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Airglow Measurements Using All-Sky Imagers

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Airglow Measurements Using All-Sky Imagers 3-2-6 New Observational Deployments for SEALION —Airglow Measurements Using All-Sky Imagers KUBOTA Minoru, ISHII Mamoru, TSUGAWA Takuya, UEMOTO Jyunpei, JIN Hidekatsu, OTSUKA Yuichi, and SHIOKAWA Kazuo An airglow all-sky imager (ASI)—a new monitoring instrument—is scheduled to be set up in February 2010 at Chiang Mai, Thailand, as part of the framework of the Southeast Asia Low-lati- tude Ionospheric Network (SEALION) being developed in the Southeast Asian region by the National Institute of Information and Communications Technology (NICT). The ASI images air- glow having an emission layer near the ionospheric F-layer in order to observe the two-dimen- sional structure and time evolution of such ionospheric disturbances as plasma bubbles. In Southeast Asia, the Solar-Terrestrial Environment Laboratory (STE) of Nagoya University has been conducting ASI-based observations at Kototabang, Indonesia, ever since 2002. This paper provides detailed insight into the formation, growth and propagation of plasma bubbles as derived from observational data recorded using ASI at Kototabang and using SEALION ionoson- des and GPS receivers. The functional requirements for another ASI to be installed and ASI- based observation schemes are then discussed based on that insight. Keywords Airglow, Ionosphere, Plasma bubble, Gravity wave 1 Introduction branches of a tree, with each branch being about 100-km wide. This phenomenon is When an image of airglow with OI induced by a plasma bubble, as discussed in 630.0 nm is taken in the equatorial anomaly this feature edition［1］［2］and often observed in region, the phenomenon-like black shadows of Japan as well during an active solar activity airglow are observed in some cases as shown period. in Fig. 1. These black shadows (with signifi- Plasma bubbles have been observed using cantly decreased airglow emission intensity) airglow all-sky imagers (ASIs) since the often drift eastward and branch out like the 1980s (see［3］ and other references). ASI- Fig.1 Plasma bubble observed at Yamagawa Observatory KUBOTA Minoru et al. 299 based observations reveal the geometry of a atmospheric wave observations. plasma bubble, its drift velocity, and how it grows. Recent advances in optics have driven 2 Principles of ASI-based obser- research to observe the microstructures and vation and overview of equip- growth process of plasma bubbles, and probe ment into their relation with plasma instability［4］［5］. Reference［6］ reports that the simultaneous 2.1 Principles of airglow-based ionos- observations of plasma bubbles at geomagnet- pheric observation ic conjugate points from Sata, Japan, and Dar- Typical airglows used in ionospheric win, Australia, had confirmed the precise con- observations from the ground are OI 630.0, jugacy in place between plasma bubbles in the 557.7 and 777.4 nm. The mechanism of air- Northern and Southern Hemispheres. glow excitation (as reported in［10］to［13］) is Electron density decreases within a plasma expressed in the following reaction formulas: bubble. It is accompanied by the growth of OI630.0 nm： ionospheric irregularities with various ＋ ＋ O ＋ O2 → O 2＋O ［7］ scales . Positioning satellite radio waves ＋ － 1 1 1 3 O 2 ＋ e → O（ S， D）＋ O（ D， P）…（＊） passing through plasma bubbles are subject to （＊）→ O（1D）→ O（3P）＋hν（630.0 nm） scintillation, resulting in degraded positioning OI557.7 nm： accuracy or sometimes poor reception. The （＊）→ O（1S）→ O（1D）＋hν（557.7 nm） construction of SEALION is intended to OI777.4 nm： establish technology for predicting the occur- O＋＋ e－ → O（5P） rence and arrival of plasma bubbles that could O（5P）→ O（5S）＋hν（777.4 nm） have a major impact on satellite positioning. Past studies have revealed the seasonal As can be seen from these reaction formu- dependence and solar activity dependence on las, the excitation of airglow with OI 630.0 the occurrence frequency of plasma and 557.7 nm is governed by the collision bubbles［8］. However, the causes of day-to-day reaction between O+ (i.e., main component ion variations in the occurrence of plasma bubbles of the ionospheric F-region) and oxygen mole- have yet to be identified. Plasma bubbles form cules (i.e., neutral atmosphere). The excitation in the vicinity of the magnetic equator, with of airglow with OI 777.4 nm is governed by some growing toward a higher-latitude region, the collision reaction between O+ and elec- and others not. The reasons for this difference trons. Figure 2 schematically shows such dif- have also not been discussed in depth. These mechanisms must therefore be explored in order to make a major step toward the fore- casting of plasma bubbles. Some previous reports have also suggested the involvement of large-scale atmospheric waves spanning an east-west wavelength of several hundred kilometers in the mechanism of plasma bubble occurrence［9］［10］. The ASI should be able to capture any such large-scale atmospheric waves existing at the ionospheric F-layer height. This paper introduces the prin- ciples of ASI-based ionospheric observation, presents detailed pictures of plasma bubbles Fig.2 Schematic view of height distribu- tions of O+, electrons and oxygen provided by the ASI, and discusses the possi- molecules, and profile of airglow bilities and planning of ASI-based, large-scale emission height 300 Journal of the National Institute of Information and Communications Technology Vol.56 Nos.1-4 2009 ferences in excitation reaction manifested as in diameter. Light leaving the objective lens is differences in the light-emitting height. This converted through a telecentric lens into paral- means that airglow with OI 630.0 and lel rays that are separated via an interference 557.7 nm have emissions peaking about 50 filter before focusing within the photodetector. km below the peak height of the F-region, as The interference filter is mounted on five to compared to airglow with OI 777.4 nm emit- six selectable channels of filter turrets to allow ting most intensely near the peak height of the observation on multiple wavelengths. A F-region. Thus, using these emissions makes it cooled CCD camera with a back-illuminated possible to collect information about two dif- CCD device is used as the photodetector for ferent heights of the ionosphere. higher sensitivity and easier handling. Such When a plasma bubble forms in the ionos- observation procedures as opening and closing phere, prevailing O+ and electrons will sharply the shutter, changing filters, and imaging and decrease. Consequently, the airglow emission saving data are performed by a program run- intensity is degraded and airglow is observed ning on an ASI control PC. The program pro- like a black shadow as shown in Fig. 1. Con- ceeds with sequential imaging by switching versely, variations in ionospheric electron den- the channels as scheduled. Figure 3 shows sity can be determined from changes in emis- sample observations of two kinds of airglow sion intensity. However, the relation between and background light imaged by a single ASI variations in ionospheric electron density and virtually at the same time (with a time differ- airglow emission intensity should be interpret- ence of up to five minutes). ed with discretion［14］. Because the airglow In this diagram, all-sky images of airglow emission intensity also depends on the with OI 630.0 nm, airglow with OI 557.7 nm, changes in the ionospheric F-layer height, and and background light are arranged from left to airglow with OI 557.7 nm is emitted even at a right. A totally different phenomenon appears height of about 95 km with another excitation in each image. The image of airglow with OI mechanism. 630.0 nm is found to have a plasma bubble forming on the southwestern side. Conversely, 2.2 Overview of ASI equipment the image of airglow with OI 557.7 nm does The ASI unit employs a fish-eye objective not exhibit an evident plasma bubble, but lens to monitor an all-sky visual field of 180 instead shows a light wave structure from the degrees, making it possible to capture in a sin- zenith to the eastern side. Such a pattern is gle view the entire range centering on an characteristic of atmospheric gravity waves at observation point several hundred kilometers the E-layer height. Neither a plasma bubble Fig.3 Two kinds of airglow observed at Yamagawa Observatory, and all-sky image of back- ground light KUBOTA Minoru et al. 301 structure nor a wave structure is found on the turbed region westward. An eastward drift background light channel, where only multiple velocity of about 110 m/s can be read from stars are mapped. This finding supports con- this diagram. This value matches the plasma jecture that the structures mapped in the first bubble drift velocity obtained from satellites two images were not caused by clouds. or GPS scintillation observations reported to NICT has developed an ASI powered by a date (［20］–［22］ and other references). This wavelength-specific focus adjustment fea- plasma bubble then shrank at around 15:00 ture［15］. It has also established a technique by UT (24:00 LT), but was continuously which to derive the absolute intensity of air- observed until around 22:00 UT (05:00 LT) glow from calibration data available from the before dawn. National Institute of Polar Research［17］. Plans Figure 5 is an example of the SEALION calls for NICT’s ASI to be set up in February integrated analysis panel in which SEALION 2010 at Sirindhorn Observatory (18.8º N lati- ionograms and GPS-TECs in effect at 16:00 tude, 98.9º E longitude, 13º dip latitude) in UT (23:00 LT) amid a plasma bubble are tiled. Chiang Mai, Thailand, in order to launch a Summary descriptions of the SEALION steady observation program, jointly with the observation points and observation equipment University of Chiang Mai and the Solar-Ter- can be found in Reference［1］.
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