The mechanisms leading to ionization of neutral gas by means of microwaves in the electron cyclotron range of frequencies in a simple toroidal plasma configuration are investigated in the TORPEX device (Fasoli A et al 2003 Bull. Am. Phys. Soc. 48 119). The role of the upper hybrid resonance, where most of the ionization events occur once the plasma is formed, is clearly demonstrated. The fast phenomena related to ionization are discriminated from the slower mechanisms leading to the stationary background profiles by measuring the density perturbation caused by a modulation of the injected microwave power. The same method permits reconstruction of the spatial profile of the particle source. The ionization processes are then related to experimental control parameters such as the injected microwave power.