Study on the Evolution Characteristics of the Return Stroke Spectra and Channel Temperatures by Using Two Spectrographs with Different Time Resolution

Temperature is one of the crucial parameters reflecting the energy and current transfer characteristics in the lightning discharge channel. According to the spectra of six lightning return strokes recorded simultaneously by two high-speed slit-less spectrographs with different time resolutions, the spectral intensity and temperature evolution of the return stroke channels over time was quantitatively analyzed. The spectral characteristics show that the ionic line intensity decayed rapidly with time as the current declined, while the atomic line intensity decreased more slowly. Additionally, it is found that ionic lines existed in the spectra of continuing current process for up to hundreds of microseconds. It further indicates that the ionic line intensities are  associated with the discharge currents and that their radiation mechanism is closely related to the collision excitation under the action of strong currents. The temperature calculated by the ionic lines can reflect the thermodynamic properties of the current-carrying channel. The temperature calculated using atomic lines is significantly lower than that calculated by the ionic lines in the same spectrum. The radiation mechanism for majority of the atomic lines differs from that of the ionic lines. During the continuing current, the channel temperatures calculated by both ionic lines and atomic lines showed a similar evolution feature which declined slowly or even basically unchanged. This property reflects the persistent heating effect of the current.