# EPP

ISSN  2096-3955

CN  10-1502/P

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SPACE PHYSICS: MAGNETOSPHERIC PHYSICS
Recently Published, doi: 10.26464/epp2021051
[Abstract](185) [FullText HTML](35) [PDF 2193KB](12)
Abstract:
We report an unusual non-storm erosion event of outer zone MeV electron distribution during three successive solar wind number density enhancements (SWDEs) on November 27−30, 2015. Loss of MeV electrons and energy-dependent narrowing of electron pitch angle distributions (PAD) first developed at L* = 5.5 and then moved down to L* < 4. According to the evolution of the electron phase space density (PSD) profile, losses of electrons with small pitch angles at L* > 4 during SWDE1 are mainly due to outward radial diffusion. However during SWDE2&3, scattering loss due to EMIC waves is dominant at 4 < L* < 5. As for electrons with large pitch angles, outward radial diffusion is the primary loss mechanism throughout all SWDEs which is consistent with the incursion of the Last Closed Drift Shell (LCDS). The inner edge of EMIC wave activity moved from L* ~5 to L* ~4 and from L ~6.4 to L ~4.2 from SWDE1 to SWDE2&3, respectively, observed by Van Allen Probes and by ground stations. This is consistent with the inward penetration of anisotropic energetic protons from L* = 4.5 to L* = 3.5, suggesting that the inward extension of EMIC waves may be driven by the inward injection of anisotropic energetic protons from the dense plasma sheet.
SOLID EARTH: SEISMOLOGY
Recently Published, doi: 10.26464/epp2021034
[Abstract](578) [FullText HTML](185) [PDF 1789KB](41)
Abstract:
Connecting earthquake nucleation in basement rock to fluid injection in basal, sedimentary reservoirs, depends heavily on choices related to the poroelastic properties of the fluid-rock system, thermo-chemical effects notwithstanding. Direct constraints on these parameters outside of laboratory settings are rare, and it is commonly assumed that the rock layers are isotropic. With the Arbuckle wastewater disposal reservoir in Osage County, Oklahoma, high-frequency formation pressure changes and collocated broadband ground velocities measured during the passing of large teleseismic waves show a poroelastic response of the reservoir that is both azimuthally variable and anisotropic; this includes evidence of static shifts in pressure that presumably relate to changes in local permeability. The azimuthal dependence in both the static response and shear coupling appears related to tectonic stress and strain indicators such as the orientations of the maximum horizontal stress and faults and fractures. Using dynamic strains from a nearby borehole strainmeter, we show that the ratio of shear to volumetric strain coupling is \begin{document}$\sim 0.41$\end{document} which implies a mean Skempton's coefficient of \begin{document}$A = 0.24$\end{document} over the plausible range of the undrained Poisson's ratio. Since these observations are made at relatively low confining pressure and differential stress, we suggest that the hydraulically conductive fracture network is a primary control on the coupling between pore pressure diffusion and elastic stresses in response to natural or anthropogenic sources.
SOLID EARTH: SEISMOLOGY
Recently Published, doi: 10.26464/epp2021026
[Abstract](883) [FullText HTML](242) [PDF 4574KB](25)
Abstract:
With the development of unconventional shale gas in the southern Sichuan Basin, seismicity in the region has increased significantly in recent years. Though the existing sparse regional seismic stations can capture most earthquakes with \begin{document}${M}_{\mathrm{L}}\ge 2.5$\end{document}, a great number of smaller earthquakes are often omitted due to limited detection capacity. With the advent of portable seismic nodes, many dense arrays for monitoring seismicity in the unconventional oil and gas fields have been deployed, and the magnitudes of those earthquakes are key to understand the local fault reactivation and seismic potentials. However, the current national standard for determining the local magnitudes was not specifically designed for monitoring stations in close proximity, utilizing a calibration function with a minimal resolution of 5 km in the epicentral distance. That is, the current national standard tends to overestimate the local magnitudes for stations within short epicentral distances, and can result in discrepancies for dense arrays. In this study, we propose a new local magnitude formula which corrects the overestimated magnitudes for shorter distances, yielding accurate event magnitudes for small earthquakes in the Changning−Zhaotong shale gas field in the southern Sichuan Basin, monitored by dense seismic arrays in close proximity. The formula is used to determine the local magnitudes of 7,500 events monitored by a two-phased dense array with several hundred 5 Hz 3C nodes deployed from the end of February 2019 to early May 2019 in the Changning−Zhaotong shale gas field. The magnitude of completeness (\begin{document}${M}_{\mathrm{C}}$\end{document}) using the dense array is −0.1, compared to \begin{document}${M}_{\mathrm{C}}$\end{document} 1.1 by the sparser Chinese Seismic Network (CSN). In addition, using a machine learning detection and picking procedure, we successfully identify and process some 14,000 earthquakes from the continuous waveforms, a ten-fold increase over the catalog recorded by CSN for the same period, and the \begin{document}${M}_{\mathrm{C}}$\end{document} is further reduced to −0.3 from −0.1 compared to the catalog obtained via manual processing using the same dense array. The proposed local magnitude formula can be adopted for calculating accurate local magnitudes of future earthquakes using dense arrays in the shale gas fields of the Sichuan Basin. This will help to better characterize the local seismic risks and potentials.

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Accepted Articles, doi: 10.26464/epp2021052
[Abstract](216) [PDF](3)
Abstract:
With the test particle simulation method, we have investigated the stochastic motion of electron with the energy of 300 keV in a monochromatic magnetosonic (MS) wave field, which is motived by the violation of the assumption of quasi-linear theory when the strong MS waves (the amplitude up to ~ 1 nT) are presented in the Earth’s magnetosphere. First of all, the electron motion can become stochastic when the wave amplitude exceeds a certain threshold. If electron initially resonates with MS wave in the way of bounce resonance, as the bounce resonance order increases, the amplitude threshold of electron stochastic motion firstly increases until it reaches the peak at about the 11 order in our study, then the amplitude threshold begins a slow decline. More interestingly, we find that the coexistence of bounce and Landau resonances between electron and MS wave will significantly reduce the amplitude threshold. In some cases, the electron motion can become stochastic in the field of MS wave with amplitudes below 1 nT. Besides, if neither the bounce nor Landau resonance condition is satisfied at the initial, then the amplitude threshold of stochastic motion also shows the increasing trend for lower frequencies and the decreasing trend for higher frequencies, but the amplitude threshold is always very large (> 5 nT). Our study suggests that the electron stochastic motion should also be considered when modeling electron dynamics regulated by intense MS waves in the Earth’s magnetosphere.
Accepted Articles, doi: 10.26464/epp2021054
[Abstract](160) [PDF](4)
Abstract:
From 2009 to 2017, parts of Central America experienced marked increase in the number of small to moderate-sized earthquakes. For example, three significant earthquakes (∼MW5) occurred near Prague, Oklahoma, the United States in 2011. On 6 Nov 2011, an Mw 5.7 earthquake occurred in Prague, central Oklahoma with a sequence of aftershocks. The seismic activity has been attributed to slip on the Wilzetta fault system. In this study, we provide a 3D fully coupled poroelastic analysis (using FLAC3D) of the Wilzetta fault system response to saltwater injection in the underpressured subsurface layers, especially Arbuckle group and the basement, to evaluate the conditions that might have led to the increased seismicity. In view of the data-limited nature of the problem, we have considered multiple plausible scenarios, and use the available data to evaluate the hydromechanical response of the faults of interest in the study area. Numerical simulations show that the injection of large volumes of fluid into Arbuckle group tends to bring the part of the Wilzetta faults in Arbuckle group and basement into near critical conditions.
Accepted Articles, doi: 10.26464/epp2022002
[Abstract](44) [PDF](0)
Abstract:
As one of the most important dynamic processes in the middle and upper atmosphere, gravity waves (GWs) play a key role in determining the global atmospheric circulation. Gravity wave potential energy (GW Ep) is an important parameter that characterizes GW intensity, so understanding its global distribution is necessary. In this paper, a deep learning algorithm (DeepLab V3+) is used to estimate the stratospheric GW Ep. The deep learning model inputs are ERA5 reanalysis datasets and GMTED2010 terrain data. The output is the estimated GW Ep averaged over 2030 km from 60°S60°N. The GW Ep averaged over 20~30 km calculated by COSMIC radio occultation (RO) data is used as the measured value corresponding to the model output. The results showed that (1) this method can effectively estimate the zonal trend of GW Ep. However, the errors between the estimated and measured value of Ep are larger in low-latitude regions than in mid-latitude regions. The large number of convolution operations used in the deep learning model may be the main reason. Additionally, the measured Ep has errors associated with interpolation to the grid, the error tends to be amplified in low-latitude regions because the GW Ep is larger and the RO data are relatively sparse, which affects the training accuracy. (2) The estimated Ep shows seasonal variations, which are stronger in the winter hemisphere and weaker in the summer hemisphere. (3) The effect of quasi-biennial oscillation (QBO) can be clearly observed in the monthly variation in the estimated GW Ep, and its QBO amplitude may be less than that of the measured Ep.
Accepted Articles, doi: 10.26464/epp2021055
[Abstract](257) [PDF](27)
Abstract:
Anthropogenic inducing seismicity has been widely reported and investigated in many regions, including the shale gas fields in the Sichuan basin, where the number of earthquakes has increased substantially since the commence of fracking in late 2014. However, the mechanism of induced earthquakes remains poorly understood, partly due to the lack of high-resolution spatial-temporal seismicity evolution. In contrast to most of the previous studies using a diffusive earthquake catalog constructed by routine methods, we constructed a high resolution catalog using a machine learning detector and waveform cross-correlation. The newly developed catalog improves the magnitude completeness and detected one-third additional earthquakes with limited data. The resulted catalog illuminates a comprehensive spatial-temporal migration of the emerging seismicity in the target area, with one of the clusters clearly delineating a potential unmapped fault trace led to the Mw 5.0 in 2019 September, by far the largest earthquake in the region. The migration of the seismicity also demonstrates a pore-pressure diffusion front, suggesting additional constraints on the inducing mechanism of the region. The patterns of the highly clustered seismicity reconcile the causal link between the emerging seismicity and hydraulic fracturing in the region and facilitate continued investigation of the induced mechanisms and associated risks.
Accepted Articles, doi: 10.26464/epp2021056
[Abstract](165) [PDF](16)
Abstract:
Locating seismic events is a central task for earthquake monitoring. Compared to arrival-based location methods, waveform-based location methods do not require picking phase arrivals and are more suitable for locating seismic events with noisy waveforms. Among waveform-based location methods, one category is to stack different attributes of P and S waveforms around arrival times corresponding to potential event locations and origin times. At correct event location and origin time, the stacking value will be maximized. In this study, to obtain high-resolution location image, we improve the waveform-based location method by applying hybrid multiplicative imaging condition to characteristic functions of seismic waveforms. In our new stacking method, stations are divided into groups and characteristic functions of seismic waveforms at stations in the same group are first summed and then multiplied among groups, which can largely eliminate the cumulative effect of noise in the summation process and thus improve the resolution of location images. We test the new method and compare it with the other three stacking methods using both synthetic and real datasets that are related to induced seismicity by oil/gas production. The results show that the new stacking method can provide higher-resolution location images.

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2021, 5(5): 365 -367. doi: 10.26464/epp2021050
Abstract:
The 2nd Equatorial Plasma Bubble (EPB) workshop, funded by the Institute of Geology and Geophysics, Chinese Academy of Sciences, and the National Natural Science Foundation of China, took place in Beijing, China during September 13–15, 2019. The EPB workshop belongs to a conference series that began in 2016 in Nagoya, Japan at the Institute for Space-Earth Environmental Research, Nagoya University, resulting in a special issue of Progress in Earth and Planetary Science that focused on EPBs. The main goal of the series is to organize in-depth discussion by scientists working on ionospheric irregularities, and solve the scientific challenges in EPB and ionospheric scintillation forecasting. The 2nd EPB workshop gathered almost 60 scientists from seven countries. A total of 20 invited and contributing papers focusing on ionospheric irregularities and scintillations were presented. Here we briefly comment on 10 papers included in this special issue.
SPACE PHYSICS: IONOSPHERIC PHYSICS
2021, 5(5): 368 -377. doi: 10.26464/epp2021047
Abstract:
Large Scale Wave Structures (LSWS) in the equatorial ionospheric F-region were observed by measuring spatial and temporal variations within detrended total electron content (dTEC) data obtained by ground-based GNSS receivers over the South American continent. By using dTEC-maps, we have been able to produce, for the first-time, two-dimensional representations of LSWS. During the period from September to December, the LSWS frequently occurred starting a few hours prior to Equatorial Plasma Bubble (EPB) development. From 17 events of LSWS observed in 2014 and 2015, wave characteristics were obtained: the observed wavelengths, periods, and the phase speeds are respectively, ~900 km, ~41 min and ~399 m/s; the waves propagated from the northeast to southeast. In some cases the front of the oscillation was meridionally aligned, extending to more than 1600 km, the first time such large extension of the wavefront has been reported. From F-layer bottom height oscillation data, measured by ionosonde, LSWS exhibit two different vertical phase propagation modes, in-phase and downward phase. The former mode indicates the presence of a polarization electric field in the F-layer bottom side; the latter suggests propagation of atmospheric gravity waves. The presence of LSWS near the solar terminator, followed by the development of EPBs, suggests that the upwelling of the F-layer bottom height produces a condition favorable to the development of Rayleigh–Taylor instability.
SPACE PHYSICS: IONOSPHERIC PHYSICS
2021, 5(5): 378 -386. doi: 10.26464/epp2021039
Abstract:
The occurrence of midnight Equatorial Plasma Bubbles (EPBs) during the June solstice period of the ascending phase of solar cycle 24, from 2010 to 2014, was studied using data from the 47 MHz Equatorial Atmosphere Radar (EAR) at Kototabang, Indonesia. The analysis shows that the occurrence of midnight hour EPBs was at its maximum during the low solar activity year 2010 and monotonically decreased thereafter with increasing solar activity. Details of the dependence of midnight hour EPB occurrence on solar activity were investigated using SAMI2 model simulation with a realistic input of E × B drift velocity data obtained from the CINDI-IVM onboard the C/NOFS satellite. Results obtained from term-by-term analysis of the flux tube integrated linear growth rate of RT instability indicate that the formation of a high flux tube electron content height gradient (steep vertical gradient) region at higher altitudes, due to the elevated F layer, is the key factor enhancing the growth rate of RT instability during low solar activity June solstices. Other factors are discussed in light of the relatively weak westward zonal electric field in the presence of the equatorward neutral wind and north-to-south transequatorial wind around the midnight hours of low solar activity June solstices. Also discussed are the initial seeding of RT instability by MSTIDs and how the threshold height required for EPB development varies with solar activity.
SPACE PHYSICS: IONOSPHERIC PHYSICS
2021, 5(5): 387 -396. doi: 10.26464/epp2021049
Abstract:
Previous studies have proposed that both the thermospheric neutral wind and the equatorial electrojet (EEJ) near sunset play important roles in the pre-reversal enhancement (PRE) mechanism. In this study, we have used observations made in the equatorial region of Southeast Asia during March–April and September–October in 2010–2013 to investigate influences of the eastward neutral wind and the EEJ on the PRE’s strength. Our analysis employs data collected by the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) satellite to determine the zonal (east-west direction) neutral wind at an altitude of ~250 km (bottomside F region) at longitudes of 90°–130°E in the dusk sector. Three ionosondes, at Chumphon (dip lat.: 3.0°N) in Thailand, at Bac Lieu (dip lat.: 1.7°N) in Vietnam, and at Cebu (dip lat.: 3.0°N) in Philippines, provided the data we have used to derive the PRE strength. Data from two magnetometers — at Phuket (dip lat.: 0.1°S) in Thailand and at Kototabang (dip lat.: 10.3°S) in Indonesia — were used to estimate the EEJ strength. Our study is focused particularly on days with magnetically quiet conditions. We have found that the eastward neutral wind and the EEJ are both closely correlated with the PRE; their cross-correlation coefficients with it are, respectively, 0.42 and 0.47. Their relationship with each other is weaker: the cross-correlation coefficient between the eastward neutral wind and the EEJ is just 0.26. Our findings suggest that both the eastward neutral wind and the EEJ near sunset are involved in the PRE mechanism. Based on the weak relationship between these two parameters, however, they appear to be significantly independent of each other. Thus, the wind and the EEJ are likely to be influencing the PRE magnitude independently, their effects balancing each other.
SPACE PHYSICS: IONOSPHERIC PHYSICS
2021, 5(5): 397 -406. doi: 10.26464/epp2021045
Abstract:
OI 630.0 nm airglow observations, from a new observatory at Bom Jesus de Lapa, were used to study the interaction between EPBs (Equatorial Plasma Bubbles) and the MSTID (Medium-Scale Traveling Ionospheric Disturbance) over the Northeast region in Brazil. On the night of September 16 to 17, 2020, an EPB was observed propagating eastward, in an apparent fossil stage, until it interacted with a dark band electrified MSTID (eMSTID). After the interaction, four EPBs merged, followed by an abrupt southward development and bifurcations. Analysis of the data suggests that an eastward polarization electric field, induced by the dark band eMSTID, forced the EPB into an upward drift, growing latitudinally along the magnetic field lines and then bifurcating.
SPACE PHYSICS: IONOSPHERIC PHYSICS
2021, 5(5): 407 -415. doi: 10.26464/epp2021044
Abstract:
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.
SPACE PHYSICS: IONOSPHERIC PHYSICS
2021, 5(5): 416 -426. doi: 10.26464/epp2021043
Abstract:
We investigated the variations of equatorial plasma bubbles (EPBs) in the East-Asian sector during a strong geomagnetic storm in October 2016, based on observations from the Beidou geostationary (GEO) satellites, Swarm satellite and ground-based ionosonde. Significant nighttime depletions of F region in situ electron density from Swarm and obvious nighttime EPBs in the Beidou GEO observations were observed on 13 October 2016 during the main phase. Moreover, one interesting feature is that the rare and unique sunrise EPBs were triggered on 14 October 2016 in the main phase rather than during the recovery phase as reported by previous studies. In addition, the nighttime EPBs were suppressed during the whole recovery phase, and absent from 14 to 19 October 2016. Meanwhile, the minimum virtual height of F trace (h’F) at Sanya (18.3°N, 109.6°E, MLAT 11.1°N) displayed obvious changes during these intervals. The h’F was enhanced in the main phase and declined during the recovery phase, compared with the values at pre- and post-storm. These results indicate that the enhanced nighttime EPBs and sunrise EPBs during the main phase and the absence nighttime EPBs for many days during the recovery phase could be associated with storm-time electric field changes.
SPACE PHYSICS: IONOSPHERIC PHYSICS
2021, 5(5): 427 -434. doi: 10.26464/epp2021046
Abstract:
This paper reports that plasma density depletions appearing at middle latitudes near sunrise survived until afternoon on 29 May 2017 during the recovery phase of a geomagnetic storm. By analyzing GPS data collected in Japan, we investigate temporal variations in the horizontal two-dimensional distribution of total electron content (TEC) during the geomagnetic storm. The SYM-H index reached −142 nT around 08 UT on 28 May 2017. TEC depletions extending up to approximately 38°N along the meridional direction appeared over Japan around 05 LT (LT = UT + 9 hours) on 29 May 2017, when TEC rapidly increased at sunrise due to the solar extreme ultraviolet (EUV) radiation. The TEC depletions appeared sequentially over Japan for approximately 8 hours in sunlit conditions. At 06 LT on 29 May, when the plasma depletions first appeared over Japan, the background TEC was enhanced to approximately 17 TECU, and then decreased to approximately 80% of the TEC typical of magnetically quiet conditions. We conclude that this temporal variation of background plasma density in the ionosphere was responsible for the persistence of these plasma depletions for so long in daytime. By using the Naval Research Laboratory: Sami2 is Another Model of the Ionosphere (SAMI2), we have evaluated how plasma production and ambipolar diffusion along the magnetic field may affect the rate of plasma depletion disappearance. Simulation shows that the plasma density increases at the time of plasma depletion appearance; subsequent decreases in the plasma density appear to be responsible for the long-lasting persistence of plasma depletions during daytime. The plasma density depletion in the top side ionosphere is not filled by the plasma generated by the solar EUV productions because plasma production occurs mainly at the bottom side of the ionosphere.
SPACE PHYSICS: IONOSPHERIC PHYSICS
2021, 5(5): 435 -449. doi: 10.26464/epp2021048
Abstract:
Observational evidence is insufficient to understand how equatorial plasma bubbles (EPBs) form over low latitudes. The mechanism of plasma-density enhancement (formation of “plasma blobs”) at low latitudes is in dispute. In this paper, we use data from multiple ground-based instruments (one all-sky airglow imager, five digisondes, and one Fabry–Perot interferometer) to investigate the evolution of an EPB event that occurred at low latitudes over China on the night of 06 December 2015 (06-Dec-2015). We provide observational evidence that an enhanced equatorward wind most likely induced by a substorm could have initiated the Rayleigh–Taylor instability (RTI) that destabilized several EPB depletions in an upwelling region of a large-scale wave-like structure (LSWS) in the bottomside ionosphere. Those EPB depletions were forced to surge poleward, from nearly 10° to 19° magnetic latitude, two hours before midnight. Smaller-scale bifurcations evolved rapidly from tips of airglow depletions by a secondary E × B instability when the aforementioned substorm-induced southwestward wind blew through. During the growth phase of the EPB depletions, a westward polarization electric field inside the LSWS is likely to have compressed plasma downward, inducing the two airglow-type blobs observed in the bottomside ionosphere, by a mechanism of LSWS-blob connection that we propose. We also provide observational evidence of brightness airglow depletions. We find that an enhanced poleward wind associated with a passing-by brightness wave (BW) is likely to have transported plasma to fill the airglow depletions, which finally evolved into brightness airglow structures. This study investigates the physical processes accompanied by the EPB event and those two-airglow blobs observed at low-latitudes over China.
2021, 5(5): 450 -461. doi: 10.26464/epp2021042
Abstract:
We estimate the zonal drift velocity of small-scale ionospheric irregularities at low latitude by leveraging the spaced-receivers technique applied to two GNSS receivers for scintillation monitoring installed along the magnetic parallel passing in Presidente Prudente (Brazil, magnetic latitude 12.8°S). The investigated ionospheric sector is ideal to study small-scale irregularities, being located close to the expected position of the southern crest of the equatorial ionospheric anomaly. The measurement campaign took place between September 2013 and February 2014, i.e. equinox and summer solstice seasons under solar maximum, during which the probability of formation of small-scale irregularities is expected to maximize. We found that the hourly average of the velocity increases up to 135 m/s right after the local sunset at ionospheric altitudes and then smoothly decreases in the next hours. Such measurements are in agreement with independent estimations of the velocity made by the Incoherent Scatter Radar located at the Jicamarca Radio Observatory (magnetic latitude 0.1°N), by the Boa Vista Ionosonde (magnetic latitude 12.0°N), and by applying a recently-developed empirical regional short-term forecasting model. Additionally, we investigated the relationship with the percentage occurrence of amplitude scintillation; we report that it is exponentially dependent on the zonal velocity of the irregularities that cause it.
SPACE PHYSICS: IONOSPHERIC PHYSICS
2021, 5(5): 462 -482. doi: 10.26464/epp2021025
Abstract:
This paper briefly reviews ionospheric irregularities that occur in the E and F regions at mid-latitudes. Sporadic E (ES) is a common ionospheric irregularity phenomenon that is first noticed in the E layer. ES mainly appears during daytime in summer hemispheres, and is formed primarily from neutral wind shear in the mesosphere and lower thermosphere (MLT) region. Field-aligned irregularity (FAI) in the E region is also observed by Very High Frequency (VHF) radar in mid-latitude regions. FAI frequently occurs after sunset in summer hemispheres, and spectrum features of E region FAI echoes suggest that type-2 irregularity is dominant in the nighttime ionosphere. A close relationship between ES and E region FAI implies that ES may be a possible source of E region FAI in the nighttime ionosphere. Strong neutral wind shear, steep ES plasma density gradient, and a polarized electric field are the significant factors affecting the formation of E region FAI. At mid-latitudes, joint observational experiments including ionosonde, VHF radar, Global Positioning System (GPS) stations, and all-sky optical images have revealed strong connections across different scales of ionospheric irregularities in the nighttime F region, such as spread F (SF), medium-scale traveling ionospheric disturbances (MSTID), and F region FAI. Observations suggest that different scales of ionospheric irregularities are generally attributed to the Perkins instability and subsequently excited gradient drift instability. Nighttime MSTID can further evolve into small-scale structures through a nonlinear cascade process when a steep plasma density gradient exists at the bottom of the F region. In addition, the effect of ionospheric electrodynamic coupling processes, including ionospheric E-F coupling and inter-hemispheric coupling on the generation of ionospheric irregularities, becomes more prominent due to the significant dip angle and equipotentiality of magnetic field lines in the mid-latitude ionosphere. Polarized electric fields can map to different ionospheric regions and excite plasma instabilities which form ionospheric irregularities. Nevertheless, the mapping efficiency of a polarized electric field depends on the ionospheric background and spatial scale of the field.
2017, 1(1): 2-12doi: 10.26464/epp2017002
[Abstract](5320) [FullText HTML](2023) [PDF](185)
2018, 2(4): 257-275doi: 10.26464/epp2018025
[Abstract](4257) [FullText HTML](1249) [PDF](171)
2017, 1(1): 13-25doi: 10.26464/epp2017003
[Abstract](4866) [FullText HTML](2551) [PDF](149)
2018, 2(1): 52-66doi: 10.26464/epp2018005
[Abstract](4282) [FullText HTML](1695) [PDF](135)
2017, 1(1): 26-34doi: 10.26464/epp2017004
[Abstract](6708) [FullText HTML](1937) [PDF](134)
2018, 2(6): 527-537doi: 10.26464/epp2018051
[Abstract](9418) [FullText HTML](1032) [PDF](125)
2018, 2(1): 67-73doi: 10.26464/epp2018006
[Abstract](4536) [FullText HTML](2500) [PDF](91)
2017, 1(1): 44-52doi: 10.26464/epp2017006
[Abstract](5484) [FullText HTML](1710) [PDF](90)
2017, 1(1): 72-76doi: 10.26464/epp2017011
[Abstract](4657) [FullText HTML](1561) [PDF](88)
2018, 2(1): 1-14doi: 10.26464/epp2018001
[Abstract](5063) [FullText HTML](1534) [PDF](87)

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