Electron Density Depletion Research Articles

Interpretation of Cluster WBD frequency conversion mode data

Abstract. The Cluster wide-band data (WBD) plasma wave receiver mounted on each of the four Cluster spacecraft obtains high time resolution waveform data in the frequency range of ~70 Hz to 577 kHz. In order to make measurements above 77 kHz, it uses frequency conversion to sample the higher frequency waves at one of three different conversion frequencies (~125, 250 and 500 kHz, these frequencies being the possible options for setting the base frequency of the frequency range being sampled) in one of three different filter bandwidths (9.5, 19 and 77 kHz). Within the WBD instrument, a down-conversion technique, built around quadrature mixing, is used to convert these data to baseband (0 kHz) in order to reduce the sample rate for telemetry to the ground. We describe this down-conversion technique and illustrate it through data obtained in space. Because these down-converted data sometimes contain pulses, which can be indicative of nonlinear physical structures (e.g., electron phase-space holes and electron density enhancements and depletions), it is necessary to understand what effects mixing and down conversion have on them. We present simulations using constructed signals containing pulses, nonlinear wave packets, sinusoids and noise. We show that the pulses and impulsive wave packets, if of sufficient amplitude and of appropriate width, survive the down-conversion process, sometimes with the same pulse shape but usually with reduced amplitude, and have timescales consistent with the filter bandwidth at the base frequency. Although we cannot infer the actual timescale of the pulses and impulsive wave packets as originally recorded by the WBD instrument before mixing and down conversion, their presence indicates nonlinear processes occurring at or somewhat near the location of the measurement. Sinusoidal waves are represented in the down-conversion timescale as sinusoids of nearly the same amplitude and at frequencies adjusted down by the conversion frequency. The original input waveforms, regardless of their shape, whether pulses or sinusoids, can never be recovered from the down-converted waveforms.

Ionospheric precursors to large earthquakes: A case study of the 2011 Japanese Tohoku Earthquake

Researchers have reported ionospheric electron distribution abnormalities, such as electron density enhancements and/or depletions, that they claimed were related to forthcoming earthquakes. In this study, the Tohoku earthquake is examined using ionosonde data to establish whether any otherwise unexplained ionospheric anomalies were detected in the days and hours prior to the event. As the choices for the ionospheric baseline are generally different between previous works, three separate baselines for the peak plasma frequency of the F2 layer, foF2, are employed here; the running 30-day median (commonly used in other works), the International Reference Ionosphere (IRI) model and the Thermosphere Ionosphere Electrodynamic General Circulation Model (TIE-GCM). It is demonstrated that the classification of an ionospheric perturbation is heavily reliant on the baseline used, with the 30-day median, the IRI and the TIE-GCM generally underestimating, approximately describing and overestimating the measured foF2, respectively, in the 1-month period leading up to the earthquake. A detailed analysis of the ionospheric variability in the 3days before the earthquake is then undertaken, where a simultaneous increase in foF2 and the Es layer peak plasma frequency, foEs, relative to the 30-day median was observed within 1h before the earthquake. A statistical search for similar simultaneous foF2 and foEs increases in 6years of data revealed that this feature has been observed on many other occasions without related seismic activity. Therefore, it is concluded that one cannot confidently use this type of ionospheric perturbation to predict an impending earthquake. It is suggested that in order to achieve significant progress in our understanding of seismo-ionospheric coupling, better account must be taken of other known sources of ionospheric variability in addition to solar and geomagnetic activity, such as the thermospheric coupling.

High-latitude ionospheric response to the solar eclipse of 1 August 2008: EISCAT observations and TRANSCAR simulation

On 1 August 2008, a total solar eclipse occurred and the footprint of its umbra passed North–East of the Svalbard archipelago, where the EISCAT Svalbard Radars (ESR) are installed. This eclipse is peculiar because it was seen at high-latitude, thus with large solar zenith angles and it was the first time that incoherent scatter radars were able to sound the ionospheric response at such a high latitude. The ESR data, after correction, show the typical signatures of a solar eclipse: depletion in electron density in the E- and F-regions, a decrease in electron temperature in the F-region, and a downflow of ions. Also, the UHF dish of the mainland EISCAT system measured a decrease in the same parameters, showing the influence of the penumbra (partial eclipse) on the ionosphere. We have modelled the response of the high-latitude ionosphere to this solar eclipse with the TRANSCAR code. Our simulations reproduce rather well the observations above the UHF radar in Tromsø and the ESR on Svalbard. In particular, we show the influence of particle precipitation above the ESR and we suspect an effect above Tromsø of the photoelectrons coming from the opposite hemisphere. Another interesting and quite unusual feature is the observation of the summer ionospheric cusp undergoing the eclipse. Radar data show structures of enhanced electron temperature, due to soft particle precipitation through the polar cusp.

Could an anisotropic molecular mechanics/dynamics potential account for sigma hole effects in the complexes of halogenated compounds?

Halogenated compounds are gaining an increasing importance in medicinal chemistry and materials science. Ab initio quantum chemistry (QC) has unraveled the existence of a "sigma hole" along the C-X (X = F, Cl, Br, I) bond, namely, a depletion of electronic density prolonging the bond, concomitant with a build-up on its sides, both of which are enhanced along the F < Cl < Br < I series. We have evaluated whether these features were intrinsically built-in in an anisotropic, polarizable molecular mechanics (APMM) procedure such as SIBFA (sum of interactions between fragments ab initio computed). For that purpose, we have computed the interaction energies of fluoro-, chloro-, and bromobenzene with two probes: a divalent cation, Mg(II), and water approaching X through either one H or its O atom. This was done by parallel QC energy-decomposition analyses (EDA) and SIBFA computations. With both probes, the leading QC contribution responsible for the existence of the sigma hole is the Coulomb contribution E(c). For all three halogenated compounds, and with both probes, the in- and out-of-plane angular features of E(c) were closely mirrored by the SIBFA electrostatic multipolar contribution (E(MTP)). Resorting to such a contribution thus dispenses with empirically-fitted "extra", off-centered partial atomic charges as in classical molecular mechanics/dynamics.

Investigation of Mars' ionospheric response to solar energetic particle events

We investigate the effects of solar energetic particle (SEP) events on the Martian ionosphere using observations from the Mars Global Surveyor (MGS) Electron Reflectometer (ER) and Radio Science (RS) experiments. Although MGS/ER is not designed to measure solar storm particles, it detects SEPs as increased instrument background. Using this proxy for SEP fluxes near Mars, we compare electron density profiles obtained from the RS experiment during periods of high and low SEP activity. Six case studies show no clear evidence for an increase in the ionospheric electron density between 200 and 100 km altitudes. However, 4 of the 6 events show a small increase in electron density below 100 km altitude during SEP events, suggesting that high‐energy (10–20 keV) electrons may cause ionization in the lower ionosphere. We also observe an ∼25% decrease in the ionospheric electron density between ∼100 and ∼120 km altitude for the two strongest events, suggesting that SEPs trigger a process that increases electron loss in this altitude range of the lower ionosphere. However, we cannot be confident from only two events that this effect is caused directly or indirectly by increased SEP fluxes. A statistical study confirms the case study results, but not over all solar zenith angles. Additionally, we observe depletions in the topside ionospheric electron density at some solar zenith angles, which can be explained by compression of the ionosphere by the passing CME.

Photoswitching of Azobenzene-Functionalized Molecular Platforms on Au Surfaces

The photo- and thermally induced switching of well-ordered molecular arrays of free-standing functional groups, formed by self-assembly on Au surfaces, were studied by photoelectrochemical methods (cyclic voltammetry, chronoamperometry) and surface plasmon resonance spectroscopy. These molecular adlayers exhibit azobenzene functions mounted vertically on the surface via molecular platforms on the basis of triazatriangulenium. Detailed quantitative studies of the switching kinetics revealed that the photoinduced trans–cis isomerization of the azobenzene groups in these adlayers proceeds very fast and highly reversible. Cis–trans backisomerization by thermal relaxation occurs surprisingly 4–5 orders of magnitude faster than in solution. A rapid thermal cis–trans relaxation that dominates over the photoinduced processes is also supported by the pronounced increase of the cis fraction in the adlayers with irradiation intensity and the weak dependence of the isomerization time constants on the intensity. In complementary density functional theory calculations of the cis isomer on a Au cluster, no significant electron density depletion of the azo moiety, but strong electronic coupling of the switchable group with the Au substrate, were found. We propose that the latter leads to a spin exchange between conduction electrons in the metal and the azo moiety, enabling a relaxation mechanism that is forbidden for the free molecule.

PMSE observations with the EISCAT VHF- and UHF-radars: Ice particles and their effect on ambient electron densities

It is now well understood that the occurrence of PMSE is closely connected to the presence of ice particles. These ice particles modify the ambient electron density by electron attachment which occasionally leads to large electron density depletions which have also been called ‘biteouts’. There has been some debate in the literature regarding the relative depth of such depletions which is usually expressed by the parameter Λ=|ZA|NA/ne. Here, |ZA|NA is the charge number density of ice particles and ne is the electron density. In this paper, we present, for the first time, the statistical distribution of Λ using measurements with the EISCAT VHF- and UHF-radars. Based on 25h of simultaneous observations, we derived a total of 757Λ values based on 15min of data each. In each of these cases, PMSE were observed with the EISCAT VHF-radar but not with the UHF-radar and the UHF-measurement were hence used to determine the electron density profile. From these 757 cases, there are 699 cases with Λ⪡1, and only 33 cases with Λ>0.5 (21 cases with Λ>1). A correlation analysis of Λ versus PMSE volume reflectivities further reveals that there is no strong dependence between the two parameters. This is in accordance with current PMSE-theory based on turbulence in combination with a large Schmidt-number. The maxima of Λ from each profile show a negative relationship with the undisturbed electron densities deduced at the same altitudes. This reveals that the variability of Λ mainly depends on the variability of the electron densities. In addition, variations of aerosol number densities may also play a role. Although part of the observations were conducted during the HF heating experiments, the so-called overshoot effects did not significantly bias our statistical results. In order to avoid missing biteouts because of a superposition of coherent and incoherent scatter in the UHF-data, we finally calculated spectral parameters n by applying a simple fit to auto-correlation functions as introduced by Strelnikova and Rapp (2010). Corresponding statistical results of the parameter n indicate that charged ice particles do exist in the vicinity of PMSE (i.e., n<1) but they did not efficiently modify ambient electron densities so that clear ‘biteouts’ are observed.

The analysis of polarization effects on the interelectronic separations in the atoms and molecules of the G1 test set

AbstractWe have calculated the differences between kinetic, nuclear attraction, and two‐electron energies as well as the position intracules, P(u), for 56 molecules and 17 atoms contained in the G1 test set, using basis sets differing in their degree of polarization. The calculations were performed using an unrestricted Hartree–Fock wave function that was expanded using the 6‐31G, 6‐31G(d,p), 6‐31G(3d,3p), 6‐31G(3df,3pd), 6‐311G, 6‐311G(d,p), 6‐311G(3d,3p), 6‐311G(3df,3pd), 6‐311++G, 6‐311++G(d,p), 6‐311++G(3d,3p), and 6‐311++G(3df,3pd) basis sets. In addition to these Pople basis sets, the Dunning's cc‐pVDZ and cc‐pVTZ bases as well as the non‐polarized portions of these basis sets were used. We observe a consistent contraction of electron pairs (as measured by the distribution in P(u)) as the number of polarization functions is increased beyond zero in molecular systems. This contraction is related to an increase in the electron density in the internuclear bonding regions of molecules as the wave functions are polarized. We note that the increased flexibility in the polarized basis set allows for a more accurate description of the chemical bond and is energetically favorable due to an increased level of attraction to the nuclei (despite the obvious increase in electronic repulsion energy). Because of the increase in electron density in internal regions of the molecule (at the expense of electron density depletions in outer regions), the probability of observing electrons closer together (i.e., at smaller u) increases due to the relationship between P(u) and its Coulomb component, J(u). © 2012 Wiley Periodicals, Inc.

Thermosphere and ionosphere response to subauroral polarization streams (SAPS): Model simulations

An empirical model of subauroral polarization streams (SAPS) has been incorporated into the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM). This SAPS driven TIEGCM is used to simulate the effect of SAPS on the global thermosphere and ionosphere during a moderate geomagnetically active period between day of year (DOY) 329 and 333 in 2008. Model results show: (1) SAPS caused an increase in global thermospheric temperature which became stronger with time. This neutral temperature increase was more significant in subauroral and auroral regions. Joule heating by the SAPS and the redistribution of this heat by dynamic processes were the primary mechanisms for the simulated global neutral temperature changes. (2) In the SAPS driven TIEGCM, the strong ion drag effect in the subauroral SAPS channel drove large changes in thermospheric winds. Zonal neutral winds had either an extra, separate channel of westward flow in the subauroral region in the afternoon‐midnight sector or a broad westward wind jet that merged with the regular duskside auroral westward zonal neutral wind driven by the high latitude convection pattern. The exact latitudinal profile of the zonal winds depended on local time. (3) The response of neutral temperature and wind to SAPS was more significant at higher altitudes and exhibited seasonal/hemispheric asymmetry. (4) The heating to the thermosphere by SAPS also resulted in changes in thermospheric composition with upwelling of molecular rich air in subauroral and auroral regions and downwelling of atomic oxygen rich air at other latitudes. These changes in thermospheric composition contributed to the deeper and more extended ionospheric electron density depletions in subauroral middle latitude regions, as well as electron density increases along the equatorward edge of the SAPS channel in the afternoon sector.

Modeling measurements of ionospheric density structures using the polarization of high‐frequency waves detected by the Radio Receiver Instrument on the enhanced Polar Outflow Probe

The Cascade SmallSat and Ionospheric Polar Explorer (CASSIOPE) satellite is to be launched in late 2012. On board this satellite will be a suite of eight scientific instruments composing the enhanced Polar Outflow Probe (ePOP). The Radio Receiver Instrument (RRI) on ePOP will be used to receive high‐frequency (HF) (10–18 MHz) transmissions from ground transmitters such as the Super Dual Auroral Radar Network (SuperDARN) array. Modeling of the characteristics of the HF signal received at ePOP for various ionospheric conditions has been undertaken in preparation for this RRI‐SuperDARN experiment. The effect of ionospheric electron density enhancements and depletions on signal parameters such as polarization and mode delay difference has been modeled. It has been found that at HF the polarization state of the received signal is highly sensitive to regions of locally enhanced or depleted electron density in the ionosphere. In particular, analysis of the orientation angle of the received signal, which changes because of Faraday rotation as the spacecraft passes over a ground transmitter, will allow detection of small‐scale electron density structures (on the order of tens of kilometers) with electron densities as little as 10% different from background values. Because of the sensitivity of the polarization of HF transionospheric waves, the signatures of these small‐scale and relatively weak ionospheric density structures will be apparent. Larger and denser structures will also be detectable from both the polarization state and other signal parameters, such as signal delay. The modeling demonstrates that detailed analysis of the signal parameters received at the ePOP satellite will allow determination of the location, size, and density of structures in the ionosphere.

Simultaneous observations at Darwin of equatorial bubbles by ionosonde-based range/time displays and airglow imaging

[1] Airglow observations of ionospheric electron density depletions made at Darwin, Australia have demonstrated that the tree-like structure of bubbles developed at the magnetic equator are mapped along magnetic field lines with considerable accuracy to the base of the ionosphere at higher latitudes. Ionosonde range-time displays made at Darwin and other equatorial sites in the Australian region show characteristic approaching and receding echoes which converge on a typical spread-F event. These off-angle echoes have often been referred to in the literature as satellite traces and associated with spread F with little recognition of their true significance. All four optical depletions previously reported in the literature as being seen at Darwin are found in this paper to be accompanied by such typical off-angle/spread F events. The zonal drift velocity of the moving reflectors can be measured from the speed at which such echoes approach and recede. Since digital ionosondes in equatorial sites have existed for many years, existing ionogram data, when suitably processed into range-time displays, may allow the occurrence of such events over several sunspot cycles to be found. A question remains as to whether all or only some of such equatorial range-time events correspond to electron density depletions.

The stability of the mesospheric plasma layer

The presence of micron and sub-micron size dust in the Earth’s summer mesopause are a possible cause of electron density depletion. Whereas electrons in this weakly ionized and weakly magnetized layer are frozen in the magnetic field, the ions and dust are highly diffusive. This relative drift between the plasma particles will cause a current in the medium. The presence of such a current can destabilize the plasma layer with a growth rate of the order of Alfvén frequency. Since required current density for the onset of this instability is on the order of J≳0.03A/m2, it is quite unlikely that such a strong current is present in the mesosphere. However, owing to the prevailing ambiguity of measurements, the existence of such a current is not completely ruled out.

Cluster observations of whistler mode ducts and banded chorus

[1] Bell et al. (2009) proposed that the source region for banded chorus consists of whistler mode ducts of depleted electron density (Ne) for upper band (UB) chorus for all wave normal angles (θ) and ducts of either enhanced or depleted Ne for lower band (LB) chorus for small θ and θ near or greater than the Gendrin angle, respectively. This paper provides support for this model using new high resolution (17 km) Ne observations from the Cluster WHISPER and EFW instruments. Data is examined from January 20, 2004, when strong banded chorus was observed over 3000 km of the Cluster 2 orbit, ending at the magnetic equator. Previous analysis of LB chorus on this day indicated the wave normal angles were larger than the Gendrin angle. Using the Ne data, we show that the LB chorus is generated within depletion ducts in the source region and that the half-width of these ducts (∼70 km) is comparable to the transverse scale (∼100 km) of the chorus source region. Making use of the fact that the group velocity of the LB chorus waves has a significant cross-L component, we show that the source region extends over 500 km near the magnetic equator.

Valence Shell Charge Concentration (VSCC) Evolution: A Tool to Investigate the Transformations within a VSCC Throughout a Chemical Reaction

Theoretical studies about reaction mechanisms are usually limited to the determination of the energetic paths that connect reactants, transition states, and products. Recently, our group proposed the structural evolution, which has provided insights about the molecular structure changes occurring along a reaction path. Structural evolution may be defined as the development of a chemical reaction system across the partitioning of the nuclear configuration space into a finite number of structural regions defined on account of the topology of a scalar field, e.g., the electron density. In this paper, we present a tool to investigate within the framework of the Quantum Theory of Atoms in Molecules the evolvement of the Valence Shell Charge Concentration, the VSCC evolution, which is the description of the changes of electron density concentrations and depletions around the bonding area of an atom. The VSCC evolution provides supplementary information to the structural evolution because it allows the analysis of valence shells within a structural region, i.e., a subset of R(Q) with the same connectivity among the atoms forming a molecule. This new approach constitutes also a complement to the Valence-Shell Electron Pair Repulsion (VSEPR) model because it gives an account of the adjustments of electron pairs in the valence shell of an atom across a chemical reaction. The insertion reaction in the hydroformylation reaction of ethylene, the reduction of cyclohexanone with lithium aluminum hydride, the oxidation of methanol with chlorochromate, and the bimolecular nucleophilic substitution of CH(3)F with F(-) are used as representatives examples of the application of the VSCC evolution. Overall, this paper shows how the VSCC evolution through an analysis of the modifications of local charge concentrations and depletions in individual steps of a chemical reaction gives new insights about these processes.

Relationship between strong range spreadFand ionospheric scintillations observed in Hainan from 2003 to 2007

Data from a DPS-4 Digisonde and an ionospheric scintillation monitor, both located at the low-latitude station Hainan (109.1 degrees E, 19.5 degrees N; dip latitude 9 degrees N), were analyzed to study the strong range spread F (SSF) and its correlation with ionospheric scintillations observed in the period of declining solar cycle 23 from 2003 to 2007. The results show that the maximum and minimum of the occurrence of SSF appeared in nearly the same months as those of the GPS L band scintillations. The variations in SSF occurrence were also similar to those of the scintillations. From 2003 to 2007, both the SSF and the scintillation occurrences decreased from the high solar activity year to the low solar activity year. The correlation coefficient between the occurrences of the SSF and the GPS L band scintillation was as high as 0.93, suggesting associated mechanisms producing SSF and scintillations. Electron density depletions extending from the bottomside to the topside ionosphere are the likely cause explaining the high correlation.

The effects of the guide field on the structures of electron density depletions in collisionless magnetic reconnection

Two-dimensional particle-in-cell simulations are performed to investigate the formation of electron density depletions in collisionless magnetic reconnection. In anti-parallel reconnection, the quadrupole structures of the out-of-plane magnetic field are formed, and four symmetric electron density depletion layers can be found along the separatrices due to the effects of magetic mirror. With the increase of the initial guide field, the symmetry of both the out-of-plane magnetic field and electron density depletion layers is distorted. When the initial guide field is sufficiently large, the electron density depletion layers along the lower left and upper right separatrices disappear. The parallel electric field in guide field reconnection is found to play an important role in forming such structures of the electron density depletion layers. The structures of the out-of-plane magnetic field By and electron depletion layers in anti-parallel and guide field reconnection are found to be related to electron flow or in-plane currents in the separatrix regions. In anti-parallel reconnection, electrons flow towards the X line along the separatrices, and are directed away from the X line along the magnetic field lines just inside the separatrices. In guide field reconnection, electrons can only flow towards the X line along the upper left and lower right separatrices due to the existence of the parallel electric field in these regions. magnetic reconnection, electron density depletion, quadrupole structures, separatrices, in-plane current

Role of defects in propene adsorption and reaction on a partially O-covered Au(111) surface

We investigate the role of defects—adatoms, vacancies, and steps—in the bonding and reaction of propene on Au(111) containing atomic oxygen, using density functional theory (DFT) calculations. The adsorption of propene is stronger on a surface containing defects compared to the flat, bulk-terminated surface, with the largest gain in binding (∼0.7 eV) on a surface with a 1/9 monolayer (ML) of Au adatoms. Charge-density difference plots reveal that the difference between defective surfaces and the bulk-terminated surface is a more pronounced depletion of electron density from the carbon–carbon π bond and a charge accumulation between the double bond and the gold atom to which the propene is bound. We calculate the energy barriers for two competing reactions that are important in determining the selectivity for propene oxidation. Allylic H abstraction by adsorbed O leads to combustion, whereas O addition to form an oxymetallacycle is the first step in propene epoxide formation. A comparison of the energetics of these two pathways on flat and defect-containing Au surfaces indicates that the reactivity depends on the nature and prevalence of surface defects. Both electronic and geometric factors, such as the path and the distance the oxygen must travel to meet the allylic hydrogen for abstraction, are important in explaining the reaction trends.

The Relationship Between Nanobubbles and the Hydrophobic Force

The formation of nanobubbles on hydrophobic self-assembled monolayers has been examined in a binary ethanol/water titration using small angle X-ray scattering (SAXS) and atomic force microscopy (AFM). The AFM data demonstrates a localized force effect attributed to nanobubbles on an immersed hydrophobic surface. This evidence is arguably compromised by the possibility that the AFM tip actually nucleates nanobubbles. As a complementary noninvasive technique, SAXS has been used to investigate the interfacial region of the immersed hydrophobic surface. SAXS measurements reveal an electron density depletion layer at the hydrophobic interface, with changing air solubility in the immersing liquid, due to the formation of nanobubbles.

Features of separatrix regions in magnetic reconnection: Comparison of 2‐D particle‐in‐cell simulations and Cluster observations

In collisionless magnetic reconnection, the in‐plane Hall currents are carried mainly by the magnetized electrons. The in‐plane Hall currents are directed toward the X line along the magnetic field lines just inside the separatrices and away from the X line along the separatrices. Such a current system leads to the quadrupole out‐of‐plane magnetic field with the peaks between the regions carrying the in‐plane currents. Simultaneously, the electron flow toward the X line along the separatrices causes electron density depletions along the separatrices. In this paper, the features of separatrix regions in magnetic reconnection and the relations between the electron density depletions and the out‐of‐plane magnetic field are investigated with both two‐dimensional particle‐in‐cell simulations and Cluster observations. We conclude that the electron density depletions are formed because of the magnetic mirror, and they are outside the peaks of the out‐of‐plane magnetic field. Such a theoretical prediction is confirmed by both simulations and observations.

COSMIC/FORMOSAT‐3 observations of equatorial F region irregularities in the SAA longitude sector

The data of ionospheric electron density and S4 scintillation index from COSMIC/FORMOSAT‐3 GPS occultation soundings are used to study the morphology of the equatorial F region irregularity (EFI) in the longitude sector (0°∼75°W) where the South Atlantic magnetic anomaly (SAA) is located. Launched into low Earth orbits on 14 April 2006, the six‐satellite constellation COSMIC provides daily global measurements of the ionosphere at altitudes below ∼800 km. The large data set enables us a statistically correlative study of electron density depletion and GPS L1 scintillation, which allows the global and altitudinal distributions of irregularity to be examined. Under solar minimum and geomagnetic quiet conditions, postsunset EFI/scintillation events are found to concentrate in the SAA longitude sector during northern winter months (D months), but nearly disappeared in the same longitude sector during the opposite season (J months). The D months’ average pattern of EFI reveals that most of irregularities occurred in the bottom side F region (200–350 km altitude) and at low magnetic latitudes adjacent to the locations of the maximum postsunset Nmax and Hmax. This is consistent with the early postsunset irregularities that are generated in the region where plasma upward drifts have been greatly enhanced. By examining the UT snapshots of the global distributions of electron density and scintillation events, strong spatial correlations are found to exist among the sunset terminator, the westward declined geo‐magnetic field, strong scintillation event, and density depletion in the SAA longitude sector only during D months, where the E region sunset terminator is most possibly parallel to the magnetic field direction at equatorial latitudes. Density depletion structures within this longitude sector were bounded by two postsunset ionization enhancement bands (EIA) that extended over central SAA and its conjugated region, respectively. Such features provide further evidence that the enhanced conductivity gradients due to particle induced ionization in the vicinity of SAA contribute additional electric field enhancements in the equatorial F region. At the walls of the depletion region near the sunset lines, prominent horizontal density gradients exist in both westward and southward directions due to sun setting and SAA ionization enhancement, respectively. The combination of horizontal density gradients with antiparallel (north‐eastward) neutral winds during D months can contribute additional growth rate to the EFI generation. These COSMIC observations not only confirm that the sunset equatorial electrodynamics plays a key role in controlling the seasonal and longitudinal occurrences of the quiet time EFI, but also reveal that seasonally dependent ionospheric responses to the energetic particle precipitation in SAA can affect considerably the morphology of EFI in the SAA longitude sector.