Magnetic component of narrowband ion cyclotron waves in the auroral zone

We report observations of waves near the local proton cyclotron frequency and its lowest harmonics, made by the Plasma Wave Instrument on board the Polar spacecraft, on orbits for which the perigee (at an altitude of 1 Earth radius) was in or near the southern auroral zone. The electromagnetic nature of these waves was revealed by measuring their magnetic components simultaneously with two independent antenna systems, one a single loop and the other a set of triaxial search coils. Waves of this kind were found in the southern auroral zone on about a third of the orbits examined. Peaks in the magnetic field power spectra occurred at frequencies both below and above the fundamental cyclotron frequency. The ratio of the amplitudes of the electric and magnetic fluctuations was usually greater than the speed of light, suggesting that we observe a magnetic component related to the wave mode customarily called electrostatic ion cyclotron waves. The fluctuating vector of the wave magnetic field was in or close to the plane perpendicular to the static magnetic field. Our main result is that its polarization, which covered a broad range from left‐hand elliptic to right‐hand elliptic, can be explained by the superposition of many linearly polarized waves.