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Pheric Study Using CRL All-Sky Imagers

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Pheric Study Using CRL All-Sky Imagers 4-3 Recent results and future plans of atmos- pheric study using CRL all-sky imagers KUBOTA Minoru, ISHII Mamoru, OYAMA Shin-ichiro, and MURAYAMA Yasuhiro As part of an international cooperative research project between Communications Research Laboratory and Geophysical Institute of University of Alaska, we developed two all-sky imagers (CRL-ASI). We had conducted pilot observations in Japan, and developed some new observation techniques for atmospheric waves. We installed the CRL-ASI at Poker Flat (magnetic lat. 65.6N), Alaska, and started aurora/airglow observations from Octo- ber 2000. CRL-ASI can be operated automatically, and controlled from CRL, Japan. Real- time summary data are opened to the public with WWW. Using these instruments, we suc- cessfully observed some new phenomena: simultaneous appearance of gravity waves and aurora, co-rotating aurora in the evening sector. These results show the advantage of sensi- tivity and spatial resolution of the CRL-ASI. Observation data by CRL-ASI will contribute to a study on the magnetosphere-ionosphere-thermosphere coupling. Keywords Airglow, Aurora, Image, Middle atmosphere, Gravity wave 1 Introduction magnetosphere (a source of auroras) and to estimate the energy of aurora particles that Very weak atmospheric emission (referred precipitate from the magnetosphere[3]. to as airglow) exists in the mesopause and In the MLT region, atmospheric molecules lower thermosphere (MLT) region. Capturing and atoms collide with one another, resulting images of airglow from the ground makes it in light-emitting excitation without the precip- possible to observe the atmosphere in this itation of electrons. After the 1980s, in the region continuously. In situ observation of the low- and mid-latitude regions (where no auro- atmosphere in this region is very difficult. ras form), cameras equipped with II were used Since around 1960, auroras in the Polar to capture monochromatic images of very Regions have been observed and studied, weak airglow. This allowed observation of using film or video cameras equipped with traveling ionospheric disturbances (TID) and 1 image intensifiers(II) [1]. In the 1980s, atmospheric gravity waves that propagate researchers attempted to take monochromatic through the region[4][5]. Recently, charge-cou- images using an interference filter in addition pled devices (CCD) without an II have begun to the II[2]. An aurora is a phenomenon to be used[6]. These devices are far superior to caused by impact excitation of atmospheric those featuring an image intensifier in terms of molecules, atoms, and ions owing to electrons durability, operability, and spatial resolution, and protons precipitating from the magnetos- but are slightly inferior in terms of time reso- phere. The emission intensity may reach tens lution. A CCD can be cooled to raise its sensi- of kilo-rayleighs (several tens as large as that tivity, and has therefore gained wide use as a of the Milky Way). Observation of auroras detector of airglow-images. makes it possible to understand the state of the As a part of the cooperative study between KUBOTA Minoru et al. 161 Communications Research Laboratory (CRL) and the University of Alaska Fairbanks, which is referred to as the Alaska Project [7], we developed two high-sensitivity all-sky imagers (CRL-ASI) by applying the above-referenced CCD-imaging technology. After conducting many modes of trial observation and develop- ing data-processing technology, we installed the equipment in Alaska in order to begin con- tinuous observation in October 2000. In the next section, we present an outline of the CRL-ASI. Section 3 describes the results of the domestic trial observation and the tech- nique of data-analysis developed during the trial observation. Section 4 presents an out- line of the observations in Alaska and the ini- tial results. Section 5 discusses future prospects. 1There are many types of two-dimensional amplifiers for illuminance: image orthicon tubes, SEM tubes, SIT tubes, devices using MCP, and so on. These are all included among the image intensi- fiers (II) described in this report. 2 Outline of apparatus During the development stage of the CRL all-sky imagers (CRL-ASI), we attempted to obtain more accurate data by adding a focus- adjustment function to conventional imagers. Fig.1 Schematic diagram of the overall This function improved data accuracy to a CRL-ASI system great extent, made it much easier to remove starlight as a source of error in measurements enables it to observe a wide area within a of airglow, and enabled us to estimate the radius of several hundreds km range around quantity of nighttime clouds from extracted the observation point. Rays of light pass star images. This latter benefit was an unex- through the objective lens and then through a pected outcome. We developed our own tech- collimator lens to become a collimated beam, nical software for automated observation and through an interference filter for spectral sepa- Internet-based remote control of the imagers. ration, and is then focused on a CCD device This allowed for the stable automatic opera- unit. This optic system is based on the all-sky tion of the CRL-ASI in Alaska. Fig.1 is a auroral imager developed by the National schematic diagram of the overall system of the Institute of Polar Research (NIPR) in CRL-ASI. 1996[8][9] as well as on the airglow-observa- As an objective lens, the CRL-ASI has a tion imager developed later at Kyoto Universi- fish-eye lens with a 6-mm focal length and an ty. Interference filters are mounted on the F1.4 made by Nikon Technologies Inc. It is five-channel filter turrets, enabling observa- capable of a 180°all-sky field of view, which tion at any of five channel-selected wave- 162 Journal of the Communications Research Laboratory Vol.49 No.2 2002 lengths. The telecentric lens has a focus- Fig.2 shows the two-dimensional distribution adjusting mechanism to enable the capture of and airglow images. A method of determining high-resolution images. The detector unit is a wave-structure heights [11] was established in cooled camera with a 512 × 512-pixel back- this experiment and has since been applied to illuminated CCD, achieving high-sensitivity the study on the mesopause and lower ther- and convenient in handling. mosphere (MLT) region[12]. A control PC is programmed to operate the In May 1998, we participated in the F- imagers so that the shutter may be opened and region Radio and Optical Measurement of closed, filters may be switched, the focus may Nighttime TID (FRONT) Campaign. We be adjusted, image data may be displayed and cooperated with Tohoku University, Niigata stored, and so on. Filter channels are switched University, Nagoya University, Kyoto Univer- to allow for continuous capture of images sity and the Geographical Survey Institute to according to a preset schedule. The order of simultaneously observe airglow at five loca- observation is described in a parameter file, tions (Moshiri, Zaoh, Kiso, Shigaraki, and which can be prepared not only with the on- Miboshi)[13][14][15]. It was found that the air- site PC but may also be prepared and sent via glow imagers served as effective tools in the Internet from a remote PC. obtaining visually comprehensible images of We cooperated with NIPR to examine the ionospheric disturbances in the mid-latitude sensitivity and other characteristics of the regions. Fig.3 shows an intensity distribution CRL-ASI. From this examination, we con- of 630-nm airglow due to oxygen atoms firmed that the CRL-ASIs are achieving obtained for a wide area from Hokkaido to expected sensitivity. For example, we suc- Kyushu in the course of the above simultane- cessfully obtained an acceptable S/N ratio for ous observation. This figure also shows the a 557.7-nm emission of oxygen atoms which distribution of total electron content (TEC) is a representative auroral emission line, with- obtained during observations using the domes- in a three-second or shorter exposure. Refer- tic GPS network. It was found that distinct ence [10] shows a method of estimating the atmospheric waves (wavelength of 100 km) absolute intensity of airglow from observation were present in airglow and were accompa- data as well as details of the CRL-ASI calibra- nied by ionospheric disturbances. tions. In January 2000, we participated in the Waves in Airglow Structures Experiment over 3 Test in Japan Kagoshima in 2000 (WAVE 2000) project. We cooperated with the Institute of Space and Development of the CRL all-sky imagers Astronautical Science, Tohoku University, the (CRL-ASI) started in 1997. These imagers University of Tokyo, Rikkyo University, and were tested until 2000 in the context of vari- others to conduct simultaneous observation ous observation projects. During this period, experiments on the ground and via rocket. we developed a suitable procedure for analysis These experiments were aimed at observation of data, producing good scientific results. of atmospheric gravity waves of horizontal Between January and March of 1998, we wavelengths between several kilometers and participated in the Planetary Scale Mesopause 100 kilometers, and succeeded in obtaining Observing System (PSMOS) Campaign. We our first data set of (1) the horizontal distribu- then cooperated with Nagoya University and tion of airglow intensities and (2) vertical pro- Kyoto University to conduct simultaneous files of oxygen atoms, electron densities, and observation of airglow from two locations. It airglow intensities[16]. These data indicate was the first successful experiment in which periodic variations accompanying atmospheric we obtained a two-dimensional distribution of waves and were analyzed to determine the wave-structure heights in the emission layer. structure of three-dimensional atmospheric KUBOTA Minoru et al. 163 Fig.2 Results of observation in the PSMOS campaign The upper panels show atmospheric gravity-wave structures in OH airglow simultaneously observed from Shigaraki and Muroh using the two ASIs.
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