Minimum sprite plasma density as determine by VLF scattering

The scattering of VLF sub-ionospheric transmissions by sprite plasma throug h horizontal angles up to 180 degrees shows that sprite plasma is highly co nducting. 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 muS/m, corresponding to an electron density at 70 km altitude of similar to 10(10) m(-3) (similar to 10(4) elec trons per cc), and so about 10(5)/cc at 55 km. The Tatter ionization densit y is about that of the daytime E-region, and over six orders of magnitude a bove 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 degr ees, suggesting that the sprite conductivity rises too slowly (as the plasm a cools) to reach adequate backscatter within the time allowed by the "eart y/fast" definition.