Spectral Observations of Upward Propagating Streamers

Abstract:

We report spectral observations of upward propagating streamers, which normally start to develop after the development of downward propagating streamers. Optical emissions from downward propagating streamers have been documented by various photometric and spectral studies. One of the significant findings of these studies is that the N₂⁺ first negative band emission – a direct evidence of ionization in sprites – is concentrated at the tips of downward propagating streamers, indicating that these narrow streamers are indeed nonlinear ionization waves, which parallels streamers observed in laboratory discharge experiments at ground pressure. However, upward propagating streamers have not been studied intensively compared to downward propagating streamers. This is mainly because it is difficult to isolate upward propagating streamers from the background luminosity: upward propagating streamers are normally spawned from existing luminous features, and the background luminosity of these features often contaminates the signals from upward propagating streamers. In spectral observation campaigns in 2009 and 2013, we have observed spectra from both downward and upward propagating streamers in two separate events. The observations were made with a high-speed slit-less spectrograph on-board the High-performance Instrumented Airborne Platform for Environmental Research (HIAPER), a research aircraft maintained and operated by NCAR. The spectra were obtained at a frame rate of 10,000 frames per second, at which the spectra from individual streamers can be temporally resolved. The data show that the fraction of blue emission – which we found mainly consisting of the N₂ second positive band system – from the upward propagating streamers is smaller than that of the downward propagating streamers. This indicates that the electron energies in the upward propagating streamers is lower than that of the downward propagating streamers, which is consistent with an earlier photometric study [Adachi et al, 2006, doi:10.1029/2006GL026495]. Although our data set is very limited, we believe that this observational study makes another step forward in our understanding of the nature of upward propagating streamers.