A possible mechanism for the formation of electron density irregularities in the magnetosphere is examined quantitatively. The mechanism involves localized E × B convection cells in which tubes of ionization with different total content are mixed. Under reasonable initial conditions, it is shown that a 0.1-mv/m electric field in the equatorial plane can produce electron density enhancements and depressions of the order of 5% at L = 4 in about one-half hour. These enhancements are large enough to trap whistler waves, whereas the depressions can trap HF waves. It is also shown that small-scale density irregularities can be produced by a much larger scale electric field. It is suggested that thundercloud electricity may be a possible source of the electric field responsible for the irregularities.