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Using high time-resolved spectra and simultaneous records of the electric field change of three negative cloud-to-ground (CG) lightning flashes with multiple return strokes, the correlations between the total intensity of ionic lines in the spectra and the corresponding amplitude of the initial electric field change, as well as between the total intensity of the spectra and the channel apparent diameter have been analyzed. The analysis shows the following: (1) The amplitude of the initial electric field change is roughly proportional to the total intensity of ionic lines. (2) The total intensity of the spectra shows a significant linear correlation with the apparent diameter of the channel. (3) The total intensity of ionic lines for seventeen analyzed return strokes decreases with increasing height along the channel, which is consistent with the current variation along the channel in the modified transmission line (MTL) model, the Master, Uman, Lin, and Standler (MULS) model, and the Diendorfer-Uman (DU) model. Meanwhile, the total intensity of ionic lines for other two analyzed return strokes along the channel without attenuation, this is consistent with the current variation along the channel in the Bruce-Golde (BG) model, the transmission line (TL) model and the Traveling Current Source (TCS) model.

According to the high time-resolved spectra of two first and a subsequent return strokes and the following continuing current processes overlapped with M-components in three natural cloud-to-ground lightning, the correlation between the total intensity of ionic lines in the spectra and the corresponding amplitude of electric field change and that between the total intensity of the spectra and the apparent diameter of the discharge channel have been investigated. Linear correlations have been found for the above two sets of parameters. The luminous properties along the channel show that the total intensity of ionic lines in the spectra for both the return stroke and M-components decreases with increasing height along the channel. The total intensity of the spectra and the apparent diameter for the return stroke also go down with increase of the height, while those for M-components do the reverse.

A natural bipolar cloud-to-ground lightning with two return strokes has been recorded with a slitless spectrograph. The subsequent positive stroke occurred retracing the preexisting channel created by the first negative stroke. They both were followed by a long continuing current going with M components. The spectra of the discharge channel for the M components were recorded first with a spectral range from 400 to 1000 nm. The spectral analysis suggests that the polarity of the discharge and the direction of the current have little effect on the structure of the spectrum, while the discharge strength, including current magnitude and duration, is the main factor determining the structure of the spectrum. The spectra of the negative and the positive return strokes are characterized by strong NII lines within the visible range, especially the two representative lines of 500.5 nm and 568.0 nm. The spectra of the M components were mainly from the contribution of OI and NI emissions in the infrared range, and typically, the four strong lines of OI 777.4 nm, NI 746.8, 821.6, and 868.0 nm were persistently observed in the spectra of the whole lightning process. In addition, the spectral characteristics in different channel sections show that background intensities for both the return strokes and all the M components have an increasing trend with increasing height along the channel. However, the intensities of NII lines for both the return strokes decrease slightly with increasing height along the channel, which should be related to the transmission characteristic of the current in the discharge channel.