Polarization of 500 kHz Electromagnetic Noise from Thunderstorms: A New Interpretation of Existing Data

Abstract

It has been widely accepted that electromagnetic noise at low radio frequencies from thunderstorms is mainly vertically polarized. This paper re-examines the existing 500 kHz data of Kohl, taking into account the electrical conductivity of the earth. The corrections involved are shown to be crucial, revealing that Kohl's data actually infer that the vertical component of the rf electric field is several times smaller than its horizontal component. This result is of importance in understanding the relationship between the electromagnetic noise spectrum generated by lightning and the details of the stroke process. The physics of the calculated correction is discussed, as well as the limits of validity for the calculation. Experiments to measure the polarization ratio for other types of terrain are suggested and the theory is used to predict a value for the expected ratio for the midwestern United States.