On 4 September 1984 the AMPTE-IRM spacecraft encountered shortly after the inbound crossing of the magnetopause a magnetic cavity of natural origin. The field strength decreased abruptly from 100 nT to values below 5 nT within the cavity. A striking feature of the data is the sudden disappearance of wave emissions that had characterized the magnetopause crossing. Simultaneously with the decrease in field strength the plasma density increased so that pressure balance is nearly maintained. A magnetic cavity of similar appearance was detected on 23 September 1984. ¿ American Geophysical Union 1987