Processes of optical and radio emissions in the upper atmosphere induced by cloud to ground discharge

点击翻译按钮获取中文摘要

In 1990 a discharge phenomenon from the stratosphere to the ionosphere was found during the tcst observation of a low-light TV camera employed by University of Minnesota. Since this discovery a lot of observations from ground, aircraft and space shuttle have been carried out. It is found that these kinds of optical emissions in the stratosphere, mesosphere and lower ionosphere, now called sprites and elves, are induced by an intense cloud to ground discharge and that the phenomena are accompanied by strong radio and gamma ray emissions. Optical emissions of sprites and elves are considered to be caused by an excitation via collision processes between atmospheric molecules/atoms and electrons that are energized by quasi-static electric field or electromagnetic impulse generated by a cloud to ground discharge. We estimated the electron energies based on the spectral observations with high-speed photometers. It is found that electron temperature has a peak of 3-25 eV for carrot-like sprites and 2-14 eV for column type sprites at the onset of the optical emission. In a body of carrot-like sprites the electron temperature tends to increase as altitude decreases with a local peak of 12-20 eV at an altitude of 〜80 km that corresponds to gap between hair and head region of sprites. On the other hand, it is confirmed that elves are caused by a negative cloud to ground discharge as well as a positive one. Electron energy of elves was found to reach up to 40 eV, which implies a strong ionization in the lower ionosphere. We conducted optical observation in Japan and showed the occurrences of sprites and elves in winter for the first time in the world. We also analyzed the "sprite halo" which appears as a diffuse fashion with a disk-like shape and revealed its time-space structure of luminous intensity and electron temperature.