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Both experimental and theoretical analysis of an {ultra-short} phenomenaoccurring during the positive streamer propagation in atmospheric pressure airis presented. {With tens-of-picoseconds and tens-of-microns precision, it isshown that when the streamer head passes a spatial coordinate, emission maximafrom N$_2$ and N$_2^+$ radiative states follow with different delays. Thesedifferent delays are caused by differences in the dynamics of populating theradiative states, due to different excitation and quenching rates.}{Associating the position of the streamer head with the maximum value of theself-enhanced electric field, a delay of 160$\,$ps was experimentally found forthe peak emission of the first negative system of N$_2^+$.} {For the firsttime, a delay dilatation was observed experimentally on early-stage streamersand clarified theoretically.} {Thus, in the case of second positive system ofN$_2$ the delay can reach as much as 400$\,$ps.} In contrast to thehighly-nonlinear behaviour of streamer events, it is shown theoretically thatemission maxima delays linearly depend on the ratio of the streamer radius andits velocity. This is found to be one of the fundamental streamer features andits use in streamer head diagnostics is proposed. Moreover, radially-resolvedspectra are synthetized for selected subsequent picosecond moments in order tovisualize spectrometric fingerprints of radial structures of N$_2$(C$^3\Pi_u$)and N$_2^+$(B$^2\Sigma_u^+$) populations created by streamer-head electrons.

journal of applied physics

Sub-nanosecond delays of light emitted by streamer in atmospheric  pressure air: analysis of N$_2$(C$^3\Pi_u$) and N$_2^+($B$^2\Sigma_u^{+}$)  emissions and fundamental streamer structure