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The role of positive ions and field emission in the negative corona triggering is discussed, in room air, at atmospheric pressure. First, an artificial positive space charge is created in a point-to-plane gap by focusing a pulsed ultraviolet laser beam at a determined distance from the point. The triggered corona current pulse is compared with the natural one. Numerical simulations which are presented take into account the insulating layers charging by positive ions on the cathode surface. The theory of electron emission is based upon a conduction mechanism in the bulk of insulating layers. Second, the role of field emission is enhanced by covering the point with a graphite coating. Field emission is modeled by introducing a field enhancement factor and current instabilities are attributed to the switch-off of emission sites on the cathode surface. © 2004 American Institute of Physics.