Temperature of apparent natural ball lightning obtained by examination of the spectra

Abstract

Based on the spectra of an apparent natural ball lightning (BL) taken by a slit-less spectrograph with a high-speed camera as a recording system in the Qinghai Plateau of China, the temperature and time-evolution characteristics of this object were investigated. We found that for most of its life, the BL maintains a generally stable luminosity with an obvious periodic oscillation that is only discernible on the spectra captured by the high-speed camera. Soil constituents (Si I, Fe I, and Ca I lines) contribute the majority of bright light, while air compositions (N I and O I lines) dominate the periodic feature of the BL. There are some differences between the temperature values calculated by the spectral lines of different elements in the spectrum. The temperature estimated by the O I lines was the highest, ranging from 7170 to 11 410 K. The mean temperatures gained by O I, Si I, and Fe I lines and continuous spectra were 8750, 4330, 4600, and 2700 K, respectively. This means that the BL has an energy source core, where the spectral lines with higher upper excitation energy should be more intense than that in its periphery. In addition, during the luminously stable stage, the temperature showed an obvious periodic oscillation with time, and its variation tendency was synchronous with the evolution of the light intensity and spectral characteristics.