Using the test particle simulation method, we investigate the stochastic motion of electrons with energy of 300 keV in a monochromatic magnetosonic (MS) wave field. This study is motivated by the violation of the quasi-linear theory assumption, when strong MS waves (amplitude up to ~1 nT) are present in the Earth’s magnetosphere. First, electron motion can become stochastic when the wave amplitude exceeds a certain threshold. If an electron initially resonates with the MS wave via bounce resonance, as the bounce resonance order increases, the amplitude threshold of electron stochastic motion increases until it reaches the peak at about the 11th order in our study, then the amplitude threshold slowly declines. Further, we find that the coexistence of bounce and Landau resonances between electrons and MS waves will significantly reduce the amplitude threshold. In some cases, the electron motion can become stochastic in the field of an MS wave with amplitudes below 1 nT. Regardless, if neither the bounce nor Landau resonance condition is satisfied initially, then the amplitude threshold of stochastic motion shows an increasing trend for lower frequencies and a decreasing trend for higher frequencies, even though the amplitude threshold is always very large (> 5 nT). Our study suggests that electron stochastic motion should also be considered when modeling electron dynamics regulated by intense MS waves in the Earth’s magnetosphere.
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.
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
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
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.
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 2030 km from 60°S60°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.
This paper studies inter-annual variations of 6.5-Day Waves (6.5DWs) in 20110 km between 52°S52°N during March 2002January 2021 and their relations with equatorial stratospheric Quasi-Biennial Oscillation (QBO). 6.5DWs’ amplitudes in temperature are calculated based on SABER/TIMED observations. QBO zonal winds are obtained from ERA5 reanalysis dataset. QBO phases are derived from Empirical Orthogonal Functions (EOF) method. Wavelet analysis of 6.5DW variations demonstrates obvious spectral maximums around 2838 months in 32°52°N and 2630 months in 32°52°S. In the Northern Hemisphere, peak periods get longer poleward, while they remain unchanged with latitude in the Southern Hemisphere. Residual 6.5DWs’ amplitudes are calculated by removing composite amplitudes from 6.5DWs’ amplitudes. Comparisons between QBO and the monthly maximum residual 6.5DWs’ amplitudes (A_Mmax) show clear relations between QBO and 6.5DWs in both hemispheres, especially in the NH. When A_Mmax is large in the NH, mean QBO profile is easterly at all levels from 70 to 5 hPa. When it’s weak, mean QBO wind is weak westerly below 30 hPa. Linear Pearson correlation coefficients between QBO phases and A_Mmax show large positive values in 60110 km between 20°52°N in April and around 64 km at 24°S in February, and large negative values from 80 to 110 km between 20°N50°N in August and at 96106 km between 20°S44°S in February. These results indicate quantitative relations between QBO and 6.5DWs and provide credible evidences for further studies of QBO modulations on long-term variations of 6.5DWs.
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.
On 21 May 2021 (UTC), an MW 7.4 earthquake jolted the east Bayan Har block in the Tibetan Plateau. The earthquake received widespread attention as it is the largest event in the Tibetan Plateau and its surroundings since the 2008 Wenchuan earthquake and in proximity to the seismic gaps on the east Kunlun fault. Here we use satellite interferometric synthetic aperture radar data and subpixel offset observations along the range directions to characterize the coseismic deformation of the earthquake. Rang offset displacements depict clear surface ruptures with a total length of ~170 km involving two possible activated fault segments in the earthquake. Coseismic modeling results indicate that the earthquake is dominated by left-lateral strike-slip motions of up to 7 m within top 12 km of the crust. The well-resolved slip variations are characterized by five major slip patches along strike and 64% of shallow slip deficit, suggesting a young seismogenic structure. Spatial-temporal changes of postseismic deformation are mapped from the early 6-day and 24-day InSAR observations, and are well explained by time-dependent afterslip models. Analysis of GPS velocity profiles and strain rates suggests that the eastward extrusion of plateau is diffusely distributed across the east Bayan Har block, but showing significant lateral heterogeneities as evidenced by magnetotelluric observations. The block-wide distributed deformation of the east Bayan Har block along with the significant co- and post-seismic stress loading from the Maduo earthquake imply high seismic risks on regional faults, especially the Tuosuo Lake and Maqin-Maqu segments of the Kunlun fault that known as seismic gaps.
The Mesozoic Yanshanian Movement affected the tectonic evolution of the North China Craton (NCC). It is proposed that Mesozoic cratonic destruction peaked ~125 Ma, possibly influenced by subduction of the western Pacific Plate beneath the Euro-Asian Plate in the Early Cretaceous. The southern Jinzhou area in the eastern block of the NCC preserves records for the tectonic events and related geological resources. Studies of the regional stress field evolution from the Cretaceous to Cenozoic can enhance our understanding of the tectonics and dynamics of the NCC. Borehole image logging technology was used to identify and collect attitudes of tensile fractures from 11 boreholes, which were subdivided into four groups according to dip directions, i.e., NNW-SSE, NWW-SEE, W-E and NE-SW. Their development was mainly controlled by the regional tectonic stress field while temperature, lithology, and depth contributed to some extent. The area was characterized by NNW-SSE- and NWW-SEE-oriented extension in 136-125 Ma in the Early Cretaceous. Subsequently, it has successively undergone W-E- and NE-SW-oriented extension in 125-101 Ma and after 101 Ma. This counterclockwise trend has persisted to the present, probably related to oblique subduction of the Pacific Plate and is characterized by ongoing nearly N-S-oriented extension and NEE-SWW-oriented compression.
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.
Energetic neutral atoms (ENAs) are produced by the neutralization of energetic ions formed by shock-accelerated gradual solar energetic particle events (SEP). These high-energy ENAs (HENAs) can reach the Earth earlier than the associated SEPs and provide information about the SEPs at the lower corona. The HENA properties observed at Earth depend on the properties of the coronal mass ejection (CME)-driven shocks that accelerate the SEPs. With a model of HENA production in a shock-accelerated SEP event, we semi-quantitatively investigate the energy-time spectrum of HENAs depending on the width, propagation speed, and direction of the shock, as well as the density and ion abundances of the lower corona. Compared to the baseline model parameters, the cases with a wider shock width angle or a higher coronal density would increase the HENA flux observed at the Earth, while the case with an Earth-propagating shock shows a softened HENA spectrum. The comparison of expected HENA fluxes in different cases with a flight-proven ENA instrument suggests that solar HENAs can feasibly be monitored with current technologies, which could provide a lead time of 2-3 hours for SEPs at a few MeV. We propose that monitoring of solar HENAs could provide a new method to forecast shock-driven SEP events that could have significant space weather impacts on the near-Earth environment.
A method for reconstructing crustal velocity structure using the optimization of stacking receiver function amplitude in the depth domain, named common conversion amplitude (CCA) inversion, is presented. The conversion amplitude in the depth domain, which represents the impedance change in the medium, is obtained by assigning the receiver function amplitude to the corresponding conversion position where the P-to-S conversion occurred. Utilizing the conversion amplitude variation with depth as an optimization objective, imposing reliable prior constraints on the structural model frame and velocity range, and adopting a stepwise search inversion technique, this method efficiently weakens the tendency of easily falling into the local extremum in conventional receiver function inversion. Synthetic tests show that the CCA inversion can reconstruct complex crustal velocity structures well and is especially suitable for revealing crustal evolution by estimating diverse velocity distributions. Its performance in reconstructing crustal structure is superior to that of the conventional receiver function imaging method.
For planetary surface materials, thermal inertia is the critical property that governs the surface’s daily thermal response and controls the diurnal and seasonal surface temperature variations. Here we use the ground measurements made by the MSL Curiosity rover and the InSight lander to determine the thermal inertia of two sites on Mars. This study compares the variation of thermal inertia during and after the Large Dust Storm (LDS) of Martian Year (MY) 34. We derive a simple approximation (using energy balance), which utilizes surface albedo, surface energy flux, and diurnal change in the surface temperature for the surface thermal inertia determination. The average thermal inertia in MY34 is about 39.2%, 3.7%, and 3.4% higher than MY35 average thermal inertia for the MSL, InSight (FOV1), and InSight (FOV2), respectively. The thermal inertia at the InSight (FOV1) is consistently lower by about 20 J m–2 s–1/2 K–1 than the InSight (FOV2) site for all scenarios, indicating notable variation in the region’s surface composition. The best-fit surface albedo in MY34 (determined using the KRC model) are about 0.08, 0.05, and 0.03 higher than MY35 surface albedo for the MSL, InSight (FOV1), and InSight (FOV2), respectively. An increase in both surface albedo and thermal inertia during the LDS indicates that the underlying surface is both more thermally resistant and more reflective than the overlying loose dust.
Planetary magnetosheaths are characterized by a high plasma wave and turbulence activity. This is also observed for the induced magnetosphere of Mars where both upstream and locally generated plasma waves have been observed in the region between its bow shock and magnetic boundary layer, the magnetosheath. In this work, 12 years (2005-2016) of Mars Express (MEX) magnetosheath crossings have been used in order to conduct a statistical study of wave activity in the magnetosheath of Mars. Electron density and temperature measured by the electron spectrometer (ELS) of the plasma analyzer (ASPERA-3) experiment on board of MEX spacecraft were used in this study. The kurtosis parameter has been calculated for these plasma parameters. This value indicates intermittent behavior in the data when it is higher than 3 (the value for a normal or Gaussian distribution). The variation of wave activity occurrence has been analyzed in relation to solar cycle, Martian orbit and distance to the bow shock. It was observed non-Gaussian properties in the magnetosheath of Mars on all analyzed scales, especially in those near the proton gyrofrequency in the upstream region of the Martian magnetosphere. We also observed that it is higher on smaller scales (higher frequencies). A significant influence of the solar cycle was also observed, where the kurtosis parameter is higher during declining and solar maximum phases, where the presence of disturbed solar wind conditions, caused by large scale solar wind structures, increases. The kurtosis decreases with increasing distance from the bow shock, which indicates that the intermittence level is higher near the bow shock. In the electron temperature data the kurtosis is higher near the perihelion due to the higher incidence of EUV when the planet is closer to the Sun, which causes a more extended exosphere, and consequently increases the wave activity in the magnetosheath and its upstream region. The extended exosphere seems to play a lower effect in the electron density data.
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.
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.
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.
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.
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.
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.
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.
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.