Diffuse aurora are found on the equatorward part of auroral oval, in the region of the so-called central plasma sheet (CPS) precipitation most likely mapping to inner plasma sheet, where magnetic field is almost dipolar (see isotropic boundary). The precipitating particles are drifting around Earth, electrons to the east and protons to the west. The strongest diffuse auroras are found on the post-midnight sector, i.e., they are produced mainly by electrons. Proton precipitation is important especially in the pre-midnight sector.
Note that even diffuse aurora are not without any discrete structures: they are just so weak and difficult to observe. First space observations were by Lui and Anger (1973).
- Lui, A. T. Y., and C. D. Anger, A uniform belt of diffuse auroral emissions seen by the Isis-2 scanning photometer, Planet. Space Sci., 21, 809, 1973.