Wind Shear Driven Double Layer Structures of E‐Region Irregularities at Low Latitudes

Previous theoretical simulations showed the generation of double‐layer structures of E‐region field‐aligned irregularities (FAIs). In this study, we report the double‐layer structures of E‐region FAIs observed by an all‐sky radar at low latitude Ledong (18.4°N, 109°E), China. These FAIs appeared at the altitudes ∼90 and 110 km respectively. Both layers displayed quasi‐periodic patterns with synchronized spatial features, that is, being manifested as spatially separated patches or wavelike structures over similar longitudes at the two layers simultaneously. The neutral wind observations revealed the existence of wind shears at two separated altitudes where the double FAI layers occurred. The two wind shears created two vertically separated sporadic E layers and possibly drove the generation of the double‐layer FAIs via gradient drift instability. The synchronized spatial features of the FAIs at the two layers could be modulated by gravity waves.


Introduction
The ionospheric E-region irregularities refer to the plasma density irregular structures at the altitudes 90-130 km.They are highly magnetic sensitive as field-aligned irregularities (FAIs) traditionally investigated based on narrow-beam very high frequency (VHF) radars (e.g., Chen et al., 2016;Ogawa et al., 2002;Patra et al., 2009;Woodman et al., 1991).According to the VHF radar echo patterns, E-region irregularities are mainly classified into the continuous and quasi-periodic (QP) types (e.g., Ogawa et al., 1995;Yamamoto et al., 1991).Both types are closely related to the sporadic E (Es) layer.Under the presence of Es, the E-region irregularities can be initially generated via the gradient drift instability (GDI), and modulated by atmospheric gravity waves (GWs) and/or Kelvin-Helmholtz (K-H) instability to develop into horizontally distributed structures (e.g., Kagan & Kelley, 1998;Larsen, 2000;Patra et al., 2009;Yeh et al., 2012).
The E-region FAIs presented in previous studies were mainly manifested as single-layer structures at the low latitudes far away from magnetic equator (e.g., Chu & Wang, 1997;Maruyama et al., 2006).Whereas E-region echoes observed by narrow-beam VHF radars may appear at different range gates in the radar Range-Time-Intensity (RTI) plots as vertically stratified structures, they could be actually due to horizontally distributed structures or sidelobe echoes (e.g., Saito et al., 2005;Sun et al., 2020).The irregularity true heights could not be well obtained unless radar interferometry technique is employed (e.g., Chau et al., 2019;Chen et al., 2022;Hysell & Larsen, 2021;Li et al., 2014).However, the interferometry employed to narrow-beam VHF radars can only obtain the spatial features of E-region FAIs in a narrow horizontal region smaller than 100 km.Recently, the allsky radar, which was traditionally employed to measure neutral winds by observing specular meteors, was developed to observe E-region irregularities in a large horizontal region up to ∼500 km (e.g., Wang et al., 2019;Xie et al., 2019).Based on the interferometry technique of all-sky radar, it was revealed that at low latitudes, the continuous-type E-region FAIs mainly correspond to the tidal Es layer, whereas the QP type were usually due to spatially separated FAI patches (e.g., Sun, Li, Wang, et al., 2023).
Under the influence of both neutral dynamics and electro-dynamics, the E-region physical processes at low latitudes are usually complex, and vary significantly with altitudes (e.g., Bernhardt, 2002;Kagan, 2002).However, previous studies with radar mainly focused on the height distribution or the horizontal spatial features of E-region FAIs in a limited region (a few tens of km).Few investigations emphasized on the FAI morphology in the vertical direction over a wide longitude region.According to the simulation by Kagan and Kelley (1998), the wind-driven gradient drift mechanisms could induce instability regions on both sides of the Es layer, and lead to vertically stratified irregularities, that is, double-layer E-region FAIs.However, few observational evidence was provided.
In this study, we reported double-layer structures of E-region irregularities observed by an all-sky radar at low latitudes far away from magnetic equator.Compared with the observations by narrow-beam VHF radars, the allsky radar provides capabilities to visualize the zonal structure of E-region FAIs over a much wider longitude region, allowing us for the first time to carry out the double-layer tomography, to trace the dynamics of doublelayer FAIs, and to study potential factors driving their generation.The observations may provide evidences, clues or implications for the existence of complex E-region irregularity structures caused by complex perturbation waves/wind system at E-region altitudes.

Data and Method
The Ledong all-sky radar (18.4°N, 109°E), which is a part of the Meteor and ionospheric Irregularity Observation System at Hainan Is (e.g., Li et al., 2022), is employed to obtain the spatial features of E-region irregularity structures.In the present study, the radar operated at 38.9 MHz with a peak power 20 kW, and covered a detection range 68.4-318.6 km with a range resolution 1.8 km.The radar uses a crossed dipole antenna for transmission and five similar antennas for reception, attaining a 3-dB beamwidth 360°in azimuth and 110°in elevation.The five receiving antennas were arranged orthogonally in the east-west and north-south directions, that is, the typical Jones configuration (e.g., Jones et al., 1998).After employing the interferometry technique, the arriving angles and locations of the backscatter echoes from plasma irregularities in both the horizontal and vertical planes can be resolved unambiguously.Details of the radar antenna array and the interferometry technique can be seen in our previous studies (e.g., Sun, Li, Wang, et al., 2023;Wang et al., 2019) and will not be repeated here.For E-region FAIs at ∼90-130 km, the observations by the all-sky radar cover a wide zonal range up to ∼500 km.
Besides FAI backscatter echoes, the all-sky radar can also provide zonal and meridional neutral winds around 80-100 km altitudes.To obtain the background winds above 100 km, observations from the Michelson Interferometer for Global High-Resolution Thermospheric Imaging instrument onboard the Ionospheric Connection Explorer (ICON) satellite were used.The ICON satellite is located at a low Earth orbit 575.3-599.7 km above the Earth, with an inclination angle 27°(e.g., Bust & Immel, 2020).Using the two emission lines 557.7 and 630.0 nm, neutral winds at 90-300 km (90-109 km) altitudes during daytime (nighttime) were obtained (e.g., Englert et al., 2017;Harding et al., 2017).The winds below 130 km with a height resolution ∼2.8 km are used in the present study.
The ionosonde collocated at Ledong is employed to get information of background Es layers.The top frequency (ftEs) and virtual heights (h'Es) of Es were derived from manually scaled ionograms.The h'Es were scaled at the bottomside of the Es traces.

Observations
Figure 1a shows the RTI plot of the irregularity backscatter echoes observed by the all-sky radar during 11:30-16:00 UT (UT = LT-7.3hr) on 15 June 2020.The irregularity echoes extended from the ranges ∼100-260 km, and exhibited complex behavior with distinct positive and negative slopes.The pattern with opposite slopes is a typical feature of QP echoes by all-sky radars, corresponding to irregularities drifting into the radar field-of-view (FOV) from one side along the zonal direction, and moving away from the other side (e.g., Wang et al., 2019).Nevertheless, the present case showed some different characteristics from previously observed QP echoes at Hainan.The SNR of QP echoes usually decreased with increasing ranges, where the peak SNR at the smallest range gate usually corresponds to the irregularity appearing due north of the radar (e.g., Chen et al., 2020;Sun et al., 2020).However, for the present case, the echo SNR showed two distinct peaks, around the ranges 105 and 130 km, respectively.The two peaks were more obvious during 13:00-14:00 UT.The observations imply that the echoes might be backscattered from two vertically separated irregularity layers.
Figures 1b and 1c present the spatial locations of the echoes in Figure 1a in different coordinates.Three main features can be noted.(a) The echoes were backscattered from the directions perpendicular to the geomagnetic field, with azimuth and elevation angles within ±70°and 25°-65°, respectively (Figure 1b).This indicates that the irregularities responsible for these echoes are FAIs.(b) The main FAIs were distributed from 100 to 150 km, covering a wide zonal region ∼250 km (Figure 1c).(c) The FAIs appeared at two distinct altitudes as two separated layers.The lower and upper layers were located around 86-96 and 105-120 km altitudes, respectively (Figure 1c).Figures 1d-1i show the spatial locations of the irregularities at three specific time intervals.The FAIs were manifested as spatially separated patches (11:45 and 13:43 UT) or wavelike structures (12:11 UT) synchronously at the two layers, that is, the FAI patches/wavelike structures could appear over similar longitudes at the altitudes around 90 and 110 km simultaneously.The zonal separations between adjacent patches/structures were ∼40-70 km.Furthermore, the supporting Movie S1 shows the whole evolution process of the FAI structures during 11:30-14:40 UT with a temporal resolution of 1 minute.More details can be seen.During most of the time, the FAIs displayed as spatially separated patches synchronously at both layers, with separations predominantly exceeding 20 km.The number of FAI patches observed simultaneously could be up to nine (e.g., 11:54 UT).
In previous studies, the QP-type E-region FAIs observed at middle and low latitudes were often embedded in Es layers (e.g., Maruyama et al., 2006;Ogawa et al., 1995).Corresponding with the double-layer FAIs in the present case, double Es layers were observed by the collocated ionosonde.As shown in Figure 2a, double Es layers were observed at 11:30, 12:00, 12:37, 13:37, and 14:00 UT.Note that the echoes at ∼190 and 285 km were multi-hop reflections rather than true layers.Figures 2b and 2c present the temporal variations of h'Es and ftEs for the double Es layers.The upper (lower) Es layer was mainly located around ∼110 km (90 km).The lower Es layer persisted from 10:00 to 16:30 UT, with gradually decreasing ftEs.The upper Es layer appeared during 11:30-14:00 UT with ftEs higher than the lower layer, but it was temporarily absent around 13:00 UT, consistent with the short absence of the upper E-region FAI echoes in Figure 1a.From Figure 2a, it could also be noticed that due to the occurrence of the double Es layers, the F layer was totally (before 12:37 UT) or partly (at 13:00 UT) blanketed.As the Es intensity characterized by ftEs enhanced (e.g., at 13:37 UT) or attenuated (e.g., at 14:00 UT), the blanketing strength also increased or decreased, respectively.This indicates the Es layers were blanketing type.
It is generally believed that blanketing-type Es is mainly composed of metallic ions and primarily formed through wind shear mechanism at middle and low latitudes (e.g., Haldoupis, 2011;Mathews, 1998;Resende et al., 2016;Whitehead, 1970).Figure 3 presents the neutral winds observed on 15 June 2020.The winds at ∼80-100 km altitudes were from the all-sky radar (Figures 3a and 3b).During the occurrence of the double-layer Es/E-region FAIs (∼11:30-14:00 UT), the wind shears favorable for Es generation, that is, northward (westward) wind above and southward (eastward) wind below, likely existed around ∼85-93 km, which may be responsible for the generation of the lower Es layer around 90 km.The neutral winds above 100 km were provided by the ICON observations, which passed near Hainan during 12:13-12:20 UT, as shown in Figures 3c-3e.Note that the decreases in the observed altitude coverage were due to the satellite transitions from daytime to nighttime across the sunset terminator.The zonal wind shear with westward wind above and eastward wind below existed around 110 km, which could be responsible for the formation of the upper Es layer around 110 km.

Discussion
The QP-type E-region FAIs observed at low latitudes far away from magnetic equator were usually characterized as single-layer structures (e.g., Maruyama et al., 2006;Ogawa et al., 2002).Few scientific papers were focused on double or multiple FAI layers at this latitude.It is relevant to mention that at Gadanki (13.5°N, 79.2°E, mag.lat.6.5°N) which is close to magnetic equator, multiple FAI layers were observed (e.g., Pan & Rao, 2004).Due to a lack of interferometry, the spatial structures of the FAI layers in horizontal plane were not investigated.It was suggested that the FAI layers, which descended at altitudes with a rate of 1-3 km/hr, could be linked with the atmospheric tides (e.g., Patra et al., 2007).At low latitude Ledong, the occurrences of such double FAI layers closely depend on whether the lower layer appearing at an earlier time could sustain until the upper layer formed or not.Typical cases of the tidal layers succeeded and failed to form double-layer FAIs observed by the Ledong narrow-beam VHF radar are presented in Figure S1 in Supporting Information S1.Similar double-layer FAIs due to atmospheric tides could also be observed by the all-sky radar, and is exemplified in Figure S1c in Supporting Information S1.As can be seen, these layers are mainly manifested as continuous-type E-region FAIs.
For the present case, atmospheric tides were actually observed on the case day, which could be clearly seen in the meridional wind contour but was not significant in the zonal wind contour (Figures 3a and 3b).However, the atmospheric tide may not be the main factor driving the generation of the double FAI layers due to the following reasons.(a) The upper layer due to tidal wind shear over Ledong usually descends rapidly, and appears later than the lower layer.For example, in Figure S1a in Supporting Information S1, the upper layer descended from the altitudes ∼120 km at 18:15 UT to ∼111 km at 19:30 UT, with an average descending rate ∼7 km/hr.However, for the double-layer FAIs in the present case, such rapid descent was not seen, and the appearance disappearance of the two FAI layers were nearly synchronized.(b) The tidal FAIs are usually manifested as continuous-type irregularities with relatively weak echo intensity by the Ledong all-sky radar.However, the FAIs in the present case were deeply modulated as QP-type irregularities with strong echo intensity.Further, based on the all-sky radar observation from April 2020 to May 2022, we made a statistical study of double-layer E-region FAIs with similar QP feature to the present case.During the period, there were only 8 cases of double layers out of 770 days with available data, attaining a very low occurrence rate ∼1%.Table S1 in Supporting Information S1 lists the details of these events.These double-layer FAIs prefer to occur during summer nighttime.The upper and lower layers mainly resided at 100-120 and 90-100 km, respectively.All the double-layer FAIs were associated with double Es layers.The statistical study indicates that for all-sky radar observations, the double-layer FAIs in a pattern similar to the present case is uncommon.For the generation of E-region FAIs, multiple mechanisms have been proposed, such as the GDI, K-H instability, and GWs.However, it is lack of solid observational evidences to discriminate the dominant mechanism in controlling the generation and evolution of E-region FAIs which have complex dynamic characteristics.Investigations on the present double-layer E-region FAIs, with their temporal and spatial variations well resolved, may provide clues and implications for understanding the complex physical mechanisms at E region.
Generally, the small-scale E-region irregularities that could be observed by VHF radars are usually suggested to be generated by the GDI mechanism (e.g., Fejer et al., 1975;Kagan, 2002;Patra et al., 2007;Resende et al., 2016).Unlike the GDI driven by electric field at the latitudes close to magnetic equator, it is suggested that the GDI process could also be driven by neutral wind at middle and low latitudes, where the neutral winds could be the same one generating the Es layer (e.g., Kagan & Kelley, 1998;Yamazaki et al., 2022).The wind-driven GDI process was suggested as follows.(a) Under the presence of an Es layer, the ions are dragged by neutral wind u n while the electrons are strongly magnetized and keep motionless, leading to a primary charge separation and a Hall current.(b) In order to keep the continuity of the Hall current, a local polarization electric field δE is generated (e.g., Cosgrove & Tsunoda, 2001).(c) The initial plasma perturbation is amplified via δE × B 0 , driving the plasma depletion move into more dense plasma and plasma clouds move into regions of decreasing plasma density.This further enhances the instability in the direction along ▽N and generates FAIs.For the present case occurring at low latitude in the northern hemisphere, the magnetic field has a horizontal component B 0 pointing north.Under the presence of an Es layer, the plasma gradient ▽N is downward (upward) at the layer topside (bottomside).The westward (eastward) wind is unstable to the downward (upward) density gradient, and is favorable for launching the GDI process and producing FAIs at the Es layer topside (bottomside) (e.g., Kagan & Kelley, 1998).
However, different from the simulation by Kagan and Kelley (1998) where FAIs were generated at both sides of the Es layer, the present double-layer E-region FAIs were not likely generated at the two sides of one single Es layer, but resided in two vertically separated Es layers.Figure 4a illustrates the overall evolution of the FAI altitudes over time, which exhibited a strong correspondence with the height variation of the double Es layers.The FAIs mainly appeared on one side (top) of the Es layer.Figures 4b-4d shows the altitude variations of the FAIs within the zonal location 50 ± 15, 25 ± 15 and 75 ± 15 km away from the radar, respectively.At different zonal locations, the altitudes of both the upper and lower FAIs, in general, showed similar patterns to the upper and lower Es layers, respectively.This further indicates that the double-layer E-region FAIs in the present case were embedded in two separated Es layers rather than two sides of one single Es layer.Regarding the h'Es which seems a little lower than the FAI echoes, there could be two possible contributors.(a) The h'Es were manually scaled at the bottomside of the Es traces, whereas Es layers usually have a thickness of several kilometers.(b) The E-region irregularities were possibly generated at the topside of the Es layer under the wind-driven GDI process.
Regarding the QP features of the double-layer E-region FAIs, there are two possible drivers, the K-H instability and GWs (e.g., Huang & Kelley, 1996;Larsen, 2000;Yokoyama et al., 2004).According to previous simulations and observations, the QP patches caused by K-H instability are relatively short in horizontal separation, for example, smaller than 20 km (e.g., Bernhardt, 2002;Larsen, 2000).However, the horizontal wavelength of GWs at the E-region altitudes can normally reach tens or hundreds of kilometers (e.g., Fritts & Alexander, 2003;Vadas, 2007).In the present study, the separations between adjacent FAIs patches could be up to ∼70 km (e.g., 11:45 UT in Figure 1g), which were more likely to be modulated by GWs.Further, the simultaneous modulation by GWs could cover up to tens of kilometers in the vertical direction (e.g., Fritts & Alexander, 2003;Shalimov et al., 2009;Vadas, 2007).However, K-H instability generally cannot induce QP structures at separated altitudes at the same time under different ambient ion density and collision frequency (e.g., Bernhardt, 2002;Larsen, 2000), unless the K-H instability arises from unstable wind shear modulated by GWs (e.g., Venkateswara Rao et al., 2008).In the present study, the appearance and disappearance of the upper and lower FAI layers were nearly simultaneous (Figure 1a), and their manifestation as QP/wavelike structures at the two layers were nearly synchronized at similar zonal locations (Figures 1g-1i).The results indicate that the generation of the structures at the two layers could be under the same driving force, that is, GWs.Besides, according to previous studies, the slant Es trace in ionograms (e.g., 12:00 UT in Figure 2a) and the wavelike manifestation of the irregularity structure (e.g., 12:11 UT in Figure 1i) may indicate the possible presence of GWs (e.g., Resende et al., 2023;Sun, Li, Han, et al., 2023).Therefore, comparing with K-H instability, GWs were more likely to provide simultaneous modulation on the two separated FAI layers in the present study.

Summary
We report a case of double-layer structures of E-region FAIs observed by an all-sky radar at low latitudes of China on 15 June 2020.Based on the interferometry analysis with all-sky field-of-view, the upper and lower layers of the E-region FAIs were located at the altitudes ∼90 and 110 km, respectively.The FAIs at both layers exhibited typical quasi-periodic features and were manifested as spatially separated patches or wavelike structures synchronously at the two layers.The neutral wind observations revealed favorable wind shears at two altitudes around 90 and 110 km, respectively, which could contribute to the generation of two vertically separated Es layers where the double-layer E-region FAIs could inhabit.It is suggested that the double-layer E-region FAIs could be generated via wind-driven gradient drift instability and modulated by gravity waves.The generation of doublelayer E-region FAIs, however, differed from the previous simulations where the double-layer FAIs were generated at the two sides of one single Es layer.The wavelike structures synchronized in the two layers provide solid evidence for the crucial role of GWs in modulating E-region irregularities.The paper also demonstrates the unique capability of the all-sky radar in resolving the tomography of complex E-region irregularity structures over a wide zonal region.With the multi-static all-sky meteor radar system currently being developed at Hainan, the regional, high temporal and spatial resolution neutral winds could be obtained.Using the fine-scale measurements of simultaneous neutral winds and E-region FAIs, together with numerical model simulations, a better understanding on potential factors controlling the complex dynamic characteristics of E-region FAIs could be achieved in future.

Figure 1 .
Figure 1.(a) The range-time plot of signal-to-noise ratio of the backscatter echoes observed by the Ledong all-sky radar on 15 June 2020.The white dashed lines mark the specific time intervals 11:45, 12:11 and 13:43 UT.The arriving angles and spatial locations of the backscatter echoes during (b)-(c) the whole period 11:30-16:00 UT, and (d)-(i) at specific time 11:45, 13:43 and 12:11 UT, respectively.The gray circles in (b and d-f) and the black dashed curves in (g)-(i) denote the locations satisfying the perpendicularity to the magnetic field, calculated using the IGRF-2015 model at 90-130 km with the magnetic aspect angle less than 0.5°.

Figure 2 .
Figure 2. (a) The ionograms at different time intervals showing the presence or absence of double Es layers.The red and green echoes represent O and X waves from the vertical direction, respectively.The red and blue arrows mark the upper and lower Es layers, respectively (b)-(c) The h'Es and ftEs variations of the double Es layers.

Figure 3 .
Figure 3. (a) The meridional and (b) zonal neutral winds observed by the Ledong all-sky radar with a temporal resolution of 15 min.The gray shaded area indicates power failure.(c) The orbital information of the ICON satellite.The arrows indicate the moving direction of ICON.The gray shaded area represents the FOV of ICON for wind measurement at 12:17 UT.The red dot and colored area denote the Ledong all-sky radar and the radar FOV for E-region FAI observation, respectively (d)-(e) The meridional and zonal neutral winds observed by ICON during 12:00-12:30 UT.Positive (negative) values indicate northward (southward) and eastward (westward) winds.The black dashed curves in the wind contours represent the wind speeds of zero.The black arrows indicate the wind shear favorable for Es generation.The red vertical dashed lines in (d)-(e) mark the duration when the FOV of ICON covered that of the all-sky radar.

Figure 4 .
Figure 4. (a) The altitude variation of the double-layer FAI echoes (b)-(d) The temporal variation of the echo altitudes at the zonal locations of 50, 25 and 75 km in the all-sky radar field-of-view, respectively.In panels b-d, the altitudes of the echoes with the largest SNR during every 5 min were employed.