[en] The tokamak plasma density has been observed to drop during ECRH, and to increase during ICRF and LHH. A mechanism is proposed here to explain these density variations. Depending on the nature of the specific wave heating, electrons or ions tend to pile up toward the weak field side. A poloidally varying electrostatic potential is expected to arise and to saturate at a level such as to balance the trapping effect due to the magnetic well and the rf heating. As a result, an E x B drift comparable to the ion vertical drift in a toroidal magnetic field may occur. Assuming that electrons move across the magnetic surfaces more freely due to the magnetic stochasticity or other turbulent diffusion mechanisms, and the particle confinement is primarily determined by the ion neoclassical transport rate, the ECRH can enhance the ion vertical drift and cause the density to drop, while the ICRF and LHH can reduce the ion vertical drift and improve the density confinement