Longitudinal characteristics of spread F backscatter plumes observed with the EAR and Sanya VHF radar in Southeast Asia

JOURNAL OF GEOPHYSICAL RESEARCH: SPACE PHYSICS, VOL. 118, 6544 6557, doi:10.1002/jgra.50581, 2013 Longitudinal characteristics of spread F backscatter plumes observed with the EAR and Sanya VHF radar in Southeast Asia Guozhu Li, 1,2 Baiqi Ning, 1,2 M. A. Abdu, 3 Yuchi Otsuka, 4 T. Yokoyama, 5 M. Yamamoto, 6 and Libo Liu 1 Received 3 June 2013; revised 4 September 2013; accepted 17 September 2013; published 1 October 2013. [1] The development of equatorial plasma irregularity plumes can be well recorded by steerable backscatter radars operated at and off the magnetic equator due to the fact that the vertically extended plume structures are tracers of magnetically north-south aligned larger scale structures. From observations during March 2012, using two low latitude steerable backscatter radars in Southeast Asia, the Equatorial Atmosphere Radar (EAR) (0.2 S, 100.3 E; dip lat 10.4 S) and the Sanya VHF radar (18.4 N, 109.6 E; dip lat 12.8 N), the characteristics of backscatter plumes over the two sites separated in longitude by ~1000 km were simultaneously investigated. The beam steering measurements reveal frequent occurrences of multiple plumes over both radar sites, of which two cases are analyzed here. The observations on 30 March 2012 show plume structures initiated within the radar scanned area, followed by others drifting from the west of the radar beam over both stations. A tracing analysis on the onset locations of plasma plumes reveals spatially well-separated backscatter plumes, with a maximum east-west wavelength of about 1000 km, periodically generated in longitudes between 85 E and 110 E. The postsunset backscatter plumes seen by the Sanya VHF radar are found to be due to the passage of sunset plumes initiated around the longitude of EAR. Most interestingly, the EAR measurements on the night of 21 March 2012 show multiple plume structures that developed successively in the radar scanned area with east-west separation of ~50 km, with however no sunset plasma plume over Sanya. Colocated ionogram measurements show that spread F irregularities occurred mainly in the bottomside F region at Sanya, whereas satellite traces in ionograms that are indications of large-scale wave structures were observed on that night at both stations. Possible causes for the longitudinal difference in the characteristics of radar backscatter plumes are discussed. Citation: Li, G., B. Ning, M. A. Abdu, Y. Otsuka, T. Yokoyama, M. Yamamoto, and L. Liu (2013), Longitudinal characteristics of spread F backscatter plumes observed with the EAR and Sanya VHF radar in Southeast Asia, J. Geophys. Res. Space Physics, 118, 6544 6557, doi:10.1002/jgra.50581. 1. Introduction [2] The Equatorial Spread F (ESF)/plasma bubble irregularity phenomenon has been extensively studied through observations of a variety of its manifestations, such as 1 Key Laboratory of Ionospheric Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. 2 Beijing National Observatory of Space Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. 3 Divisão de Aeronomia, Instituto Nacional de Pesquisas Espaciais, São Paulo, Brazil. 4 Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan. 5 National Institute of Information and Communications Technology, Tokyo, Japan. 6 Research Institute for Sustainable Humanosphere, Kyoto University, Uji, Japan. Corresponding author: G. Li, Key Laboratory of Ionospheric Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China. (gzlee@mail.iggcas.ac.cn) 2013. American Geophysical Union. All Rights Reserved. 2169-9380/13/10.1002/jgra.50581 spread echoes in ionograms [Abdu et al., 2012, and the references therein], backscatter plumes from VHF radar range-time-intensity (RTI) maps [Woodman and LaHoz, 1976], pronounced ion-density depletions observed by satellites, and magnetic field line elongated airglow depletions observed by optical imagers [e.g., Kil and Heelis, 1998; Otsuka et al., 2002]. The plasma bubbles and the associated irregularity plumes are known to result from nonlinear evolution of plasma instability by the generalized Rayleigh- Taylor (R-T) instability mechanism that depends on the external driving forces (e.g., neutral wind and electric fields) and the background ionospheric properties (e.g., flux tube integrated Pedersen conductivity) [Kelley, 1989; Sultan, 1996; Abdu, 2001, and the references therein]. When some initial seeding perturbations exist, the R-T instability may be excited at the upward density gradient region of the F layer bottomside, so that the density perturbations evolve into plasma depleted flux tubes that penetrate the F layer peak to the topside ionosphere, reaching altitudes as high as 1500 km or more, and extending to dip latitudes greater than ±15 [e.g., Otsuka et al., 2002; Kil et al., 2009]. That the towering radar plasma 6544

(a) Geo.Lat (deg) 20 15 10 5 0 5 10 Sanya magnetic equator EAR 95 100 105 110 115 Geo.Lon (deg) Geo. Lon (deg) (b) Height (km) 350 300 250 200 150 100 50 100 105 110 16 beams 5 beams 200 0 200 400 600 800 10001200 West Zonal distance (km) East (c) Height (km) EAR Sanya EAR Sanya 800 600 400 200 southward beam magnetic field 650 km 520 km northward beam 10 5 0 5 10 15 20 25 Geo.Lat (deg) Figure 1. (a) Locations and (b) beam directions of the Equatorial Atmosphere Radar (EAR) and Sanya VHF radar. (c) Magnetic field geometry depicting how different altitude (latitude) regions are connected to each other through the conducting magnetic field lines. Note that for the southward (northward) beam of EAR (Sanya radar), the 300 km altitude is connected to 520 km (650 km) altitude over the magnetic equator (apex altitude). irregularity plumes do indeed represent equatorial plasma bubbles has been confirmed through overwhelming evidence from observation by combinations of multiple instruments [e.g., Tsunoda and White, 1981]. The zonal scales of the plumes are dependent on the wavelength of the initial perturbations, which generally range from tens of kilometers to a few hundred kilometers [Ossakow, 1981]. The plumes occur most often during the postsunset hours driven dominantly by the evening prereversal enhancement in the eastward electric field (PRE) [Fejer et al., 1999; Yokoyama et al., 2004], which develops under the eastward thermospheric zonal wind and when the E region conductivity rapidly decreases so that a large westward longitudinal gradient of the conductivity is established across the terminator. They can occur successively at a large longitudinal span of more than 180 [Li et al., 2010] as the result of enhanced eastward electric fields associated with geomagnetic activity. The plumes can occur also later at night even near sunrise [e.g., Fukao et al., 2003]. [3] With respect to the occurrence characteristics of the equatorial plasma plumes (bubbles), much work has been done related to their global longitudinal and seasonal distributions. For example, in situ observations from satellites have revealed that in Southeast Asia, a high occurrence probability of plasma bubbles exists in equinoctial months. This can be well understood since the sunset terminator makes a good alignment with the local magnetic meridian, thereby enhancing the PRE and hence the ESF occurrence [Abdu et al., 1992; Su et al., 2008]. It is well known that, in general, the bubble occurrence on a global scale is correlated climatologically with the postsunset rise of the equatorial F layer due to the PRE [e.g., Fejer et al., 1999; Li et al., 2008; Oyekola, 2009]. However, from the viewpoint of day-to-day variability, the plasma bubble development is known to be related also to additional important factors other than the PRE. Measurements by transequatorial HF radio wave propagation, airglow imager, and coherent and incoherent scatter radars have revealed quasi-periodic occurrence of equatorial plasma plumes in the east-west direction, with zonal scales of a few hundred kilometers [e.g., Röttger, 1973; Tsunoda and White, 1981; Singh et al., 1997; Makela and Miller, 2008], which are far less than the longitudinal dimension of the PRE (>3000 km) [Saito and Maruyama, 2007]. Specifically, using east-west scans of the Advanced Research Project Agency Long-Range Tracking and Instrumentation Radar, Tsunoda and White [1981] found that electron density contours in the bottomside of F layer were altitude modulated with an average zonal wavelength of about 400 km, similar to the zonal spacing of the periodic plasma plumes. Considering the close association between the development of backscatter plumes and the presence of wave structures in the bottomside of F layer, the spatially separated plumes in the east-west direction are believed to be caused by the large-scale wave structures (LSWS). Tsunoda and Ecklund [2007] found that there is a beam-dependent appearance for the LSWS upwellings, which led them to suggest that the polarization electric fields associated with LSWS contributed to the local plasma vertical drift around sunset. From a statistical study of ionosonde data at two sites, Chumphon and Bac Lieu separated by ~740 km in longitude, Saito and Maruyama [2007] demonstrated a significant influence of LSWS on the sunset rise of the F layer. The strong variability of the virtual height of F layer (h F) detected in their study was related to whether ESF existed at one or both stations. However, the ionosonde is unable to diagnose some important aspects of the generation and evolution processes of equatorial plasma plumes. The differences in the characteristics of ESF that must be present in such a longitudinal range were not investigated. [4] In the present work, we have investigated the temporal and spatial evolutions of plasma irregularity plumes and their smaller scale longitudinal differences in Southeast Asia, using the beam steering capability of the two radars, the Equatorial Atmosphere Radar (EAR) located at Kototabang (0.2 S, 100.3 E; dip lat 10.4 S) and the Sanya VHF radar (18.4 N, 109.6 E; dip lat 12.8 N), that were operated in an observational campaign conducted in March 2012. The two radars are separated in longitude by 9.3 (~1000 km, Figures 1a 1b). It is worth pointing out that due to the magnetic north-south aligned characteristics of the plasma depletions, both radars (in opposite hemispheres) can detect in sequence the plume structures associated with the zonally drifting bubbles that rise to high enough altitude. Based on 6545

Table 1. The Parameters of Equatorial Atmosphere Radar (EAR) and Sanya VHF Radar Used for the Present Observations of Equatorial Plasma Plumes Value Parameter EAR Sanya Location 0.2 S, 100.3 E 18.4 N, 109.6 E Frequency 47 MHz 47.5 MHz Peak power 100 kw 24 kw Pulse repetition frequency 250 Hz 160 Hz Number of coherent 1 4 integrations Range resolution 2.4 km 4.8 km Time resolution 1.5 min 5 min Beam directions 125.0, 37.5) (130.0, 34.3) (45, 33 ) (azimuth, zenith) (137.0, 30.9) (144.0, 28.4) (32, 29 ) (151.0, 26.6) (158.0, 25.3) (0, 23 ) (165.0, 24.5) (172.0, 24.0) (328, 29 ) (180.0, 23.8) (188.0, 24.1) (315, 33 ) (195.0, 24.7) (202.0, 25.7) (209.0, 27.2) (216.0, 29.3) (223.0, 32.1) (230.0, 35.9) the IGRF-2010 model, an altitude of 300 km over the EAR and Sanya stations can be mapped to magnetic equatorial apex heights of 520 km and ~650 km, respectively (Figure 1c). In the beam steering mode of operation, the scanned area at a height of 300 km covers approximately 360 km and 280 km in east-west direction for the EAR and the Sanya radar, respectively (Figure 1b). Thus, the beam steering measurements by the two radars provide a good spatial coverage and can be used to study the occurrence and dynamics of equatorial plasma plumes in the region of Southeast Asia and possible short longitudinal scale differences in their characteristics. Understanding the spatial structures of plasma plumes and their variations with longitude is very important for a better understanding of the ESF vis-à-vis, the LSWS since recent observations revealed that the ESF do not always occur following the appearance of LSWS at a single site [e.g., Narayanan et al., 2012; Li et al., 2012]. In this study, we present observations of periodic backscatter plume structures with the EAR and Sanya radar during geomagnetic quiet days and examine the mechanism responsible for the generation of these structures. We use large scale and small scale to describe structures with the wavelength of several hundreds to one thousand kilometers and tens of kilometers, respectively. The overall analysis focuses mainly on the following issues: (1) Can the initiation of sunset plasma plumes and the premidnight periodic plasma plume structures, which is often observed at a single site, be continuously detected at two longitudes separated by about 1000 km? How the periodic plume structures seen in RTI maps of the two radars depend on longitude. (2) Whether there are any differences in the generation of sunset plasma plumes at stations closely spaced in longitude and what can cause such differences. 2. Instrumentation [5] The Sanya VHF radar, with an operating frequency of 47.5 MHz and a peak power of 24 kw, has the capability to detect 3 m scale field-aligned irregularities of the low-latitude E region, and of the valley and the F region [Li et al., 2011]. The radar beam can be steered within ±45 in azimuth around north. The antenna pattern, with a 3 db beam width of 10 in east-west and 24 in north-south, satisfies the radar beam-magnetic field line perpendicularity condition in the height regions of interest. During the March 2012 observations, the radar beam was steered among five directions from east to west through north. Unlike fixed beam radar measurements, the multiple beam steering observations can uniquely distinguish between a locally forming backscatter plumes from a plume that drifted from west and can obtain the accurate drift velocity without slit-camera interpretation. Details of the observational parameters for Sanya radar are listed in Table 1. [6] The EAR operates at 47 MHz with a peak power of 100 kw and a 3 db beam width of 3.4. The EAR uses an active phased array antenna system and can steer the beam on a pulse-to-pulse basis. The rapid beam scanning ability enables the EAR to detect the temporal and spatial variations of backscatter plumes [Fukao et al., 2003]. In March 2012, the radar was operated with two sets of an eight-beam mode. Combining the backscatter echo intensity from all 16 beams summarized in Table 1, a two-dimensional map of equatorial plasma plumes in a fan sector can be obtained every 2 3 min. [7] At the EAR station, ionograms were received also from the Southeast Asia low-latitude ionospheric network. At Sanya, the colocated digital ionosonde (Digital Portable Sounder 4d, DPS-4d) was operated every 5 min to obtain an ionogram. The satellite traces shown in ionograms [Abdu et al., 1981] are oblique echoes, produced by reflections from a tilt in isodensity contours associated with LSWS [Tsunoda et al., 2013]. The Spread F types and the occurrence of satellite traces in ionograms were manually identified. The virtual heights of F layer (h F) at5mhzweremanuallyscaled. 3. Results [8] Out of 26 nights of simultaneous observations by the EAR and the Sanya radar in March 2012, equatorial plasma irregularity plumes have been detected on 15 nights. Out of the 15 plume events, we found 12 events in which plasma plumes were observed by both radars and three events in which plasma plumes were observed only by one of the radars. Most days of the month are geomagnetically disturbed days. In order to eliminate the geomagnetic storm effects on the ESF plume development, we have selected two examples typical for geomagnetic quiet days to present here: 21 and 30 March 2012. 3.1. Observations on 30 March 2012 [9] Figure 2 shows an example of equatorial plasma plume event observed on 30 March 2012. A series of isolated plume structures were observed by both radars. The Kp indices on that day were 3, 2, 1+, 1, 1,1, 0, and 0+. Backscatter echo intensity profiles obtained from the EAR and Sanya radar on that day are shown in Figures 2a 2e and 2f 2j, respectively. These height-time-intensity (HTI) plots show the signal-to-noise ratio (SNR) as a function of height and universal time (UT). The black solid and red dashed lines represent the local and apex sunset terminators (the sunset terminator at the apex altitude of the geomagnetic field line connected with the observed area), respectively. The superposed red vertical axes show apex altitude. The local time 6546

Figure 2. Height-time-intensity (HTI) plots of backscatter plume echo observed with the (a e) EAR and the (f j) Sanya radar during 1100 2000 UT on 30 March 2012. The superposed red vertical axes indicate apex altitudes. The black solid and red dashed lines in the left of each panel indicate local and apex sunset terminators, respectively. The beam direction (azimuth and zenith) is shown in the right top of each panel. The zones of rapidly intensified backscatter echoes within plume group A are produced by plasma bubble bifurcation. The well-separated plume groups (marked with A to D and A to C ) inhtimaps,corresponding the spatially separated plumes in the east-west direction, are triggered by large-scale wave structures (LSWS). for EAR (LT = UT + 6 h 41 min) and Sanya (LT = UT + 7 h 18 min) stations are shown Figures 2e and 2j, respectively. It is relevant to mention that for EAR observations, only echo measurements from five selected beams are shown in the figure. The azimuth and zenith angles for each beam are shown in the right top of each panel. [10] As seen in Figure 2, in general, the morphology of backscatter plume groups observed by the EAR, in a periodic behavior, is quite similar to that of the plume groups observed at Sanya. The isolated, periodic plumes without having bottomside structures are divided into several groups. For the EAR, the backscatter plumes that occurred around 1230 1350 UT, 1330 1500 UT, 1545 1645 UT, and 1700 1800 UT are labeled A, B, C, and D shown in Figures 2a and 2c, respectively. For the Sanya radar, the backscatter plumes that occurred around 1200 1300 UT, 1400 1610 UT, and 1700 1800 UT, are labeled A, B, and C shown in Figures 2f and 2h, respectively. Two features of interest may be noted here. Notice first that for group B, C, B, and C, the plume structures appeared from the west beam and were sequentially detected by other beams to the east. This indicates that the initial appearance of plasma plumes must have occurred to the west of the radar field of view. They drifted eastward (as indicated by the slant dashed lines) into and out of the radar beam following their development at a westward longitude. By the time the plume groups B and C passed over the EAR, however, the smaller plasma plume D disappeared in the radar field of view. Earlier radar observations at equatorial and low-latitude sites have shown that the periodic plume structure is a regular feature. Observations from Gadanki [Patra and Phanikumar, 2009], Kototabang [Fukao et al., 2006], and Kwajalein [Hysell et al., 1994] revealed that periodic plume structures could be observed during postsunset and postmidnight hours. Specifically, Hysell et al. [1994] observed many plume structures, as large as 11 in number, at Kwajalein. The present observations by the EAR and Sanya radar show that somewhat similar periodic backscatter plume groups (A to D, and A to C ) can be detected at two stations separated by about 1000 km in longitude. It is of further interest to note that the overall intensity of the eastward drifting plume structures show a gradual decay from the westernmost to the easternmost beam, both in the EAR and Sanya radar results. This may indicate the rate of decay of the 3 m scale irregularities in their eastward drifting phase (that follows their initial growth phase). The zonal drifts of plasma plumes observed by both radars are estimated according to the plume pattern time delay and the horizontal separations of the easternmost and 6547

Figure 3. Fan sector maps of the EAR backscatter echo power observed between 1920 LT and 1947 LT on 30 March 2012. The secondary plume A2, which initiated from the primary plume A1, was produced by bifurcation. westernmost beams around 300 km altitude. The zonal drift velocities of plume structures B, C, D, B, and C are approximately 110 ms 1,100ms 1,55ms 1, 105 ms 1, and 95 ms 1, respectively. Using the time it takes for a plume to drift through the radar beam and the drift velocity, the zonal dimensions of the plumes B, C, B, and C are estimated to be about 600 km, 350 km, 850 km, and 580 km in the east-west direction, respectively. [11] Another feature is the beam-dependent appearance of the backscatter plumes. The plumes labeled as A and A were not detected by the west beams of EAR and Sanya radar, respectively. In contrast to the feature of plume groups B to D and B to C (that the corresponding echo strength is most intense in the westernmost beam), the intensity of the irregularity plumes (A and A ) near the apex sunset terminator is the strongest in the easternmost beam decreasing in intensity toward the western beams where the influence of solar radiation and hence the E layer conductivity tends to increase as to be expected. As is evident from Figures 2a and 2f, both the plumes AandA initiated at a height of ~250 km, and at around 1930 LT which corresponds to apex sunset time. This feature agrees with the statistical results presented by Yokoyama et al. [2004] that the plume structures are exclusively generated around apex sunset. Although the onset and beam-dependent characteristics of the plume structure A are broadly similar to those of the plume structure A, the multiple plumes (the zones of rapidly intensified backscatter echoes) within plume structure A are additional features. Such multiple plumes within a plume group shown in HTI map are often suggested to be due to bifurcation of larger equatorial plasma bubbles. To see the plasma plume bifurcation more clearly, temporal variations in the spatial distribution of plume group A, as represented in fan sector maps, are shown in Figure 3. Each map in this figure is constructed by combining the echo intensity from all the 16 beams of EAR. The first map, taken at 1920 LT, shows a backscatter plume (labeled A1) that appeared at the east edge of the fan sector, near 300 450 km. A noticeable change in the bottomside of the western wall of plume A1 is seen at 1927 LT. The enhanced backscatter region may be a developing head of a new plume. As is evident from the maps taken at 1927 and 1930 LT, the bottomside structure extended upward to an altitude more than 400 km, 6548

Figure 4. Same as Figure 2, except the data obtained during 1100 2000 UT on 21 March 2012 are plotted. No sunset plume was detected by the Sanya radar. indicating the appearance of a new developing backscatter plume (identified as A2). We believe that the appearance of this secondary plume A2 did result from the primary plume A1 seen at 1920 LT. The two plumes evolved to look like a Y shaped structure shown in the maps taken between 1940 and 1947 LT. With regard to the plume bifurcation, there are two possible ways: If a plume stream is significantly wide, it would suffer bifurcation due to differential motion, or if a plume stream encounters an obstruction, it could also lead to the bifurcation [Aggson et al., 1996]. [12] On the other hand, the beam-dependent appearance of the sunset plasma plumes near the terminator detected by both radars could be associated with the beam dependence of sunset upwelling of bottomside F layer. Using the PRO radar multiple beam steering measurements, Tsunoda and Ecklund [2007] found a close relationship between the upwelling layers and radar backscatter plumes. They suggested that both the modulated upwelling depth and beam-dependent plasma plume structures were produced by LSWS. The satellite traces and multiple reflected echoes on ionograms are evidence for the existence of large-scale wave structures in ionization and can be taken as a signature of LSWS at the F region heights [Tsunoda et al., 2013, and the references therein]. Further, it has been identified that the satellite traces can occur simultaneously at low-latitude conjugate points [Abdu et al., 2009b]. By examining the digisonde observations made on 30 March 2012, multiple satellite traces are found to occur near sunset at both radar sites (Figure is not shown here). The presence of satellite traces could indicate that the upwellings or crests of LSWS occurred around the Sanya and EAR longitudinal sectors. Considering that the LSWS is a necessary precursor for the occurrence of plasma plumes [e.g., Tsunoda and Ecklund, 2007; Abdu et al., 2009; Li et al., 2012; Patra et al., 2013], the sequences of the sunset plasma plumes and the periodicity of plume groups simultaneously detected by the EAR and Sanya radar may suggest a wide span of consecutive LSWS. 3.2. Observations on 21 March 2012 [13] In the case presented above, we described the periodic occurrence of plasma plumes that have comparable features as detected by both radars. We will now present a case of significant difference in the backscatter plume occurrence over the EAR and Sanya stations. Figures 4a 4e and 4f 4j show the HTI maps of the EAR and the Sanya radar echoes observed on 21 March 2012, respectively. The Kp indices on this day were 1+, 1, 1,1,2, 1, 2+, and 2. We note that the backscatter plumes detected by the EAR during 1250 1420 UT, as shown in Figures 4a 4e, are significantly intense. The echo SNR, which trace the gradients in the background plasma density, is more than 30 db near the center of each plume structure. Note that the plumes generally attained a maximum altitude of 400 km (apex altitude ~630 km). Further, the plume shown in Figures 4d 4e extended upward to an altitude higher than 450 km, corresponding to an apex altitude of ~700 km. The HTI plots (Figures 4f 4j) from the Sanya radar do not, however, exhibit presence of any plasma plume near sunset. In contrast, postmidnight descending thin irregularity echo layer was observed by the west beams of Sanya radar (but not over EAR). Such postmidnight echoes were often observed with 6549

Figure 5. Same as Figure 3, except the data obtained between 1928 LT and 2028 LT on 21 March 2012 are plotted. The multiple structures labeled 1 to 4 are individual plumes separated by ~50 km in east-west direction. the EAR during June solstice of the solar minimum period [Yokoyama et al., 2011]. A comparison of postmidnight echoes between EAR and Sanya stations should be investigated in the future. It is worth pointing out that if sunset equatorial plasma plume developed upward to ~600 km at the magnetic equator over Sanya longitude on 21 March (as that observed over EAR), it should be detected by the Sanya radar. The absence of sunset plumes by the Sanya radar indicates that no sunset equatorial plume was generated over Sanya longitude. Further, the colocated ionogram measurements showed that the bottom type spread F did not evolve into strong range type spread F (which will be shown in Figure 6). [14] Notably, the plume patterns shown in Figures 4a 4e are significantly different from those well-separated plume groups observed on 30 March 2012 (Figure 2). In this case (on 21 March 2012), a series of plumes that are closely spaced to each other in time were observed. A time gap of several minutes between successive plumes was detected. At a first glance, the multiple closely located plasma plumes in HTI map may appear to be a case of a plasma plume bifurcation. However, a careful look on the temporal and spatial variations of the plumes suggests that the multiple structures are individual plumes separated by ~50 km in east-west direction. The earlier plumes did not participate in the generation process that produced new plumes (as indicated also by the dashed slant lines in Figures 4a 4e). A sequence of fan sector maps of the radar echo power made at close intervals during 1928 2028 LT presents a more clear-cut evidence for this. As shown in Figure 5, the fan sector map at 1928 LT shows that a weak echo was present in the EAR field of view around 300 km altitude. In the next two maps taken at 1931 and 1935 LT, the strengthened old echo, labeled as 1, together with a newly generated stronger echo (labeled 2), 6550

Figure 6. Ionograms observed on the nights of 21 March 2012 at Sanya. The arrows show the appearance of satellite trace close to the main and second hop of F layer traces, indicative of the appearance of LSWS. attests to the stronger irregularity growth conditions. The map taken at 1941 LT shows two more new echoes, one (labeled 3) generated within the scanned area and the other (labeled 4) moved in from outside of the fan sector. The east-west spacing among the plumes 1 to 4 is about 50 km. Examination of the maps taken at later times 1945 1951 LT revealed that the tilt of the echoing regions 2 4 is generally identical (~ west of vertical) to each other. Such tilted structures are often attributed to the vertical shears of F region zonal drifts [Zalesak et al., 1982]. As is evident from the map taken at 2002 LT, the plume 4 penetrated up to much higher altitudes than the other plumes shown in the fan sector maps. After 2012 LT, one may note that the structure of plume 4 presents a reversal of the initial westward tilt to become increasingly eastward below ~380 km, while above this altitude, the westward tilt progressively increased. This produced a reversed C shaped plume. It is relevant to point out that the present C shape is different from that seen in the optical data which comes from the mapping of the equatorial plasma plumes (above the F peak) to both hemispheres [e.g., Kil et al., 2009]. The development of east-west spaced plumes separated by 50 km as seen in Figure 5 may reveal the presence of small-scale wave-like seed structure in the bottomside F layer. [15] As mentioned above, the backscatter plume occurrence was quite different from one site to the other on 21 March 2012 in that no postsunset plasma plume was detected by the Sanya radar. To examine if there is any bottomside spread F or LSWS at Sanya, we present the colocated ionogram measurements on that night. Figure 6 shows a time sequence of ionograms on the night of 21 March 2012. As can be seen from the first ionogram taken at 1230 UT, no satellite trace, indicative of LSWS, was observed over Sanya. However, 10 min later (1240 UT), a satellite trace (marked with arrows) was found near the first and second hop of F layer trace. The range separation between the satellite trace and the main F layer trace is about 50 km. This is a normal one since earlier measurements from Ibadan show that the range separation varied between 25 and 160 km with an average of 80 km [Lyon et al., 1961]. The feature that the satellite traces appeared only at a short frequency interval could indicate the modulation in plasma density (associated with LSWS) was limited to a small altitude range with a small deviation in the plasma frequency with respect to the background ionosphere. On the other hand, what appears to be an initiation of bottomside spread F (BSF) in the ionograms taken at 1250 UT, with the echo range and top frequency of spread traces less than 100 km and 9 MHz, respectively, did 6551

Table 2. The Parameters of Backscatter Plume Groups A to D and A to C Used for Estimating the Onset Locations of the Plumes in East-West Direction a Stations Plumes T 0 (UTh) V 0 (ms 1 ) Value East-West Dimension (km) EAR A 12.9 * * B 14.2 110 600 T sun 12.4 UTh C 16.2 100 350 D 17.5 55 * Sanya A 12.5 * * B 15.0 105 850 T sun 11.9 UTh C 17.5 95 580 a T sun, T o, and V o are sunset time (300 km altitude), the time when the plume was detected, and the eastward drift velocity of the plume over the radar site, respectively. The asterisks represent plume initiated or decayed within the radar scanned area and the associated parameters cannot be resolved from the data set. not evolve further in the subsequent ionograms. By ~1340 UT, the BSF totally disappeared. This indicates that the irregularities that caused the BSF over Sanya did not evolve into postsunset strong range type spread F on this evening. Also, it may be noted that the ionograms taken at 1730 UT (Figure 6, bottom right) showed the appearance of postmidnight spread F. While in the 1250 UT ionogram (near sunset), the top frequency of the spread trace extended to ~9 MHz, in the postmidnight ionogram, it was more than 13 MHz. This ionogram feature is consistent with the presence (absence) of postmidnight (sunset) backscatter echo shown in Figures 4f 4j. It is relevant to point out that the postmidnight spread F (that is seen as a single descending echo layer in radar HTI maps) is not induced by processes related equatorial plasma bubbles but seems to be associated with the middle latitude type spread F, which could be triggered by medium-scale traveling ionospheric disturbance [Yokoyama et al., 2011]. The ionograms taken over the EAR site (not presented here) show that multiple satellite traces were present at 1225 UT after which both the range type spread F and plasma plumes were detected. From these observations, it seems unlikely that the longitudinal difference in plasma plume occurrence between the two sites can be ascribed simply to the appearance or not of LSWS. 4. Discussion [16] In the preceding sections, we presented case studies of two events of backscatter plumes, one on 30 March and the other on 21 March 2012, observed with the EAR and the Sanya VHF radar that are separated in longitude by ~1000 km. It is noted that on 30 March 2012, the sunset backscatter plumes, preceded by LSWS, was concurrently detected by both radars around 1920 2020 LT. This was followed by periodic backscatter plume structures with similar characteristics at the two sites, which continued to exist until midnight with inter periods of about 30 60 min. For the event of 21 March 2012, however, the plume occurrence is significantly different from one site to the other. Different from the EAR observation that showed the bottomside spread F evolving into multiple backscatter plumes (separated by 50 km in the east-west direction) after sunset, the bottomside spread F (shown in ionograms) over Sanya promptly decayed and disappeared, not developing to topside and forming backscatter plumes. On the other hand, postmidnight irregularity structure in the form of thin descending layer was observed over Sanya with no irregularities present over the EAR site. These results illustrate the existence of significant regional scale differences in the plasma plume generation and evolution, and the potential for studying them using the two closely spaced radars. Possible causes responsible for the similarities or the differences in the characteristics of equatorial backscatter plumes are discussed in the following section. 4.1. Zonal Structure of Periodic Backscatter Plumes [17] From a statistical study, Yokoyama et al.[2004]demonstrated that the plasma plumes initiate exclusively near apex sunset during geomagnetic quiet conditions. The radar backscatter plumes observed later at night might be due to the passage of the sunset plume structures that were generated westward of the radar and then drifted eastward into the radar beam. The periodic backscatter plumes as usually seen by a single beam radar HTI map may represent equatorial plasma irregularity formations spatially separated in east-west direction, through an interpretation of slit-camera or frozen-in assumption of plasma plumes. Because the eastward drift velocity and duration of plasma plumes are generally on the order of 100 ms 1 and 3 h, respectively, parts of the plumes would decay and disappear before being seen by a radar far away from the onset location. With the scanning ability of the EAR and the Sanya radar, a much larger span of plume structure in east-west direction can be obtained without the slit-camera assumption. Coming back to the periodic plume structures observed on 30 March 2012 (Figure 2), we note that the structures A and A were generated within the radar scanned area, and the others were due to spatially separated plasma plumes moving into the radar field of view by their eastward drifts. To investigate the zonal irregularity structures associated with the generation of plasma plumes, we used the method developed by Fukao et al. [2006] to trace the generation location of periodic backscatter plumes. Briefly, through assuming that the plume has an eastward drift velocity of V g at the initial stage (T g ) and the velocity linearly decay, the onset location D g of the plume structure can be determined using the equation D g ¼ V sunðt o T sun ÞðV o þv g Þ=2,where ðv o þv g Þ=2 V sun V sun is the westward velocity for the sunset terminator, about 464 ms 1, T sun, T o,andv o are sunset time, the time when the plume was detected, and the eastward drift velocity of the plume over the radar site, respectively. The V g can be estimated from the linear regression of drifts and V o at radar site. By using the linear regression value (about 12 ms 1 /h) presented by Fukao et al. [2006] and the characteristic parameters (T o and V o )ofbackscatterplumes observed by the EAR and Sanya radar (summarized in Table 2), we estimated the onset locations of the periodic plumes observed on 30 March 2012. [18] As shown in Figure 7, the symbols A to D, A to C correspond to the plume structures shown in Figure 2. Interestingly, the plume structure B detected by the Sanya radar and the plume A detected by the EAR are found to 6552

West Zonal distance (km) East 1600 1200 800 400 0 400 800 1200 1600 20 Sanya 15 Geo.Lat (deg) 10 5 0 D C C B A B EAR A 5 Height 10 LSWS upwelling 90 95 100 105 110 Geo.Lon (deg) Figure 7. Onset locations of equatorial plasma plume groups A, B, C, and D (marked as ), and A, B, and C (marked as ). The symbols A to D and A to C correspond to the plume groups shown in Figure 2. The horizontal bar shows the east-west dimensions of the plumes (also shown in Table 2). Zonal distance is shown in the top axis. The dashed-rectangle indicates the plume groups B and C (A and B ) initiated around the same longitude ~93 E (~100 E), with a zonal distance of ~1800 km (~1000 km) away from Sanya. A schematic illustration representing the upwellings or crests of LSWS is shown at the bottom of the plot. initiate from the same longitudinal sector (100 E). This suggests that the premidnight plume structure (B ) observed over Sanya is the continuation of the generation and dynamics of the sunset plasma plume (A) over EAR. Based on the bifurcation of plume group A shown in Figures 2 and 3, together with the numerical simulations by Huang and Kelley [1996] showing that the bifurcations occur when the plume dimension is large and do not when it is small, the sunset plume group A may be a big plume. This is supported by the eastwest dimension of plume B, about 800 km, estimated from the echo duration in HTI map (about 2 h 10 min shown in Figure 2h) and the zonal drift (105 ms 1 ). Further, one may note that the zonal distance between EAR and Sanya stations is only about 1000 km. Such a big plume structure with eastwest width of ~800 km, should be detected by Sanya radar shortly after its onset at EAR site, as evidenced by Figure 2 showing that the sunset plume A initiating around 1230 UT (in the easternmost beam of EAR) was detected by the westernmost beam of Sanya radar near 1310 UT. On the other hand, it is quite possible that the plume groups B and C originated from the same longitude (~93 E) since only four groups of plumes were detected by EAR. The periodic plume structures, with maximum separation of about 1000 km, covered longitudes between 85 E and 110 E. Also, we note that in this case, the plumes initiated at the west of 90 E disappeared before being seen by the Sanya radar. [19] With regard to the source mechanisms responsible for the present plume structures spatially separated in longitude, the seed perturbations associated with LSWS are suggested to play important roles on the plume development [Tsunoda and Ecklund, 2007]. The perturbations were amplified by the R-T instability under favorable conditions and evolved into towering plumes seen in radar HTI maps. In this context, it may be mentioned that on 30 March 2012, satellite traces (indicative of LSWS) appeared in ionograms at both the EAR and Sanya stations (not shown here) before the generation of sunset backscatter plumes. This probably indicates that the spatially separated plume structures might grow from the upwellings or crests of consecutive LSWS in longitudes 85 110 E. If so, the interdistances of the plume structures in longitude could be similar to the wavelengths of LSWS. A schematic illustration representing the LSWS is shown in the bottom of Figure 7. It may be pointed out here that the equation for the irregularity plume onset location D g discussed above assumes that the plume initiation always occurs at the apex sunset region, which appears to be a general statement. In specific cases such as in the western radar beams in Figure 2 such plume initiation is not observed. Therefore, the estimation of the D g as presented in Figure 7 can suffer some degree of variability. Recent results from the GNU radio beacon receiver (GRBR) network support the importance of the LSWS in the formation of spatially separated plume structures. Using the GRBR total electron content measurements in Southeast Asia, Thampi et al. [2009] reported the presence of LSWS in the longitudinal sector 95 120 E, with zonal scales on the order of several hundred kilometers. More recently, Tulasi Ram et al. [2012] developed a method to derive the characteristics of zonal LSWS on a regular basis and pointed out that the GRBR network lays an excellent platform to study the LSWS. In this respect, simultaneous measurements from the GRBR network, the EAR and the Sanya radar may provide direct evidences for seed role of consecutive LSWS on the spatially separated backscatter plumes in Southeast Asia. While considering the generation of LSWS, two possibilities exist, the collisional shear instability (CSI) [Hysell and Kudeki, 2004] that is driven by a velocity shear in the presence of a density gradient near sunset, and the large-scale polarization electric field that is 6553

Height (km) 400 350 300 250 the virtual height of the bottomside of F layer at 5 MHz 21 March (EAR) 30 March (EAR) 30 March (Sanya) 30 March (Chiang Mai) 21 March (Chiang Mai) 21 March (Sanya) 18 19 20 21 LT (hours) Figure 8. Variations of the virtual height (h F)ofF layer at 5 MHz observed on 21 and 30 March 2012. The asterisks, crosses, and solid dots represent the measurements at EAR, Chiang Mai, and Sanya stations, respectively. generated by gravity waves (GW) [e.g., Abdu et al., 2009a; Tsunoda, 2013]. In terms of the horizontal scales (hundred kilometers) of periodic plume structures and the onset time of satellite traces on 30 March 2012, both the CSI and GW could seed the R-T instability and produce the observed zonal structures of backscatter plumes. [20] In this context, it needs to be noted that the plume generation near the apex sunset (originally discussed by Yokoyama et al. [2004]) is evident in the eastern beams of both the EAR and Sanya radar. But, interestingly, such plume initiation does not occur in the western beams of both radars. This has to do with the fact that the instability growth favored by the combined presence of the PRE and the LSWS is operative in the eastern beams of both radars simultaneously. In view of the fact that the PRE has much larger zonal scale than the LSWS and is tied to the local time, whereas the LSWS, originating from zonally propagating gravity wave perturbation (of lesser scale sizes), has no clear coherence with the PRE, the simultaneous plume onsets in the eastern beams would suggest that the LSWS crest region happened to occur in those beams at both the radar sites also simultaneously. The LSWS crest appears to have been shifted unfavorably with respect the PRE for the western beams of both radars where plumes did not develop. This might suggest that there is an integral number of gravity wavelengths in the 1000 km separation between the EAR and Sanya radar. 4.2. Longitudinal Difference in the Occurrence of Sunset Backscatter Plumes [21] Returning to Figure 4, it is noted that on 21 March 2012, over the EAR, the plumes attained an apex altitude of ~700 km around 1310 UT, but over Sanya no sunset plumes were detected. Further, the plumes observed over EAR on 21 March decayed at earlier local time (as shown in the bottom panels of Figure 5) than they did on 30 March and did not propagate enough eastward to be observed over Sanya. This could indicate that the background condition to initiate/sustain radar plumes on 21 March was favorable only around EAR longitude near sunset. On the basis of our current understanding derived from model and observational studies, the generalized R-T instability mechanism is believed to be responsible for the generation and growth of ESF. The primary causes for the equatorial plasma plume development at a given longitudinal sector can be stated briefly intermsof(1)evening vertical plasma drift and height of the F layer ascribed to the prereversal enhancement of zonal electric field (PRE); (2) density perturbations for seeding the R-T instability at the bottomside F layer; and (3) the flux tube integrated Pedersen conductivity controlled by thermospheric meridional winds (TMWs). The TMW, whether directed northward or southward, has the potential to suppress or even totally inhibit the nonlinear development of the instabilities to form postsunset ESF plumes [Maruyama, 1988;Abdu et al., 2009b], even when the linear growth rate of R-T instability is sufficiently positive for the initiation of bottomside irregularities. This could indicate the presence of weak or nearly zero meridional wind on the two evenings of 21 and 30 March over equatorial latitude at the EAR longitude, where the meridional wind during the equinoctial month (March) is expected to be very weak, as verified also from the horizontal wind model 93 (HWM93) [Hedin et al., 1996]. The HWM93 also shows that the TMWs at the longitudinal sectors of EAR and Sanya (separated by ~1000 km in longitude) are very similar. Therefore, it is unlikely that the absence of equatorial plasma plumes on 21 March 2012 (Figure 4) over Sanya longitude can be caused by an enhanced TMW. Even so, the possible role of the meridional component of a zonally propagating gravity wave wind in modulating the TMW and thus to contribute to the h F variation cannot be excluded. [22] To investigate the possible dependence of backscatter plumes on sunset F layer height increase, we present in Figure 8 the variations of the virtual height of F layer (h F) at 5 MHz, on 21 and 30 March, over EAR and Sanya. Also shown in this figure are the h F measurements from Chiang Mai (18.8 N, 98.9 E; Dip lat 12.7 N) ionosonde, which is located at the same latitude as Sanya but at the longitudinal sector of EAR. It is evident from Figure 8 that the evening h F variations at Sanya (solid dots) and Chiang Mai (crosses) are identical on 30 March 2012, but the h F over the EAR station (asterisks) was 50 km higher than that over Sanya and Chiang Mai. In view of the nearly similar geomagnetic locations (dip angle) of the EAR and the Chiang Mai (Sanya) sites, the difference between their h F values (such as that seen on 30 March 2012) may not result from any E B drift difference at the two different geographic latitudes. The lower h F values of Sanya and Chiang Mai should be explained mainly by the meridional component of the TMW that must be significant and poleward at their lowlatitude (approximately 20 N) locations as can be verified from the HWM93. Another important feature shown in Figure 8 is that the sunset h F values observed at Chiang Mai were significantly higher than those at Sanya on 21 March 2012, the maximum difference being about 50 km (and also slightly different from its values on 30 March). It is important to note that on 21 and 30 March, when backscatter plume structures were detected, the h F values over the EAR station are identical. The apparent connection between the h F variation and backscatter plume occurrence as seen in the results on 21 and 30 March 2012 over Sanya and the EAR might call attention to a dependence of plasma plumes on the postsunset rise of the F layer (the above item 1). The rapidly rising F layer at the longitudinal sector of EAR, favoring the growth of R-T instability due to lower ion-neutral 6554

collision frequency, becomes unstable to density perturbations leading to the growth of equatorial plasma plumes. However, the h F difference may not have a proportional impact on the longitudinal occurrence of bottomside spread F. Then a question arises as to what caused the sunset h F difference between the two longitudinal sectors of EAR (Chiang Mai) and Sanya. [23] According to theoretical studies, the PRE that drives the sunset F layer to high altitudes is controlled mainly by two factors, the thermospheric zonal wind (TZW) and the longitudinal gradient in the integrated E layer conductivity near sunset. Due to the same magnetic declination angle and the magnetic equator offset from the geographic equator in Southeast Asia, and the close longitudinal proximity of the EAR and Sanya stations, the PRE should be, in the first order, identical for the two stations on the same day. Thus, it seems unlikely that the h F difference (on 21 March) in the two longitudinal sectors can be explained by the background PRE. As pointed out above, due to their large latitudinal separation from the geographic equator, the Sanya/Chiang Mai sites should experience significant meridional wind which is normally poleward in the evening hours so that the h F should be generally reduced over these sites as compared to that of the EAR site, as the observations also show. But on the evening of 21 March, there is an additional decrease in the evening h F over Sanya that is associated with the absence of radar plumes, but with the presence of bottomside spread F. The cause of the reduced h F is not clear to us. But we may speculate on the following possibilities. Depending upon its propagation direction, a gravity wave can modify the TZW and/or the TMW thereby modifying also the PRE and/or the h F, respectively. For example, a gravity wave from a local (tropospheric convective) source propagating in slant upward direction can produce zonal as well as meridional wind perturbations at thermospheric heights. The TZW modulated by such a gravity wave can produce a corresponding zonal modulation in the PRE in the required sense to cause a reduced h F over Sanya longitude. At the same time, the meridional component of the gravity wave perturbation wind could modify more directly the h F variation in the longitude of Sanya. These or other possible causes, however, need to be investigated further using direct TZW/TMW measurements. Another source of the h F difference may be seen in terms of sunset polarization electric fields associated with the presence of LSWS [e.g., Saito and Maruyama, 2007; Tsunoda and Ecklund, 2007]. The Satellite traces, indicative of LSWS in the bottomside F layer, were observed at the EAR and Sanya stations on 21 March 2012. The h F measured overhead would depend on the upwelling depth and zonal distance away from the crest of LSWS. Therefore, the presence of LSWS and the longitudinal variations in TZW and TMW (in varying degrees depending upon the nature of the gravity waves) might have played roles on the h F difference between the longitudinal sectors of EAR and Sanya. In any case, the reduced h F over Sanya appears to be an important factor that contributed to the inhibition of plume development over this station on 21 March. [24] As regard the item 2, the seed perturbations that must exist in the bottomside F layer is an important factor for the development of the plasma bubbles that manifest as backscatter plumes seen in radar HTI maps. It has been established from multi-instrument measurements that ESF occur exclusively on the days when LSWS exist. Considering that the satellite traces indicative of LSWS were detected at both the EAR and Sanya stations, we could expect that the difference in plumeoccurrenceon21march2012isunlikelytobeassociated with the LSWS. Using coordinated observations with an all-sky airglow imager, narrow bandwidth photometer, VHF radar, and ionosonde, Narayanan et al. [2012] have shown that though the presence of LSWS is important, they alone are not sufficient for the triggering of plasma bubbles. However, the coexistence of both LSWS and small-scale waves may have the potential to trigger plasma bubbles. Numerical simulations by Sekar et al. [2001] revealed that the evolution of well developed plasma bubbles is possible while the shorter and long wavelength perturbations coexist, and the spatial separation of the plasma plumes is decided by the shorter-wavelength perturbation. For the EAR backscatter plumes observed on 21 March 2012, one note that the interdistances between successive plumes are about 50 km (Figure 5), indicating the presence of small-scale waves. This could mean that while long wavelength gravity waves and/or evening vortex shear flow [e.g., Tsunoda, 2013] produced the large-scale plasma upwellings in the bottomside F layer (LSWS) over a larger longitudinal span, the small-scale waves, which might occur at the longitude of EAR but not at Sanya, riding on the LSWS, might have aided further intensification of the seeding process and triggered the development of multiple backscatter plumes. Moreover, the observations indicate that unlike the LSWS which continuously occurred at a larger longitudinal span of more than 1000 km (since satellite traces were detected over EAR and Sanya) on 21 March, the small-scale waves might occur at a narrower longitudinal range only around EAR site. It is quite possible that a longitudinal difference in the occurrence of small-scale waves may complement our explanation for the difference in the backscatter plume development observed over EAR and Sanya longitudes. This, however, may bring a further question as to whether the occurrence of small-scale waves is always confined to a narrower longitudinal range or not. This issue needs to be addressed in future studies with a larger database of simultaneous beam steering measurements by the two radars. 5. Conclusions [25] The simultaneous observations by the EAR and Sanya VHF radar offer a new insight into our understanding of the 1000 km scale longitudinal differences in the occurrence of VHF backscatter plumes and their variability. The scanning capability available for both the radars makes it possible to study the changes in the generation and evolution of the equatorial plasma irregularity plumes at their closely located longitudinal sectors. Case study by the simultaneous radar beam steering measurements on 30 March 2012 revealed that sunset plasma plumes could be initiated concurrently at the apex sunset region for eastern beams of both the EAR and Sanya VHF radar. The postsunset plasma plume observed over Sanya was due to the passage of sunset plasma plume that was generated over EAR longitude. This provides further evidence that the premidnight periodic towering plumes seen in radar HTI maps do represent equatorial plasma irregularity formations spatially separated in east-west direction. Moreover, the observations that ionogram satellite traces occurred prior to the onset of sunset plumes and the presence of periodic plume structures at both sites indicate that the LSWS could trigger, in this case, the sunset equatorial plasma 6555