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(Englisch)

A rapidly expanding disk-shaped region of red luminosity in the ionosphere, lasting less than a thousandth of a second, which occurs high above energetic cloud-to-ground lightning of positive or negative polarity.

From e(mission of) l(ight and) v(ery low-frequency perturbations from) e(lectromagnetic pulse sources), and to avoid clashing with the acronym ELF (extremely low frequency) used in the same field of science.