Abstract
Streamers, thin, ionized plasma channels, form the early stages of lightning discharges. Here we approach the study of extraterrestrial lightning by studying the formation and propagation of streamer discharges in various nitrogen-methane and nitrogen-oxygen mixtures with levels of nitrogen from 20% to 98.4%. We present the friction force and breakdown fields Ek in various N2:O2 (Earth-like) and N2:CH4 (Titan-like) mixtures. The strength of the friction force is larger in N2:CH4 mixtures whereas the breakdown field in mixtures with methane is half as large as in mixtures with oxygen. We use a 2.5 dimensional Monte Carlo particle-in-cell code with cylindrical symmetry to simulate the development of electron avalanches from an initial electron-ion patch in ambient electric fields between 1.5Ek and 3Ek. We compare the electron density, the electric field, the front velocities as well as the occurrence of avalanche-to-streamer transition between mixtures with methane and with oxygen. Whereas we observe the formation of streamers in oxygen in all considered cases, we observe streamer inceptions in methane for small percentages of nitrogen or for large electric fields only. For large percentages of nitrogen or for small fields, ionization is not efficient enough to form a streamer channel within the length of the simulation domain. In oxygen, positive and negative streamers move faster for small percentages of nitrogen. In mixtures with methane, electron or streamer fronts move 10–100 times slower than in mixtures with oxygen; the higher the percentage of methane, the faster the fronts move. On Titan with methane percentages between 1.4% and 5%, a successful streamer inception would require a large electric field of 4.2 MV m−1 (3Ek). Such large fields might not be present and explain the non-detection of Titan lightning by the Cassini/Huygens mission.
Highlights
Lightning on Earth is a highly complex phenomenon involving physical processes on various spatial and temporal scales (Villanueva et al, 1994; Ebert et al, 2006; Ebert and Sentman, 2008; Luque and Ebert, 2012; Rakov, 2013; da Silva and Pasko, 2013) starting from electron avalanches and resulting in the formation of hot, conducting lightning leaders
We have found that the attachment rate is reduced for increasing concentration of N2 in both N2:O2 and N2:CH4 mixtures over the entire range of electric fields considered
We have investigated the motion of electrons and the streamer inception in N2:CH4 and in N2:O2 mixtures with number densities of 2.9 ⋅ 1025 m−3 with different percentages κ of nitrogen in ambient fields of
Summary
Lightning on Earth is a highly complex phenomenon involving physical processes on various spatial and temporal scales (Villanueva et al, 1994; Ebert et al, 2006; Ebert and Sentman, 2008; Luque and Ebert, 2012; Rakov, 2013; da Silva and Pasko, 2013) starting from electron avalanches and resulting in the formation of hot, conducting lightning leaders. Using Monte Carlo simulations, Dwyer et al (2006) simulated the runaway breakdown, which is the discharge initiation through high-energy electrons whose friction is significantly smaller than for low-energy electrons (Gurevich et al, 1992), in the atmospheres of Jupiter and Saturn They found that the runaway breakdown field lowered by the presence of hydrometeors is ten times smaller than the conventional breakdown field and suggest that this might facilitate lightning inception on these planets. We perform Monte Carlo particle-in-cell simulations of electron avalanches in mixtures of N2:CH4 with different percentages of nitrogen in various electric fields and determine for which conditions the electron avalanches transition into streamer discharges.
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