It's Not Easy Being Green: Kinetic Calculations Simulating the Emission Spectra of the Picket Fence

STEVE is a rare ionospheric optical phenomenon characterized by a narrow mauve arc extending thousands of kilometers east/west across the subauroral sky[1]. Vibrant green streaks, referred to as the "picket fence" for their vertical elongation and fairly regular spacing, sometimes form below STEVE. While several early studies suggested that the picket fence is produced via precipitation of magnetospheric particles[2], subsequent analysis revealed distinct spectral features that challenged this hypothesis[3]. Specifically, the absence of N2+ first negative emissions and the presence of N2 first positive emissions suggest that electrons with energies between 7.35 eV and 18.75 eV provide the energy source for picket fence emissions. Recent studies modeling the structure of the picket fence invoke parallel electric fields, suggesting that these fields may generate electrons with the required energies[4,5]. However, it is yet to be shown that parallel electric fields at picket fence altitudes can generate emissions consistent with the observed spectra. In this study, we present additional picket fence spectral data from the TREx spectrograph[6] to better define the spectral features for our modeling analysis. We also solve the Boltzmann equation for the electron energy distribution function under the influence of local electric fields parallel to the magnetic field in a realistic neutral atmosphere using BOLSIG+ software[7]. Using the resulting electron energy distribution functions, we determine the volume emission rates for the oxygen green line, N2 first positive, and N2+ first negative emissions. Our results indicate that parallel electric fields are capable of accelerating electrons to energies sufficient to reproduce the picket fence spectra, and we provide constraints on the magnitude of the required parallel electric fields. With our findings, we hope to advance our understanding of the picket fence, offer insights into its underlying mechanisms, and provide results verifiable by future models and experiments.

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[3] Mende, S. B., Harding, B. J., & Turner, C. (2019). Subauroral green STEVE arcs: Evidence for low‐energy excitation. Geophysical Research Letters, 46(24), 14256-14262.

[4] Lynch, K. A., McManus, E., Gutow, J., Burleigh, M., & Zettergren, M. (2022). An ionospheric conductance gradient driver for subauroral picket fence visible signatures near STEVE events. Journal of Geophysical Research: Space Physics, 127(12), e2022JA030863.

[5] Mishin, E. V., & Streltsov, A. V. (2022, December). The Kinetic Theory of STEVE and Picket Fence. In AGU Fall Meeting Abstracts (Vol. 2022, pp. SM25E-2016).

[6] Gillies, D. M., Donovan, E., Hampton, D., Liang, J., Connors, M., Nishimura, Y., ... & Spanswick, E. (2019). First observations from the TREx spectrograph: The optical spectrum of STEVE and the picket fence phenomena. Geophysical Research Letters, 46(13), 7207-7213.

[7] Hagelaar, G. J. M., & Pitchford, L. C. (2005). Solving the Boltzmann equation to obtain electron transport coefficients and rate coefficients for fluid models. Plasma sources science and technology, 14(4), 722.