The Earthรionosphere cavity is characterized by a number of disturbances which cause departures from the behaviour expected for the spherical symmetric case usually applied in model calculations[ The main e}ects are the inhomogeneity of the upper atmosphere which is characterised by di}erent conductivity pro_les during day and night\ and the conductivity anisotropy introduced by the Earth|s magnetic _eld[ Standing waves\ excited by the global thunderstorm activity\ with an eigenfrequency of ยผ7 Hz develop in this cavity[ This phenomenon\ which is called Schumann resonance\ su}ers a frequency splitting due to the removal of the spherical symmetry[ At the Institut fu ร r Geophysik we measured the electromagnetic _eld components for a whole year at frequencies up to 19 Hz[ We present the results of the analysis of the polarization of individual events of the horizontal magnetic _eld components[ With the help of the polarization parameters as indicators\ we show the existence of the aforementioned splitting[ A frequency di}erence of 9[0ร9[1 Hz between the two elliptical modes has been estimated[ ร 0888 Elsevier Science Ltd[ All rights reserved[