It is shown that ac discharges propagating at an air / dielectric interface, though of planar structure, behave, till a critical voltage Vcrit, as corona discharges in an air gap, with similar propagation fields for the filamentary discharge components and similar glow components. This leads to consider the surface discharges as gas discharges propagating above the dielectric surface. Beyond Vcrit, the retention of charges by the dielectric surface becomes ineffective, due to the gas heating in the filamentary channels and to the heat subsequently transferred from these channels to the surface. In return, the surface gives its energy excess back to the discharge, so opening the way, on the surface, to leader-like discharges of higher conductivity, needing about 10 times lower fields to propagate. Key words: surface discharge, gas/surface interface, discharge propagation, surface modification. 1. INTRODUCTIVE BACKGROUND Lichtenberg was the first in 1778  to report observations on surface discharges. His approach consisted in sprinkling a mixture of two powders on dielectric surfaces after exposure to the discharge: red lead oxide positively charged (thus attracted by the negative charges of the surface), and yellow sulphur powder negatively charge
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