Temporal Evolution Feature on Spectrum and Temperature of Lightning return stroke Channel

Temperature is one of the crucial parameters reflecting the energy and current transfer characteristics in the lightning discharge channel. According to the spectra of eight lightning return strokes recorded simultaneously by two high-speed slitless spectrographs with different time resolutions, the spectral-structure and temperature evolution of the return stroke channels over time was quantitatively analyzed. Different from the previous report, one of the spectrographs have recorded the ionic lines in the spectra within approximately 200 microseconds during the return stroke. The ionic line intensity decayed rapidly with time as the current declined, while the atomic line intensity decreased more slowly. The spectral-structure evolution characteristics indicate that the ionic lines in the spectra existed throughout the discharge current process (including the continuing current (CC) stage). Additionally, it further suggests 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 of the atomic lines differs from that of the ionic lines. Compared with the decay of the ionic-line intensity, the decline of the channel temperature calculated by ionic lines is slower. The temperature calculated by atomic lines shown the similar feature, and it is even basically unchanged in the CC stage. This property reflects the persistent heating effect of the current.温度是反映雷电放电通道能量与电流传输特性的关键参数之一。基于两台不同时间分辨率的高速无狭缝摄谱仪（high-speed slitless spectrographs）同步记录的8次雷电回击（lightning return stroke）光谱数据，研究人员对回击通道的光谱结构及温度随时间的演化规律进行了定量分析。与以往研究报道不同的是，其中一台摄谱仪记录到了回击过程中约200微秒内光谱中的离子谱线（ionic lines）。离子谱线强度随电流衰减而快速随时间衰减，原子谱线（atomic lines）强度的衰减则相对缓慢。光谱结构演化特征表明，光谱中的离子谱线贯穿整个放电电流过程（包括持续电流（CC）阶段）。此外，该结果进一步揭示，离子谱线强度与放电电流密切相关，其辐射机制与强电流作用下的碰撞激发（collision excitation）紧密相关。通过离子谱线计算得到的温度，能够反映载流通道的热力学特性。同一光谱中，通过原子谱线计算得到的温度显著低于离子谱线计算结果。原子谱线的辐射机制与离子谱线存在差异。相较于离子谱线强度的衰减，通过离子谱线计算得到的通道温度衰减更为缓慢。原子谱线计算得到的温度也呈现出类似特征，在CC阶段甚至基本保持不变。该特性反映了电流的持续加热效应。