Abstract
<p>The Earth is surrounded by the ionosphere and magnetosphere that can roughly be seen schematically as two concentric shells. These two composed and inhomogeneous structured shells around the Earth selectively affect electromagnetic (EM) waves propagation. Both ionosphere and magnetosphere interact also with particles and waves coming from external sources, generating electromagnetic phenomena that in turn might become sources of EM waves. Conversely, EM waves generated inside the ionosphere remain confined at various altitudes in this region, up to a so-called critical frequency limit, depending on frequency, EM waves can escape out of the ionosphere and magnetosphere or get through. The EM waves generated inside the magnetospheric cavity mainly originate as a result of the electrical activity in the atmosphere. It is well known that also man-made sources, now widely spread on Earth, are a fundamental source of EM waves; however, excluding certain frequencies employed in power distribution and communication, man-made noise can be dominant only at local scale, near their source. According to recent studies, EM waves are also generated in the Earth’s lithosphere; these waves were sometimes associated with earthquake activity showing, on the Earth’s surface, intensities that are generally orders of magnitude below the background EM noise. In this review paper, we illustrate EM waves of natural origin and discuss their characterization in order to try discriminate those of lithospheric origin detectable at or near the Earth’s surface.</p>
Highlights
The electromagnetic (EM) spectrum is subdivided into regions, according to wavelength (Table 1)
External sources bring EM radiations inside the magnetospheric cavity but they can be generated inside the cavity, for example by energetic particles acting as forcing mechanisms that perturb and modify the ionospheric and magnetospheric configuration
The most intensive near Earth’s sources are associated with the atmospheric electrical activity, while the external sources mainly originate on the Sun
Summary
The electromagnetic (EM) spectrum is subdivided into regions, according to wavelength (Table 1). The natural electromagnetic background noise in the Earth’s environment originates from different phenomena that involve both Earth’s external and internal sources and their possible interaction with the ionospheric and magnetospheric cavities. At lower frequency a primary source of noise is the interaction between particles and waves coming from outer space and the magnetosphere, while a secondary source originates inside the ionospheric cavity: in this case atmospheric lightning discharges produce several remarkable phenomena such as sferics, tweeks and so on [e.g., Wait 1982, Helliwell 2006, Bianchi and Meloni 2007, and references therein]. The solar wind, the magnetospheric foreshock, the magnetospheric bow shock, and the magnetopause, are sources of ULF waves Many of these waves pass through the magnetopause and propagate through the magnetosphere interacting with waveguides, magnetic field lines and the ionosphere generating the pulsations detected at the Earth’s surface. In the frequency range from ELF/VLF to LF, the PSD varies roughly as the inverse squared frequency [Fraser-Smith 1995]
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