) followed in smaller dominance by the semidiurnal (12 h), terdiurnal (8 h) and quarter-diurnal (6 h) components. In the magnetic equatorial sector, similar behavior was seen, with the exception of the semidiurnal tide, which in general does not appear to have influenced the IL's dynamics (except in summer). Additionally, the IL mean descent velocity over São Luís and Cachoeira Paulista shows a day-to-day variability that may be associated with a wave-like perturbation with a periodicity of some days. Some peculiarities in the IL dynamics were noted, such as the presence of the ILs during the night hours. Ascending and descending ILs appeared to have been formed from some connection with the ionospheric F layer. Quite often, these characteristics are observed in the presence of strong signatures of the gravity wave propagation as suggested by the F layer traces in the ionogram. The descending intermediate layer over Brazil appears to have been formed through a process of F1 layer base detachment. An interesting case study showed that an ascending ILs, initially detected at ∼130 km, reached the base of the F2 layer, due probably to the gravity wave propagation and/or the effect of a prompt penetration electric field.

Long‐Lasting Penetration Electric Fields during Geomagnetic Storms: Observations and Mechanisms

AbstractPenetration electric fields originating from the solar wind/magnetosphere, shielding electric fields produced by the Region 2 field‐aligned currents, and disturbance dynamo electric fields produced by disturbance neutral winds are the major disturbance electric fields in the middle‐ and low‐latitude ionosphere during geomagnetic storms. Understanding the interplay of electric fields from different sources is critical for understanding storm time ionospheric electrodynamics. In this study, we analyze ionospheric data measured by five Defense Meteorological Satellite Program satellites in the dusk‐evening sector during a magnetic storm. A peculiar feature in this case is that the vertical ion drift was enhanced in the upward direction by up to 180 m/s, corresponding to the occurrence of an eastward penetration electric field in the equatorial ionosphere, for 14 hr, including 3 hr of the storm main phase and the first 11 hr of the recovery phase. The variations of other ionospheric parameters (the ion density, ion composition, electron temperature, and zonal drift) were well correlated with the variation of the vertical ion drifts, providing further evidence of prolonged penetration electric fields. Observations of short‐lived penetration electric fields are also presented, and the causes for short‐lived penetration electric fields are discussed. Important issues on the shielding effects are addressed. Scenarios for occurrence of long‐lasting penetration electric fields are proposed. Some outstanding problems associated with penetration electric fields are listed for future studies.

Weak Magnetic Storms Can Modulate Ionosphere‐Plasmasphere Interaction Significantly: Mechanisms and Manifestations at Mid‐Latitudes

AbstractA comprehensive study of the response of the ionosphere‐plasmasphere system at mid‐latitudes to weak (Dstmin > −50 nT) magnetic storms is presented. For the first time, it is shown that weak magnetic disturbances can lead to significant modulation of ionosphere‐plasmasphere H+ ion fluxes. It is found that this modulation is caused by the enhancements/reductions of the topside O+ ion density, which is induced by F2‐layer peak height rise and fall during the storms. The F2‐layer motion is caused by thermospheric wind changes and by a penetration electric field. Both drivers are closely related to the changes in the Bz component of interplanetary magnetic field. The most prominent manifestation of the H+ ion flux modulation is strong changes in H+ ion fraction in the topside ionosphere. This study also indicates that the NRLMSISE‐00 model provides the correct relative changes of neutral H density during weak magnetic storms and also that there is a compelling need to include geomagnetic activity indices, in addition to solar activity (F10.7), as input parameters to empirical topside ionosphere models.

Ionospheric Response to Disturbed Winds During the 29 October 2003 Geomagnetic Storm in the Brazilian Sector

AbstractModeling the ionosphere during disturbed periods is one of the most challenging tasks due to the complexity of the phenomena that affect the electric fields and the whole thermosphere environment. It is well known that both, prompt penetration electric fields and large amounts of energy deposited in the polar region during disturbed periods, produce significant disturbances in the global electron density distribution, in particular, in the equatorial ionization anomaly development. Besides, the disturbance dynamo, traveling atmospheric disturbances, and traveling ionospheric disturbances also affect the equatorial ionization anomaly density distribution. In this work we use the Sheffield University Plasmasphere‐Ionosphere Model at Instituto Nacional de Pesquisas Espaciais, to simulate the drastic effects that were observed at the low‐latitude ionosphere in the Brazilian region during a very intense magnetic storm event, the so‐called 2003 Halloween storms. In the absence of measured vertical drift during the storm, a new vertical drift deduced from the interplanetary electric field combined with the time variation of the F region virtual height is used as input. The simulation results showed that, in the case of the disturbed thermospheric wind, the ionospheric observations are better explained when a novel traveling wave‐like disturbance propagating from north to south, at a velocity equal to 300 m/s, is considered.

Characteristics of prosthetic vision in rats with subretinal flat and pillar electrode arrays

Objective. Retinal prostheses aim to restore sight by electrically stimulating the surviving retinal neurons. In clinical trials of the current retinal implants, prosthetic visual acuity does not exceed 20/550. However, to provide meaningful restoration of central vision in patients blinded by age-related macular degeneration (AMD), prosthetic acuity should be at least 20/200, necessitating a pixel pitch of about 50 µm or lower. With such small pixels, stimulation thresholds are high due to limited penetration of electric field into tissue. Here, we address this challenge with our latest photovoltaic arrays and evaluate their performance in vivo. Approach. We fabricated photovoltaic arrays with 55 and 40 µm pixels (a) in flat geometry, and (b) with active electrodes on 10 µm tall pillars. The arrays were implanted subretinally into rats with degenerate retina. Stimulation thresholds and grating acuity were evaluated using measurements of the visually evoked potentials (VEP). Main results. With 55 µm pixels, we measured grating acuity of 48 ± 11 µm, which matches the linear pixel pitch of the hexagonal array. This geometrically corresponds to a visual acuity of 20/192 in a human eye, matching the threshold of legal blindness in the US (20/200). With pillar electrodes, the irradiance threshold was nearly halved, and duration threshold reduced by more than three-fold, compared to flat pixels. With 40 µm pixels, VEP was too low for reliable measurements of the grating acuity, even with pillar electrodes. Significance. While being helpful for treating a complete loss of sight, current prosthetic technologies are insufficient for addressing the leading cause of untreatable visual impairment—AMD. Subretinal photovoltaic arrays may provide sufficient visual acuity for restoration of central vision in patients blinded by AMD.

Conjugate hemispheric response of earth’s ionosphere due to geomagnetic storms occurred during two equinox periods

The ionospheric response of two geomagnetic storms of 2016 occurred during spring equinox (5-8 March, 2016) and autumn equinox (12-15 October, 2016) is investigated using the total electron content (TEC) data derived from Global Positioning System (GPS) receivers located in the equatorial ionization anomaly (EIA) crest regions at northern hemisphere (Tripura University, Agartala, India) and southern hemisphere (Karratha, Australia). While in southern EIA station ionospheric responses for the two storms are found to be symmetric but in northern EIA station the responses are completely asymmetric. The observations are explained by the contribution of storm-time prompt penetration electric fields (PPEFs), disturb dynamo electric fields (DDEFs), disturbed meridional (equatorward) winds as well as the neutral compositional changes over low latitudes.

Response of the Earth’s equatorial ionosphere during the severe G4-class geomagnetic storm of 8th September 2017

Equatorial and low latitude ionospheric response of the 8th September 2017 severe G4-class geomagnetic storm is investigated using the total electron content (TEC) data from a longitudinal chain of global positioning system (GPS) receivers over Asian, African and American sectors. During the main phase, a positive storm effect is observed over Asian sector, a complete negative storm effect over African sector and both are observed over American sector. A sharp increase in peak TEC is observed over the complete longitudinal chain during the recovery phase. The results show the decisive contribution of prompt penetration electric fields (PPEFs) and disturbance dynamo electric fields (DDEFs), storm time disturbed meridional (equatorward) wind as well as the neutral compositional changes over equatorial and low latitudes in the observed ionospheric storm effects.

Electric‐field penetration depth and dielectric spectroscopy observations of human skin

BackgroundThe dynamic behavior of water molecules remains an important subject for understanding human skin. The change in the dynamics of water molecules from those in bulk water can be effectively observed by dielectric spectroscopy. To study water in the human skin in vivo, non‐invasive and non‐destructive measurements are essential. Since many unknowns remain from previous research, in this report we employ a two‐layer dielectric model to evaluate the penetration depth of the electric field and use the results in measurements on human skin.Materials and MethodsWe used open‐ended coaxial probes with different diameters to perform time‐domain reflectometry (TDR) measurements for an acetone‐Teflon double‐layer model and for human skin from various parts of the body.ResultsThe electric‐field penetration depth obtained from model measurements increases with the increasing outer diameter of open‐ended coaxial electrodes. For skin measurements, the relaxation strength corresponding to the water content shows a clear dependence on the epidermal thickness of the measured body parts.ConclusionWe determined the depth distribution of the water content of skin from results of dielectric measurements obtained using electrodes with various electric‐field penetration depths. We found exponential decays with the thickness of the epidermis of each body part for several examinees. This study suggests an effective method for detailed evaluations of human skin.

Effects on the equatorial and low latitude thermosphere and ionosphere during the 19–22 December 2015 geomagnetic storm period

In this paper, the critical frequency of F2 layer (foF2) and their peak height (hmF2) recorded at equatorial and low latitude stations of the South American sector were analyzed during the 19–22 December 2015 geomagnetic storm period. We have observed initially decreases of foF2 at equatorial and low latitudes over the North crest of the equatorial ionization anomaly (EIA) and almost no effect over the South crest. These initial negative storm effects can be fully explained by the penetration of a westward electric field to low and equatorial latitudes. The positive storm effects that replaced the negative storm effects during the development of the main phase may have occurred possibly because of an intensification of the “fountain effect” due to an increase of the upward vertical drift at equatorial latitudes. Changes in the O/N2 ratio (increases or decreases) may be the most probable cause for the positive and negative storm effects observed during the recovery.

Low Latitude Ionospheric TEC Oscillations Associated With Periodic Changes in IMF Bz Polarity

AbstractAn Ionospheric Observation Network for Irregularity and Scintillation in East/Southeast Asia was recently deployed. Using ionospheric total electron content (TEC) from the two crossed Beidou geostationary satellite receiver chains of the network along 110°E and 23°N and Doppler velocity measurements from the Sanya (18.3°N, 109.6°E) portable digital ionosonde, we report first observations of low latitude TEC oscillations synchronized over a wide longitude range in East/Southeast Asia, which occur at nighttime, after the main phase of the geomagnetic storm on 20 April 2018. A comparison among TEC and Doppler velocity and interplanetary magnetic field (IMF) Bz component shows that the periodic TEC enhancements correlate with F region downward plasma drifts and IMF Bz southward turnings. The results suggest that the quasiperiodic southward turnings of IMF Bz could produce multiple short‐lived westward prompt penetration electric fields, which contribute to driving the nighttime low latitude TEC oscillations simultaneously over the wide longitude range.

A parallel electric field penetration technique for identifying hazardous water sources beneath coal seam mining face floors

Abstract In this work, a highly applicable multi-channel parallel electric field penetration technique is proposed to facilitate identification of hazardous water sources beneath coal seam mining face floors. The effects of various parameters on the characteristics of the apparent resistivity curves of a spherical model were analyzed through numerical simulation. It was determined that the proposed technique was sensitive to several model parameters (resistivity, size and spatial location). In addition, physical model experiments were also performed. Resistivity was determined from a three-dimensional inversion of the measurements. The results showed that the proposed technique was highly effective in determining the electrical characteristics and spatial distribution range of the anomalous bodies in the physical model. An engineering application of the proposed technique further demonstrated its effectiveness and reliability. The proposed technique can provide a basis for formulating water disaster prevention and control measures for mining faces.

Dynamic Response of Ionospheric Plasma Density to the Geomagnetic Storm of 22‐23 June 2015

AbstractOn 21–22 June 2015, three consecutive interplanetary shocks slammed into the Earth's magnetosphere. Immediately after the third shock at 18:36 UT on 22 June, marked by an exceptional sudden storm commencement with an amplitude of ΔSYM‐H = ∼106 nT, a major geomagnetic storm commenced. In the present study, a multi‐instrument approach comprising observations, data analysis, and modeling is used to examine the global ionospheric response. Results show that enhanced storm time processes produced major total electron content (TEC) variations at different latitudes, longitudes, and phases of the storm. A closer inspection of the TEC observations reveals strong longitudinal and hemispherical asymmetry. In addition, multiple equatorward and poleward propagating traveling ionospheric disturbances (TIDs) were detected in the TEC data. Equatorward propagating TIDs are consistent with vertical neutral winds simulated from Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model; however, poleward TIDs were not reproduced in the model. We find that a combination of driving processes including enhanced high‐latitude injection, prompt penetration electric fields, disturbance dynamo effect, neutral winds, and composition changes were acting at different stages of the storm.

Interhemispheric Asymmetry in Response of Low‐Latitude Ionosphere to Perturbation Electric Fields in the Main Phase of Geomagnetic Storms

AbstractStructures of sudden enhancements/depressions and associated interhemispheric asymmetry in low‐latitude total electron content (TEC) during the main phase (MP) of geomagnetic storms have remained unpredictable majorly due to oscillating equatorial vertical E×B drifts and resultant redistribution of plasma in low latitudes in a given seasonal background. Robust analysis of 7 major and 30 moderate ionospheric storms during the years 2000–2018 is performed with comprehensive literature review encompassing various sources of asymmetry in magnetosphere‐ionosphere coupling. Taking advantage of simultaneous long‐term observations of E×B drift from Jicamarca, H component from magnetometers, and global ionospheric map vertical TEC (VTEC) and TEC observations across the dip equator from the South American sector, simultaneous formation of peaks and valleys in VTEC and associated asymmetry are studied. Additionally, a three‐layer neural network‐based E×B drift model is developed using delta‐H observations that provide drift estimates in the absence of Jicamarca drifts. The main results establish simultaneous high‐magnitude short‐lived (1–2 hr) enhancements and depression in VTEC during the MP in daytime in both hemispheres with varying differences of −30 to 100 TECU with respect to quiet time mean and along with prominent existence of interhemispheric asymmetry in TEC during the MP regardless of seasons. Maximum VTEC in the northern and southern low latitudes is found to occur at different times during storms. Large difference of VTEC is found ranging between 10 and 30 TECU between the near conjugate locations of the hemispheres. Coincident global episodic peaks marked by steep VTEC falls show dominance of episodic eastward and westward penetration electric fields in the low‐latitude daytime ionosphere.