A new mechanism of transformation of long-wavelength acoustic waves into electromagnetic in partially ionized plasmas is suggested. The range of geomagnetic pulsation frequencies has been studied. It is shown that in collective motion acoustic wave involves the charged particles of media by means of the collisions. Relative motion of charged particles excites the alternating current with arbitrary phase, and consequently, the parametric generation of electromagnetic fields occurs. The growth rates, excitement thresholds, spectra, and polarization of electromagnetic disturbances are determined. The suggested mechanism not only has generalphysical importance, but it can also explain the observed data and the prediction of expected character of atmospheric waveprocesses before the earthquake. ¿ 1998 American Geophysical Union