Pc 1 waves observed at synchronous orbit are studied using magnetic field data from the flux gate magnetometer on board ATS 6. The data were obtained in the interval of June to September in 1975 when the satellite was located at the geographic equator at 36¿ east longitude near the geomagnetic equator (geomagnetic latitude ~-3¿). It is found that spectral structures of Pc 1 waves are significantly affected by variations of the magnitude of the local magnetic field near the geomagnetic equator. Spectral bands of Pc 1 waves are usually confined to the frequency range below the helium gyrofrequency (fHe+) when Pc 1 waves appear on the afternoon side accompanied by a decrease in the ambient magnetic field during substorms. In the daytime, Pc 1 waves often occur in association with an increase in the ambient magnetic field caused by a magnetospheric compression. In such cases, Pc 1 waves tend to appear in the frequency range above fHe+. The sense of polarization is left-handed for most waves as was noted in previous studies. It is suggested from the linear growth rate analysis that the absence of Pc 1 waves in the frequency range above the helium gyrofrequency during substorms is mainly due to an increase in the energetic proton flux.