Lightning is a very common natural phenomenon and there has been growing interest in diagnosing lightning plasma. As a major source of lightning spectral emissions, N ii ions have been attracting considerable attention over the past decades [Phys. Rev. Lett. 112, 035001 (2014)]. In this work, we aim to propose a scheme for lightning plasma diagnostics by exploring the alignment of upper levels of strong lightning spectra populated by electron impact excitation, the dominant population mechanism, of N ii ions. To this aim, the magnetic-substate-resolved excitation cross sections of the upper levels are calculated, from which the relevant second- and fourth-rank alignment parameters ${\mathcal{A}}_{20}$ and ${\mathcal{A}}_{40}$ are obtained. It is found that these excited upper levels are highly aligned (that is, the relative population of the magnetic substates significantly deviates from the statistical distribution) at certain impact energies, leading to highly anisotropic and polarized lightning spectral lines. Also, the alignment parameter ${\mathcal{A}}_{20}$ is found to be strongly dependent on the impact energy, whereas such a dependence is relatively weak for ${\mathcal{A}}_{40}$. The strong energy dependence of ${\mathcal{A}}_{20}$ is expected to present strongly energy-dependent angular anisotropy and linear polarization of the lightning spectral lines. In particular, for some of the upper levels the parameter ${\mathcal{A}}_{20}$ undergoes a sign reversal as the impact energy changes, which can qualitatively alter angular anisotropy pattern and polarization behavior of the relevant lightning lines. The discovered strong energy dependence and particularly the sign reversal of the level alignment are expected to serve as a scheme of diagnosing the relevant lightning N ii plasmas such as electron energy and temperature.

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• Atomic & molecular collisions
• Atomic spectra
• Electronic excitation & ionization
• Ions
• Polarization
• Hartree-Fock methods
• Quantum chemistry methods