Low-frequency transverse electric atmospheric noise: Measurement and theory

This paper presents measurements of both transverse electric (TE) and transverse magnetic (TM) LF atmospheric noise made aboard a free-floating balloon at altitudes up to 20 km. Data are given for rms noise level, TE/TM noise ratio, amplitude probability distribution, and impulsiveness parameter VD. The TE/TM noise ratio was greater than reported by other authors and implies major contributions from horizontally polarized sources. Calculations of the fields generated by lightning discharges as a function of altitude and orientation show that horizontal discharges are weak TE radiators at VLF/LF unless their altitude exceeds a few kilometers. Above several kilometers, however, horizontal discharges can radiate TE noise almost as efficiently as vertical ones radiate TM noise. Most horizontal lightning channels do, in fact, occur above 3 km and are more frequent than vertical channels. Moreover, the horizontal component of lightning is often two or three times greater in extent than the vertical component, even for cloud-to-ground discharges. It therefore appears that TE noise might well be caused primarily by direct radiation from horizontal lightning rather than by geomagnetically converted radiation from vertical lightning.