It is shown that amplitude modulated HF heating in the ionospheric E-region, can produce substantial power in the ELF/ULF range even in the absence of ambient-ionospheric currents. The signals are associated with spontaneous generation of magnetic fields caused by the coupling of the hot spot temperature gradient to the ambient ionospheric density gradient (∇n¿∇T). The mechanism produces values for PC5 micropulsations consistent with those observed by Stubbe and Kopka (1981).