Takehiko Aso

A note on the semidiurnal non-migrating tide at polar latitudes

In the Antarctic upper mesosphere and lower thermosphere around 90 km, meteor radar observations at the South Pole have detected a significant semidiurnal wind component in summer which is found to be non-migrating with zonal wavenumber s = 1. It has been surmised that this component might possibly be excited through the non-linear interaction of the migrating semidiurnal tide with stationary planetary waves with zonal wavenumber s = 1 prevailing at stratospheric heights. The Kyushu University GCM has been successful in elucidating very unambiguously this conjecture. In the present paper, linearized steady tidal modeling is carried out in this connection to reproduce, by a fairly simplified but explicit model, trans-equatorial propagation of non-migrating semidiurnal tide forced in the opposite winter hemisphere and to compare latitudinal structures of migrating and non-linearly excited intermittent tides in view of polar latitudes where non-migrating tide tends to dominate over migrating tide. It is also shown that the re-analysis meteorological data for almost 10 years clearly supports the well-known N-S asymmetry in stationary planetary wave activity in winter polar stratospheric regions, possibly due to the difference in surface topography between two hemispheres. We suggest that there might be significant asymmetry in the summertime enhancement of semidiurnal non-migrating tide between both polar regions. This phenomenon may be in a context similar with the N-S asymmetry of gravity wave activity in the polar regions, possibly giving rise to N-S disparity of cold summertime mesopause temperature. Clarification awaits intensive bi-polar studies by coordinated radar and optical observations which are running both in the Arctic and Antarctic regions.

Japanese research project on Arctic and Antarctic observations of the middle atmosphere

Abstract An all-sky optical imager is in routine observation at the South Pole. Monochromatic images of aurora and air glow at N 2 + 427.8nm, OI 557.7nm, OI 630nm and OH 730nm are supplying significant information on the magnetospheric process in the polar cap and cusp/cleft region along with atmospheric wave signature at this particular point. SuperDARN radars in Antarctica make observations over the South Pole. At Syowa Station, Antarctica, a multi-instrumental observation project is now being implemented for the study of the polar upper atmosphere from the mesosphere to the thermosphere, where complex physical and chemical processes take place making the region very attractive for scientific research. Two HF radars, which are part of SuperDARN radars, have been already installed and started observations. By the end of 1999, all-sky imagers, photo meters, a Na temperature Lidar, an MF radar and a Fabry-Perot interferometer will be introduced and start collecting various physical parameters on a routine basis. In the Arctic region, we are planning to deploy coordinated ground-based observations with optical, radio and radar sensing of the polar middle and upper atmosphere in conjunction with EISCAT radars. Scientific goals are versatile to shed light on the tangled coupling processes in response to magnetospheric disturbances from above and bi-lateral interactions with high-density lower atmospheric layers. These are outlined in this paper.

Sensitivity calibration of digital colour cameras for auroral imaging

A method to sensitivity calibrate Digital Single Lens Reflective (DSLR) cameras is outlined. A low intensity calibrated light source tunable in wavelength is described. 31 monochromatic lines from 4000 to 7000 A with a bandpass of approximately 12 A were used to find the spectral responses for the D70 and the D200 cameras manufactured by Nikon. The source radiance ranged from about 300 to 1.6k R/A. The cameras were operated in manual mode with 4 seconds exposure time at ISO 1600, which are typical settings required for night time photography of the aurora. For the Nikon D200 camera, the blue, green and red spectral responsivities peak at 4600, 5300 and 5900 A, respectively. The response was high for the blue colour channel with a clear cut-off at 4100 A for the UV part of the spectrum. The red channel response indicates low sensitivity above 6600 A. The D70 shows similar spectral responsivity, except that it peaks in the green colour channel and it is more sensitive to both UV and NIR radiation. Both cameras are capable of detecting night- and dayside auroral at 4 second exposure time. For optimal auroral imaging capability, the green and red spectral responsivities need to be shifted up by about 300 and 400 A in wavelength.

First results of auroral tomography from ALIS-Japan multi-station observations in March, 1995

Auroral tomography observations have been carried out in March, 1995, as a joint international campaign between Sweden and Japan. Three unmanned Swedish ALIS stations (Kiruna, Merasjärvi, Tjautjas) and two Japanese JICCD sites (Abisko, Nikkaluokta), geographically separated by about 50 km at higher latitudes, were operated to capture multi-station monochromatic tomography images at 557.7 nm wavelength using CCD cameras. All cameras were pointing to one of the predetermined directions to secure a common field of view. Several images of auroral arcs, mostly for the core region right above Kiruna, have synchronously been taken by the multi-station imaging system. Tomographic inversion analysis for four-point images was carried out using the algebraic reconstruction technique. Reconstructions of a curved arc and of a double arc system suggest promising application of this technique to the retrieval of three-dimensional auroral luminosity.