Terrestrial Natural and Man-made EM Noise

By Cesidio Bianchi and Antonio Meloni

3.II. Atmospheric noise in ELF/VLF band and related electromagnetic phenomena

Lightnings are the main source of energy for the electromagnetic background inside the ionospheric cavity. Starting from the lower band ELF (few Hz) up to several VHF (hundreds MHz) the noise originates from the energy radiate by the lightning. Several million lightning strokes occur daily from an estimated 2000 storms worldwide, and the Earth is somewhere struck 100 times a second by a lightning. The discharge is very violent and can easily reach 10.000 Amperes. The amount of energy released by each discharge can vary from units to tenth of GJ. Hence, for the duration of the discharge (less than 1 s), the powers involved in this phenomena are of the order of 1- 10 GW. The annual total released energy is in the order of 1019 J. If only 10% of this energy will be radiated as electromagnetic energy (figure 4) this amount of electric energy is comparable to the energy produced in 1970 by the electric power stations over the world.

FIGURE 4 Lightning frequency domain electromagnetic spectrum Lightning frequency domain electromagnetic spectrum

The main relevant phenomena in the ELF lower band are the Schumann resonances. This phenomenon consists of a wide spectrum electromagnetic signal, composed by dumped waves of frequencies below 60 Hz. Schumann resonances occur because the Earth and the ionosphere form a natural wave guide that shows a fundamental resonance frequency at 7.8 Hz and upper harmonic components at about 15.6, 23.4 and 31.2 Hz (figure 5). The Earth-atmosphere system can be seen, from an electromagnetic point of view, as a series of shell layers of different electrical conductivity. The Earth and the ionospheric layers appear as perfect conductors with the air of negligible conductivity in between; they form an Earth-ionosphere cavity, in which electromagnetic radiation is trapped. Lightning strikes within the troposphere radiate energy into this system and the waves travel around the Earth. In the case of constructive interference, Earth-ionosphere cavity resonances are excited in the above mentioned frequency range (6-60 Hz).

FIGURE 5 Schumann resonance peaks Schumann resonance peaks

A series of propagating phenomena that are relevant in this band of frequency are the so called spherics, tweeks and whistlers. Radio atmospherics (or spherics for short and sometime statics) are impulsive signals generated by lightning strokes that travel with a low attenuation in the Earth-ionosphere wave guide. These impulsive signals (few ms) propagate for thousand kilometers. As in a real wave guide, the Earth-ionosphere guide, can sustain the propagation of this signals with very low attenuation values. Since only the upper part of this channel varies with time the sferic propagation is determined by ionosphere conditions. Almost all AM receivers detect spherics as disturbances (sounds like pops and crashes). The more powerful the lightning stroke, or the closer the distance is to the VLF receiver's location, the louder the disturbances given by the spherics will sound. Sparks of lightning strokes are generally powerful sources of electromagnetic (radio) emission throughout the radio frequency spectrum from the very low radio frequencies up to the microwave frequency ranges and the visible light spectrum, even if the radio power is concentrated in VLF range from 0.1 to 10 kHz (figure 6).

FIGURE 6 Typical sferic frequency spectrum Typical sferic frequency spectrum

Tweeks are spherics dispersed in frequency. Their sound is similar to a bird’s song in a frequency range of 1-7 kHz (figure 7). When spherics propagate for long distances in a dispersive medium like the ionosphere their harmonic components separate along the travel (Helliwell R.A. 1965). These components penetrate at various depths the ionosphere, in such a way that, higher frequencies penetrate more than lower ones and as consequence travel for longer distances. These different paths imply different arrival times at an observer. In a spectrogram they result like a descending tone with duration of the order of 25 to 150 ms. Tweeks are normally heard in the evening after the sunset.

FIGURE 7 Tweeks that traveled for different distances in the Earth-ionosphere waveguide shown for about 60.000 km (1), more than 10.000 km (2) and more than 14.000 km (3) paths Tweeks that traveled for different distances in the Earth-ionosphere waveguide shown for about 60.000 km (1), more than 10.000 km (2) and more than 14.000 km (3) paths

The whistlers are remarkable burst generated by lightning discharge. In fact part of the this energy escapes the ionospheric barrier and propagate through the magnetosphere. They can be heard in radio receivers as a relative long whistle decreasing in frequency, from about 6 kHz to few hundred Hz (figure 8). In the magnetosphere whistlers interact with free electrons and are forced to propagate along the Earth’s magnetic field lines. The harmonic components of the signal identified as whistlers correspond to electromagnetic waves that have traveled several Earth radii arriving at different times to the observer. Lower frequencies are delayed 3-6 second with respect to the higher ones. The dispersion of a whistler depends on the length of the path over which the signal travels as well as the characteristics of the propagation medium such as its electron density.

FIGURE 8 Whistler spectrum Whistler spectrum