  It is by now well understood that large sprite discharges at the low air
densities of the mesosphere are physically similar to small streamer discharges
in air at standard temperature and pressure. This similarity is based on
Townsend scaling with air density. First the theoretical basis of Townsend
scaling and a list of six possible corrections to scaling are discussed; then
the experimental evidence for the similarity between streamers and sprites is
reviewed. We then discuss how far present sprite and streamer theory has been
developed, and we show how streamer experiments can be interpreted as sprite
simulations. We review those results of recent streamer research that are
relevant for sprites and other forms of atmospheric electricity and discuss
their implications for sprite understanding. These include the large range of
streamer diameters and velocities and the overall 3D morphology with branching,
interaction and reconnection, the dependence on voltage and polarity, the
electron energies in the streamer head and the consecutive chemical efficiency
and hard radiation. New theoretical and experimental results concern
measurements of streamer spectra in air, the density dependence of streamer
heating (hot leaders are unlikely at 80 km altitude and cold streamers are
unlikely in liquids), and a discussion of the influence of magnetic fields on
thermal electrons or on energetic electrons in streamers or sprites.
