Gradient driven electrostatic instabilities are investigated in TORPEX [A. Fasoli, B. Labit, M. McGrath, S. H. Muller, M. Podesta, and F. M. Poli, Bull. Am. Phys. Soc. 48, 119 (2003)], a toroidal device (R=1 m, a=0.2 m) in which plasmas are produced by microwaves (P <= 20 kW) with f(rf)=2.45 GHz, in the electron cyclotron frequency range. Typical density and temperature are n(e)<= 10(17) m(-3) and T-e similar or equal to 5 eV, respectively. The magnetic field is mainly toroidal (<= 0.1 T), with a small vertical component (<= 4 mT). Instabilities that can be generally identified as drift-interchange waves are observed and characterized for different levels of collisionality with neutrals. The frequency spectrum and the spatial profile of the fluctuation-induced flux are measured. An 86-tip probe is used to reconstruct the spatio-temporal evolution of density structures across the plasma cross section. The measured structures are characterized statistically, and related quantitative observables are constructed. (c) 2006 American Institute of Physics.