Magnetic elliptical polarization of Schumann resonances

Abstract

An analysis of the expressions for the magnetic signatures of the transverse magnetic (TM) normal modes of the earth‐ionosphere cavity shows that the side multiplets are elliptically polarized traveling waves. Measurements of the orthogonal, horizontal components of the magnetic field in the extremely low frequency (ELF) range 3–60 Hz performed during September 1985 reveal that the Schumann resonance eigenfrequencies determined separately from the north–south and east–west magnetic components often differ by as much as 0.5 Hz, indicating that the underlying magnetic signal is not linearly polarized at such times. Determination of the corresponding magnetic polarization reveals a high degree of magnetic ellipticity, with both the sense of polarization and the ellipticity varying diurnally. The dominant sense of polarization over the measurement passband is identified to be right‐handed during local daylight hours, with the maximum ellipticities greater than 0.5 occurring just after local sunrise and before local sunset. The frequencies of maximum ellipticity during the daylight hours coincide with the Schumann resonance intensity peaks at 7, 14, 20 and 26 Hz. During the local nighttime hours the dominant sense of polarization is identified to be left‐handed, with a maximum ellipticity greater than 0.5. The frequencies of maximum ellipticity during the nighttime coincide with the intensity nulls between the Schumann resonances. The high degree of magnetic ellipticity suggests that the side multiplets of the Schumann resonances corresponding to azimuthally inhomogeneous normal modes are strongly excited in the highly asymmetric earth‐ionospheric cavity.