Spatial relationship of equatorial plasma bubbles and field-aligned irregularities observed with an all-sky airglow imager and the Equatorial Atmosphere Radar



[1] We report, for the first time, simultaneous two-dimensional observations of 630-nm airglow depletions and radar backscatter from field-aligned irregularities (FAI) associated with equatorial plasma bubbles. Spatial distributions of backscatter were obtained by performing east-west scans with the 47-MHz Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia. A 630-nm airglow depletion, caused by a plasma bubble, was simultaneously observed with an all-sky airglow imager. Both the airglow depletion and backscatter region appeared as band-like structure elongated in the meridional direction with a zonal width of about 100 km. To compare the spatial structures of backscatter with that of airglow depletion, the backscatter was projected onto a horizontal plane at 250-km altitude. Backscatter was found to occur within the entire airglow-depleted region.

1. Introduction

[2] Plasma bubbles are depletions that develop in the nighttime equatorial F region plasma. OI 630-nm airglow depletions caused by plasma bubbles have been observed with all-sky airglow imagers at equatorial regions and low latitudes, since Weber et al. [1978]. Airglow imagers have the ability to observe two-dimensional structures of plasma bubbles.

[3] VHF, UHF and L-band radars near the geomagnetic equator have been used for observations of backscatter caused by Bragg scatter from field-aligned irregularities (FAI) with a spatial scale-size of one half of the radar wavelength [e.g., Woodman and LaHoz, 1976; Tsunoda, 1980a, 1983]. Several coordinated observations of plasma bubbles and FAI have been conducted. Electron density derived from in situ measurements with rockets and satellites has been compared with FAI observed simultaneously with ALTAIR radar located in the Kwajalein in the Central Pacific [Rino et al., 1981; Tsunoda et al., 1982]. Tsunoda [1980b] has compared spatial maps of plasma bubbles and FAI by scanning the antenna beam in both incoherent and coherent measurements with the ALTAIR radar. They have shown that FAI are collocated with plasma-depleted regions.

[4] Recently, Fukao et al. [2003] have observed F-region backscatter from FAI with the Equatorial Atmosphere Radar (EAR), located in West Sumatra, Indonesia. Since the EAR has an active phased-array system, the antenna beam can be steered on a pulse-to-pulse basis, and consequently, observe different directions essentially simultaneously. This capability enables to provide the spatial distribution of FAI with higher temporal resolution compared with the ALTAIR radar.

[5] The current paper presents the first coordinated observations of plasma bubbles obtained in OI 630-nm airglow images with an all-sky imager and 3-meter-scale FAI with EAR, and shows time evolution of two-dimensional structures of both plasma bubbles and FAI.

2. Observations

[6] The EAR is a monostatic pulsed Doppler radar with a carrier frequency of 47.0 MHz located at the equator in West Sumatra, Indonesia (0.20°S, 100.32°E; dip latitude 10.36°S) [Fukao et al., 2003]. Using the unique capability for viewing different directions almost simultaneously, Fukao et al. [2003, 2004] have revealed the spatial distribution and temporal development of FAI.

[7] On the night of April 1, 2003, the EAR was operated with a mode which consists of 90-s FAI observation and 85-s tropospheric observation. In the FAI observations, eleven beams were steered at azimuths of ±50° around geographic south (130°–230°) to map the two-dimensional pattern of backscatter structures. This arrangement covered approximately 600 km in east-west direction at 500-km altitude. Since the beam directions were perpendicular to the geomagnetic field lines at 350-km altitude, zenith angle of the beams varied between 24°–30°. This observation mode was the same as that reported by Fukao et al. [2003, 2004].

[8] An all-sky imager has been operated at the EAR site since October 2002 as part of the Optical Mesosphere Thermosphere Imagers (OMTIs) [Shiokawa et al., 1999]. A two-dimensional image of 630-nm airglow intensity is obtained every 4.5 min with exposure time of 105 s.

[9] Figure 1 shows geometry of the radar and airglow observations in the meridional plane. The 630-nm airglow layer generally exists at 250-km altitude. Since field-of-view (FOV) of the imager is an area within a radius of approximately 500 km (70° off-zenith), the observation covers the geomagnetic field lines whose apex altitudes over the geomagnetic equator are approximately from 300 to 700 km. Zenith angle of the EAR beam that points due south geographically is 24.0°. Perpendicularity of the beam with the geomagnetic field lines is achieved at the altitude range from 100 to 600 km within the half-power beam width of 3.4° [Fukao et al., 2004]. The geomagnetic field line pointed by the southward beam at an altitude of 350 km passes the airglow layer at 3.4°S, which is within FOV of the imager. We assume that plasma depletions are elongated along the geomagnetic field lines. This assumption is considered to be reasonable because Otsuka et al. [2002] have presented clear geomagnetic conjugacy of plasma depletions with zonal width of 40–100 km between the northern and southern hemispheres and suggested that the plasma depletions are elongated along the geomagnetic field lines. Therefore, this setup allows investigating spatial relationship between FAI and plasma depletions.

Figure 1.

Geometry of radar and airglow observations in the meridional plane. Solid and dashed curves represent geomagnetic field lines and altitude, respectively. The EAR beam is perpendicular to the geomagnetic field at 350 km altitude. Field-of-view (FOV) of the all-sky imager (70° off-zenith) at EAR site is also shown in the figure. 630-nm airglow layer is assumed to exist at 250 km altitude.

3. Results

[10] Backscatter, extending in altitude from 200 to 400 km, were observed with the EAR at 2200–2330 LT on April 1, 2003. We projected the backscatter intensity observed in the eleven radar beams of the EAR, along the geomagnetic field lines, onto the surface at 250-km altitude, where the 630-nm airglow layer is assumed to exist. Figure 2a shows a time sequence of the backscatter map from 2220 LT to 2250 LT. The region has a band-like structure elongated in the meridional direction with little tilt to the west with increasing latitude. The FAI region has a zonal width of about 100 km and moves eastward at about 80 m/s. Figure 2b shows 630-nm airglow images simultaneously observed with the all-sky imager at the EAR site. The all-sky images were converted to a map in geographical coordinates, assuming that the airglow layer exists at 250 km altitude. An airglow depletion elongated in the meridional direction can be seen in each image. From time sequence of the image, the depletion is found to move eastward without changing its structure. In Figure 2c, the radar echoes whose signal-to-noise ratio exceed zero dB are superposed on the airglow image of Figure 2b. The backscatter is seen to exist throughout the entire plasma-depleted regions. The echo is most intense near the center of the plasma depletion in the longitudinal direction, where the airglow depletion is deepest. In the latitudinal direction, location of maximum echo intensity is shifted from that of maximum airglow depletion. This would be caused by altitude dependence of the echo intensity.

Figure 2.

(a) Two-dimensional distribution of FAI echo intensity (signal-to-noise ratio) observed with the EAR between 2220 and 2250 LT on April 1, 2003. The echo intensity is projected along the geomagnetic field lines onto a surface of 250-km altitude. Eleven solid curves indicate the antenna beams. (b) 630-nm airglow images observed with an all-sky imager at the EAR site. (c) FAI echo intensity shown in Figure 2a is superposed on the each airglow image shown in Figure 2b. The contour lines are drawn with levels greater than 0 dB at 3-dB interval. It is noted that the FAI echo coincides with the airglow depletion.

[11] Figure 3a shows longitudinal and temporal variations of the 630-nm airglow intensity at 3°S between 2200 and 2315 LT. Color levels in each image show percentage of airglow intensity deviations from 1-hour average to the background. An airglow depletion can be seen to move from 98.5°E to 100.5°E between 2200 and 2245 LT. Velocity of this eastward movement is about 80 m/s. Amplitude of the depletion is 10–20% for about 1 hour during passage through the imager's field of view. This figure also clearly shows that the backscatter is spatially collocated with the airglow depletion and that their spatial relationship is maintained during their passage. Intensity of the backscatter decreases with time by 10 dB for 1 hour. The e-folding time of the decreasing rate is approximately 20 min.

Figure 3.

(a) Temporal and longitudinal variations of 630-nm airglow intensity at 3°S between 2200 and 2315 LT on April 1, 2003. The airglow variation is shown as percentage of deviation from 1-hour running average to the background. Decrease and increase of the intensity after 2300 LT are due to the cloud. (b) FAI echo intensity simultaneously observed with the EAR is superposed on Figure 3a.

4. Discussion

[12] 630-nm airglow depletion and backscatter were simultaneously observed with an all-sky airglow imager and the EAR on the night of April 1, 2003. From comparison between spatial distributions of backscatter and airglow depletion, we have found that 3-meter-scale FAI occurred within the entire airglow-depleted region. This result is consistent with that derived from the one-dimensional comparisons in previous works [e.g., Szuszczewicz et al., 1980; Tsunoda et al., 1982]. Tsunoda et al. [1982] conducted simultaneous observations of FAI and background electron density profiles using the ALTAIR radar, and showed that backscatter occurs at the same altitude as plasma-depleted regions. The present paper clearly shows spatial coincidence of FAI with a plasma-depleted region, identified by reduced 630-nm emissions, on the zonal and altitude/longitudinal plane. Further, we have shown that the most intense backscatter was coincident with regions of deepest depletion in plasma density.

[13] Huba et al. [1978] showed that the lower-hybrid-drift instability could account for FAI with spatial scales less than about a meter, but that the larger-scale FAI cannot be explained by this instability on the basis of linear theory. The ions must be unmagnetized for this instability to operate, and demagnetization is achieved through ion-ion collisions, which requires a high plasma density. N > 105 el/cm3, where N is plasma density, is required for the demagnetization in the case of 3-meter-scale FAI, whereas N inside plasma bubbles is typically 104 el/cm3 [Huba and Ossakow, 1981]. For these high N values, however, unrealistically sharp density gradients are required to excite the lower-hybrid-drift instability.

[14] On the other hand, the low-frequency-drift instability operates at larger spatial scales (larger than 10 meters), but is damped at 3-meter-scale by ion viscosity [Huba and Ossakow, 1979]. Ossakow [1981] concluded that 3-meter-scale FAI cannot be linearly excited by known instabilities, and that an explanation would require some form of nonlinear cascade process. Although a nonlinear theory has not yet been proposed, we envision that larger-scale FAI could be excited by low-frequency-drift instability along gradients of even larger-scale irregularities. Once these irregularities are generated, 3-meter-scale FAI would be generated by that unidentified nonlinear cascade process. The larger-scale FAI still require low N to overcome the ion viscous damping. Our observations, that 3-meter-scale FAI tend to favor regions of lowest N, is consistent with the low N requirement to generate the larger-scale FAI by the low-frequency-drift instability.

[15] By analyzing ALTAIR radar data, Tsunoda and White [1981] have presented that FAI echoes developed at the bottomside F region along the west wall of the plasma density upwelling with an east-west wavelength of about 400 km while it did not appear in the east wall. Further, Tsunoda [1983] have shown that the secondary FAI echo, called plume, which had zonal extent of the order of ten to hundred kilometers, also grew from the west wall. The echo regions would be collocated with plasma density depletions. These results indicate that the west wall of the upwelling is unstable for the spatial scales on the order of ten to hundred kilometers. These unstable conditions are caused by the gradient drift instability driven by the eastward neutral wind (upward electric current) near the evening terminator [Tsunoda, 1983]. At the west wall, where the plasma density gradient is antiparallel to the neutral wind direction, the conditions for the gradient drift instability are satisfied. The present paper shows that 3-meter-scale FAI occurred not only at the wall but also within the entire regions of the plasma depletion. This suggests that the FAI are generated by mechanisms different from the gradient drift instability.

[16] Airglow intensity in the plasma bubbles was depleted approximately 10–20% from the background. Amplitude of the depletion was almost unchangeable for about 1 hour. On the other hand, backscatter intensity decreased as propagating eastward with e-folding time of approximately 20 min. Further, backscatter was not observed after midnight on this night, while airglow depletions were observed with the all-sky imager (not shown). These results indicate that large-scale structures such as plasma bubbles which have several ten-kilometer scale, are maintained long after small scale irregularities disappear. This feature is consistent with that reported by Basu et al. [1978]. They have shown that during generation phase of the equatorial irregularities in the evening hours, the kilometer and meter-scale irregularities coexist, whereas in the later phase, the meter-scale irregularities decay but the large-scale ones continue to exist.

5. Conclusions

[17] We have conducted coordinated observations of the Equatorial Atmosphere radar (EAR) and an all-sky airglow imager in West Sumatra, Indonesia on April 1, 2003. Two-dimensional distributions of field-aligned irregularities (FAI) were revealed using east-west scans with the EAR. A 630-nm airglow depletion caused by plasma bubbles was simultaneously observed with the all-sky airglow imager. Both the backscatter region and airglow depletion had band-like structure elongated in the meridional direction with zonal width of about 100 km. Comparing the FAI structures with the airglow depletions, FAI were found to occur within the entire airglow-depleted region. The most intense backscatter was coincident with regions of deepest depletion in plasma density. This result indicates that 3-meter-scale FAI would be collocated with the larger-scale irregularities generated by the low-frequency-drift instability. Backscatter intensity decreased with time constant of approximately 20 min while amplitude of the airglow depletion was almost unchangeable for 1 hour. This result suggests that the small-scale plasma structures decay earlier than large-scale structures, and is consistent with the observations reported in the previous literature.


[18] We thank Y. Katoh, M. Satoh, and T. Katoh of the Solar-Terrestrial Environment Laboratory, Nagoya University, for their kind support of airglow imaging observations. The operation of EAR is based upon the Agreement between RISH and LAPAN signed on September 8, 2000. We are grateful to R. Tsunoda for his helpful comments on the present work. This work is supported by Grant-in-Aid for Scientific Research (11440145, 13573006, and Priority Area-764) and on the 21st Century COE Program (Dynamics of the Sun-Earth-Life Interactive System, No.G-4) of the Ministry of Education, Culture, Sports, Science and Technology of Japan.