Generation of short-scale field-aligned electron density irregularities on a short timescale (≪1 s) by ionospheric topside sounder pulses is studied. It is a four-wave parametric coupling process, in which a sounder-launched Z mode pump wave decays into two electron Bernstein sidebands and a geomagnetic field-aligned density irregularity. The dispersion relation of the parametric coupling process has been derived and analyzed. The results show that meter-scale field-aligned electron density irregularities (FAI) can be excited with growth rates of the order of 105 s-1 under the resonance conditions that f0≅fp≅nfc, where f0, fp, and fc are the pump wave frequency, electron plasma frequency, and electron cyclotron frequency, respectively, and n is an integer. The excited FAI extend along the magnetic field toward the satellite path, causing the incoherent scattering of the subsequently transmitted sounding pulses. This is suggested to be a possible cause of long-duration Z mode echoes appearing on the Alouette 2 ionograms when f0≅fp≅nfc, as reported by Benson [1997>. It is found that the threshold field of the instability decreases with increased n. It may explain why long-duration Z mode echoes were only observed when n≥4. ¿ 1999 American Geophysical Union