Terrestrial Natural and Man-made EM Noise

By Cesidio Bianchi and Antonio Meloni

3.I. Magnetic noise in ULF/ELF band and other propagating phenomena

The terrestrial static magnetic field includes also very slow time variations: secular, annual, 27 days, diurnal, and magnetic bay type variations that have very low frequency (< 1mHz); all this can be considered a sort of “magnetic” noise. Variations can be quantified in tenth of nT for the diurnal variation and hundreds of nT in the case of strong magnetic storms. At higher frequencies in this band various other phenomena, like geomagnetic pulsations take place. Geomagnetic pulsations, i.e., ultra-low-frequency (ULF) waves cover roughly the frequency range from 1 mHz to 1 Hz, i.e., from the lowest the magnetospheric cavity can sustain, up to the various ion gyrofrequencies. Pulsation frequency is considered to be "ultra" low when is lower than the natural frequencies of the plasma, like plasma frequency and the ion gyrofrequency. Geomagnetic pulsations were first observed in the ground-based measurements of the 1859 Great Aurora events (Stewart, 1861). Lower frequency pulsations are generally related to the Kelvin Helmholtz instability that takes place at the magnetopause, as generated by the solar wind interaction with the magnetosphere, or by upstream waves in the foreshock region. In the following (figure 3) geomagnetic pulsations with the intensity variation, frequency and other information are represented (Lanzerotti et al. 1990).

FIGURE 3 Magnetic noise sources Magnetic noise sources

Other electromagnetic phenomena originate by the impinging particles on the magnetosphere that give rise to electromagnetic emission that propagate inside the magnetosphere cavity. Chorus emissions and auroral Hiss are two relevant phenomena. Chorus emissions are among the most intense plasma waves in the outer magnetosphere that propagate as far as the Earth surface and are observed at intermediate invariant latitudes. The Chorus spectral features (from 500 Hz to 1.2 kHz) consist in the succession of predominantly rising tones which resemble a chorus of chirping birds from which these emissions take their name. Auroral hiss emissions are broad, intense electromagnetic emissions which occur over a wide frequency range from a few hundred Hz to several tens of kHz occurring mainly in the auroral zone. This spreading at high frequencies is caused by the anisotropic character of whistler mode propagation (see later). The resulting tones are strongly modulated hiss-like tones.