M. K. Ejiri

Development of Optical Mesosphere Thermosphere Imagers (OMTI)

The Optical Mesosphere Thermosphere Imagers (OMTI) have been developed to investigate the dynamics of the upper atmosphere through nocturnal airglow emissions. The OMTI consist of an imaging Fabry-Perot interferometer, three all-sky cooled-CCD cameras, three tilting photometers, and a Spectral Airglow Temperature Imager (SATI) with two container houses to install them in. These instruments measure wind, temperature and 2-dimensional airglow patterns in the upper atmosphere at multi-wavelengths of OI (557.7 nm and 630.0 nm), OH (6–2) bands, O2 (0, 1) bands, and Na (589.3 nm), simultaneously. Examples of the data are shown for the cameras, the photometers, and the SATI based on the airglow observation at a mid-latitude station in Japan. Good correlation of the photometer and SATI observations is obtained. A comparison is shown for small- and large-scale wave structures in airglow images at four wavelengths around the mesopause region using four cooled-CCD cameras. We found an event during which large-scale bands, small-scale row-like structures, and large-scale front passage occur simultaneously.

Integrating-sphere calibration of all-sky cameras for nightglow measurements

Abstract Three all-sky cooled-CCD cameras designed for mid-latitude nightglow measurements were developed at the Solar-Terrestrial Environment Laboratory and calibrated using a 2-m integrating-sphere and a spectrometer at the National Institute of Polar Research, Japan. Absolute sensitivity and image-flattening data are obtained together with filter transmission functions. We find that filters with highly-flattened image-quality surfaces may cause serious Newtons Ring pattern on final images.

Statistical study of short‐period gravity waves in OH and OI nightglow images at two separated sites

[1]xa0Using airglow images of the near-infrared OH band (720–910 nm) and OI (557.7 nm) line, we investigated seasonal, latitudinal, and local time variations of short-period gravity waves. The images were obtained at two locations in Japan that are ∼1200 km apart, Rikubetsu (43.5°N, 143.8°E) and Shigaraki (34.9°N, 136.1°E), between October 1998 and October 1999. Our analysis has focused on small-scale gravity waves with wavelengths less than 40 km and dominant phase speeds of ∼20–50 m/s. Wave occurrences for both OH and OI at Rikubetsu and Shigaraki are significantly higher than 60%, with a slightly larger value in summer. The occurrences increase from evening to midnight. There are no obvious local time dependencies in horizontal wavelength, propagation direction, and phase speed. The propagation directions in summer are either northward or northeastward at both locations. However, in winter the propagation directions at Rikubetsu are generally westward (NW, W, and SW), whereas those at Shigaraki are only southwestward. From simultaneous wind observation by the MF radars at Wakkanai (45.4°N, 141.7°E) and Yamagawa (31.2°N, 130.6°E), we discuss possible influences of Doppler and thermal ducting, wind filtering, and source distribution of gravity waves propagating from the lower atmosphere to the airglow heights in the mesopause region.

Traveling ionospheric disturbances observed in the OI 630‐nm nightglow images over Japan by using a Multipoint Imager Network during the FRONT Campaign

Pilot observations using a network of five all-sky imagers (ASIs) were conducted during the new moon period of May 19–22, 1998 as part of the F-region Radio and Optical measurement of Nighttime TID (FRONT) campaign. The network observation enabled us to track propagation of medium-scale traveling ionospheric disturbances (TIDs) in the OI 630-nm nightglow over a distance of more than 2500 km. The TIDs were observed every night during the campaign period, but occurrence was limited from evening to midnight. They have horizontal wavelengths of 200–600 km, travel a horizontal distance of more than 1000 km, and last for more than three hours. In every case, the TIDs moved southwestward with a velocity of 83–137 m/s. Using dual-site TID images, the altitude of the TID structures in the 630-nm nightglow was calculated to be ∼260 km, which corresponds to the bottom side of the mid-latitude ionospheric F layer.

Imaging observations of the equatorward limit of midlatitude traveling ionospheric disturbances

This paper reports the first attempt to observe the equatorward limit of medium-scale traveling ionospheric disturbances (TIDs) in the middle latitudes. The TIDs usually propagate southwestward in the northern hemisphere. An all-sky cooled-CCD imager measured 630-nm airglow at a southern island of Japan, Okinawa (26.9°N, 128.3°E, geomagnetic latitude (MLAT) = 17.0°), during the FRONT-2 campaign of August 4–15, 1999. The TIDs were detected at the mainland of Japan (∼21°–36° MLAT) by the total electron content (TEC) observations of more than 1000 GPS receivers. In the August 4 event, the TIDs moving southwestward was seen only in the northern sky of Okinawa as a depletion band in the 630-nm airglow images. In the August 6 event, the TIDs were not seen in the 630-nm images at Okinawa, although weak TID activity was observed by the GPS network at the mainland of Japan. The TEC data also showed weakening of the TID activity below 18° MLAT. Based on these observations, we suggest that there is a possible limit of medium-scale TID propagation around ∼18° MLAT.

Height measurements of nightglow structures observed by all-sky imagers

Abstract To determine altitudes of nightglow wave structures, we have carried out simultaneous observations of OH Meinel bands and OI 557.7-nm emissions using four all-sky CCD imagers at two separated site. Two imagers were set up at the Shigaraki MU Observatory (34.85°N, 136.11°E), and two others were set up at the Muroh Observatory (34.57°N, 136.01°E). The range between the two sites is about 32 km. Observations were conducted during the new moon period from 21 January to 3 February 1998 as a part of the Planetary Scale Mesopause Observing System (PSMOS) campaign in Japan. On the night of 24–25 January, 1998, two orthogonal wave structures were observed at both sites. To determine the altitudes of nightglow structures, we use a new technique which is an application of a triangulation method. From the obtained images, we succeeded to derived a distribution of the altitudes differing by as much as 10 km across the field of view apparently in association with a large scale intensity variation.

Multi‐point observation of short‐period mesospheric gravity waves over Japan during the FRONT Campaign

Simultaneous observations of short-period gravity waves were carried out using three all-sky cooled-CCD imagers of OH airglow at Moshiri (northern edge of Japan) and at Shigaraki and Bisei (middle part of Japan) during the FRONT campaign of May 1998. These stations were separated by horizontal distances of 250–1300 km, and they offered a unique opportunity to investigate the spatial extent of gravity waves in the mesopause region. The images from Shigaraki and Bisei showed short-period gravity waves (horizontal wavelength: 20–40 km, phase front: ∼east-west aligned) moving northward (phase velocity: 20–40 m/s) on May 19, 21 and 22. At Moshiri, similar east-west waves moving northward were observed on May 19 and 21, indicating a broad horizontal extent of the gravity waves. Waves at Moshiri were not evident on May 22, however. The horizontal wind measured by the MF radar at Wakkanai (near Moshiri) showed height profiles that were similar to those at Yamagawa (southern edge of Japan) on May 19 and 21 but quite dissimilar on May 22. On the basis of these data, we suggest that short-period gravity waves in the mesopause region can have a broad spatial extent of more than 1000 km. The generation and propagation of internal gravity waves are discussed as a cause of the spatial extent of the waves.

Dual‐site imaging observations of small‐scale wave structures through OH and OI nightglow emissions

[1]xa0Four all-sky CCD imagers have been simultaneously operated at Shigaraki (34.9°N, 136.1°E) and Murou (34.6°N, 136.0°E), Japan from January to March 1998 to obtain OH and OI (557.7 nm) nightglow images. We determined height of observed small-scale wave structures (horizontal wavelengths of ∼7–9 km) by a triangulation technique. For an event of January 26, the structures seen in both nightglow images are estimated to have existed at around 91 km, suggesting that the structures occur at altitudes where these emission layers overlap. For another event of January 30, the structures with a height of 82 km were observed only in the OH images. These results indicate that the height extent of the small-scale wave structures is fairly small, suggesting that such structures are instability features rather than gravity waves. Even if we assume that the structures are gravity waves, their vertical wavelengths estimated by using the dispersion relation are very small.

A localized structure in OH airglow images near the mesopause region

[1]xa0An intense localized airglow-depletion structure was observed in OH airglow images (emission altitude ∼86 km) at Shigaraki (34.9°, 136.1°) on 19 December 1998. The structure was a single-band airglow depletion with a small horizontal width (∼15 km) but a large amplitude (∼34% from the average intensity). Airglow rotational temperature indicated a large temperature lapse rate at this time as a result of strong semidiurnal variation. We suggest that nonlinear wave-wave interaction is the most likely cause of this localized structure.