Topside sounders observe electron-density irregularities in the ionosphere with dimensions of hundreds of meters to many kilometers across the magnetic field. Most irregularities are elongated and may extend thousands of kilometers along the field. The variation of electron density is normally a few percent or less. The irregularities tend to occur in patches, in which the individual irregularities vary greatly in size. Irregularities can produce radio echoes by scattering, ducting, or complex refraction, depending on their dimensions. The different kinds of echoes produce different characteristic patterns on topside ionograms, and therefore irregularities in various size ranges may be distinguished and studied.