Toru Adachi

VLF/ELF sferic evidence for in‐cloud discharge activity producing sprites

[1] Sprite luminosities produced by winter lightning in Japan were found to be associated with simultaneous occurrences of clusters of radio atmospherics as observed in the VLF range, suggesting an in-cloud discharge activity. Concurrent ELF data show transient perturbations, indicating continuous charge transfer in causative lightning. These data provide the first evidence that an in-cloud discharge activity plays an important role in the generation mechanism of sprites.

Electric fields and electron energies in sprites and temporal evolutions of lightning charge moment

The fundamental electrodynamical coupling processes between lightning and sprites are investigated. By combining the observed spectral data with the Monte Carlo swarm experiments, reduced electric fields and electron energies in sprite streamers and halos are estimated. The obtained fields inside sprite halos (70‐97 Td with an analysis error of ±5 Td) are lower than the conventional breakdown field, Ek ∼ 128 Td, indicating a significant reduction of electrons associated with halos while those in sprite streamers (98‐380 Td with an error of ±50 Td) are higher than Ek, suggesting that a significant ionization process drives their formation and development. A combined analysis of photometric and electromagnetic data makes it possible to estimate temporal evolutions of lightning charge moment. It is found that lightning discharges with a short time scale (∼1 ms) and a moderate amount of charge moment (∼400 C km) produce discernible halos. On the other hand, lightning discharges with a large amount of charge moment (∼1300 C km) produce streamers regardless of their time scale. The results obtained are comprehensively interpreted with both the conventional breakdown field necessary for the formation of streamers and the electric field necessary for the production of optical emissions of halo which is sensitive to the time scale of the thundercloud field due to the significant reduction of electrons.

Activities of sprites and elves in the winter season, Japan

Abstract Sprites and elves observation has been carried out in three winter seasons in Japan since December 1998. Thirtyfive sprites’ features have been imaged with CCD cameras and 95 optical emission from elves have been captured with hi-speed array photometers. The number of events observed on a night is dependent on the occurrence of intense cloud to ground lightning activity and cloud condition above the observation site. Most of the sprites/elves are directly associated with storms produced by a cold front passing over Japan. It is found that sprites/elves appear not only in the west coast of Japan but also above the Pacific Ocean. The average vertical length of winter sprites in the west cost is shorter than that of summer sprites in the US by about a factor of 2. The altitude of the causative thunderstorm for winter sprites and elves is comparatively lower than that of the summer events in the US and the horizontal width of the cloud is sometimes smaller than 30 km . The sprites’ detection in winter in Japan suggests the possibility of winter sprite occurrence in Scandinavia and/or Israel where the climate condition is similar to Japan.

Global distribution of intense lightning discharges and their seasonal variations

In order to study the temporal and regional variation of lightning occurrences and their relation to sprite activity and climate variability, we have analysed the 1?100?Hz ELF magnetic field waveform data obtained at the Syowa station in Antarctica, Onagawa in Japan and Esrange in Sweden for a one year period from September 2003 to August 2004. We have selected totally 1.7 ? 105 events of transient Schumann resonances from the ELF magnetic field data whose amplitude exceeds 40?pT at all stations. Then, the lightning locations are estimated by a triangulation method with an estimation error of 0.5?Mm. It is found that in the summer season (from June to August) the lightning occurrence rates are higher in the northern hemisphere than in the southern hemisphere with large enhancements in North America, South-East Asia and the northern part of Africa. On the other hand, in the winter season (from December to February) these rates are higher in the southern hemisphere with large enhancements in South America, Australia and the southern part of Africa. These features are consistent with the results of global lightning measurements from space conducted by the Optical Transient Detector and the Tropical Rainfall Measuring Mission satellite. Then, we have also calculated the charge moment value (Q ? dl) of lightning discharges using ELF magnetic field waveform data and have estimated the distribution function of charge moments for positive cloud-to-ground (+CG) discharges and for negative cloud-to-ground (?CG) discharges. It is found that the shape of the distribution function for both +CG and ?CG discharges is almost the same for all seasons and that the distribution function of ?CG discharges has a steeper slope at high Q ? dl than that of +CG discharges.

High-Speed Volumetric Observation of a Wet Microburst Using X-Band Phased Array Weather Radar in Japan

AbstractThis paper reports a high-speed volumetric observation of a wet microburst event using X-band phased array weather radar (PAWR) in Japan. On 10 September 2014, PAWR observed the three-dimensional structure of a convection cell, which had a vertical extent of 5–6 km and a horizontal dimension of 2–10 km, moving toward the east-northeast. At 2310 Japan standard time (JST), a precipitation core with a radar reflectivity of >40 dBZ appeared at 3–5 km above ground level. The core then increased in size and intensity and rapidly descended to the ground. During this time, a reflectivity notch associated with midlevel inflow was initially formed near the top of the precipitation core and, subsequently, at lower altitudes. A strong low-level outflow with a radial divergence of >4 × 10−3 s−1 appeared just below the notch at around 2321 JST. The outflow lasted for approximately 13 min and eventually disappeared after 2333 JST along with dissipation of the causative storm cell. These results suggest that, in ...

First satellite‐imaging observation of medium‐scale traveling ionospheric disturbances by FORMOSAT‐2/ISUAL

On the night of May 16, 2007, a satellite limb imager of FORMOSAT-2/ISUAL observed wave-like structures of the 630-nm airglow simultaneously with an all-sky imager deployed at Darwin in Australia. The height of the airglow layer was estimated as 220 km, and the structures were aligned in the northeast-southwest orientation with a wavelength of ∼300 km and propagated toward the northwest with a phase velocity of ∼100 m s−1, showing typical characteristics of the nighttime medium-scale traveling ionospheric disturbances (MSTIDs). We conclude that ISUAL for the first time succeeded in observing the airglow layer altitude and airglow structures modulated by MSTID from space. Such a satellite limb airglow imaging could be a new tool to characterize ionospheric irregularities on a global level.