Minimum sprite plasma density as determined by VLF scattering

The scattering of VLF sub-ionospheric transmissions by sprite plasma through horizontal angles up to 180/spl deg/ shows that sprite plasma is highly conducting. Following a simple transmission-line (one-dimensional-wave) model in 1997, two-dimensional and three-dimensional models have been produced. Here, we compare the results of the three models, and show that all require a uniform conductivity of at least 30 μS/m, corresponding to an electron density at 70 km altitude of /spl sim/10/sup 10/ m/sup -3/ (/spl sim/10
⁴
electrons per cc), and so about 10
⁵
/cc at 55 km. The latter ionization density is about that of the daytime E-region, and over six orders of magnitude above the ambient density at 55 km. By contrast, the "early/fast" events, as defined by the Stanford group, do not exhibit scatter angles above 15/spl deg/, suggesting that the sprite conductivity rises too slowly (as the plasma cools) to reach adequate backscatter within the time allowed by the "early/fast" definition.