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

<p>Temperature is one of the crucial parameters reflecting the energy and current transfer&nbsp;characteristics in the lightning discharge channel.&nbsp;According to the spectra of eight&nbsp;lightning&nbsp;return strokes recorded simultaneously by two high-speed slitless&nbsp;spectrographs with different time resolutions, the spectral-structure&nbsp;and&nbsp;temperature evolution of the return stroke channels over&nbsp;time was quantitatively analyzed.&nbsp;Different from the previous report,&nbsp;one of the spectrographs&nbsp;have&nbsp;recorded the ionic lines&nbsp;in the spectra&nbsp;within approximately&nbsp;200 microseconds during the return stroke. The ionic&nbsp;line intensity decayed rapidly with time as the current&nbsp;declined, while the atomic line intensity decreased more slowly. The spectral-structure evolution characteristics indicate that&nbsp;the ionic lines in the spectra existed throughout the discharge current process&nbsp;(including&nbsp;the continuing&nbsp;current&nbsp;(CC)&nbsp;stage). Additionally, it further suggests&nbsp;that the ionic line intensities are associated with the discharge currents and that their radiation mechanism is closely related to the collision&nbsp;excitation under the action of strong currents.&nbsp;The temperature calculated by&nbsp;the&nbsp;ionic&nbsp;lines&nbsp;can reflect the thermodynamic properties of the current-carrying channel.&nbsp;The temperature calculated&nbsp;using&nbsp;atomic lines is significantly&nbsp;lower than that calculated by&nbsp;the ionic lines in the same spectrum. The radiation mechanism of the atomic lines&nbsp;differs from&nbsp;that of the ionic&nbsp;lines.&nbsp;Compared with the decay of the ionic-line intensity,&nbsp;the decline of the channel temperature&nbsp;calculated by ionic&nbsp;lines&nbsp;is slower.&nbsp;The temperature&nbsp;calculated by atomic&nbsp;lines&nbsp;shown the similar feature,&nbsp;and&nbsp;it is even basically unchanged in the CC&nbsp;stage.&nbsp;This property reflects the persistent&nbsp;heating effect of the current.</p>