A corona is a process by which a current develops between two high-potential electrodes in a neutral fluid, such as air,
by ionizing that fluid creating a plasma around one electrode, and by using the ions generated in plasma-processes as
the charge carriers to the other electrode.
Corona discharge usually involves two asymmetric electrodes, one highly curved (such as the small diameter electrode in an ozone generator) and one of low curvature (such as the outer tube of an ozone generator). The high curvature ensures a high potential gradient around one electrode for the generation of a plasma.
Coronas may be positive or negative. The charge is determined by the polarity of the voltage on the highly-curved electrode. If the curved electrode is positive with respect to the flat electrode, it is positive corona, if negative, it is a negative corona. The physics of positive and negative coronas are strikingly different. A negative corona generates much more ozone than the corresponding positive corona.