ANNALS OF GEOPHYSICS, 58, 3, 2015, G0330; doi:10.4401/ag-6745 Background electromagnetic noise characterization: the role of external and internal Earth sources Antonio Meloni, Cesidio Bianchi*, Giuliana Mele, Paolo Palangio Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy Article history Received January 29, 2015; accepted June 9, 2015. Subject classification: Radio noise characterization, Ionosphere-magnetosphere, Background EM noise, Crustal/lithospheric EM sources. ABSTRACT but they can also be generated inside the cavity, for ex- The Earth is surrounded by the ionosphere and magnetosphere that can ample by energetic particles acting as forcing mecha- roughly be seen schematically as two concentric shells. These two com- nisms that perturb and modify the ionospheric and posed and inhomogeneous structured shells around the Earth selectively magnetospheric configuration. The EM Power Spectral affect electromagnetic (EM) waves propagation. Both ionosphere and Density (PSD) at the Earth’s surface was deeply stud- magnetosphere interact also with particles and waves coming from ex- ied and reported since the first extensive measurements ternal sources, generating electromagnetic phenomena that in turn might were available, as discussed for example by Lanzerotti become sources of EM waves. Conversely, EM waves generated inside the [1978] and Lanzerotti et al. [1990]. The most intensive ionosphere remain confined at various altitudes in this region, up to a so- near Earth’s sources are associated with the atmos- called critical frequency limit, depending on frequency, EM waves can es- pheric electrical activity, while the external sources cape out of the ionosphere and magnetosphere or get through. The EM mainly originate on the Sun. Relevant information about waves generated inside the magnetospheric cavity mainly originate as a the external EM noise can be found in the CCIR/ITU result of the electrical activity in the atmosphere. It is well known that (Comité Consultatif International pour la Radio; now also man-made sources, now widely spread on Earth, are a fundamental also International Telecommunication Union) Radio- source of EM waves; however, excluding certain frequencies employed in communication Sector (ITU-R) reports, published in power distribution and communication, man-made noise can be domi- 1964, 1988, and 1990 [CCIR/ITU 1960, 1988, 1990], as nant only at local scale, near their source. According to recent studies, EM well as in Davies [1990], Helliwell [2006], and Bianchi waves are also generated in the Earth’s lithosphere; these waves were and Meloni [2007]. sometimes associated with earthquake activity showing, on the Earth’s Sudden fluxes of particles that enter the magne- surface, intensities that are generally orders of magnitude below the back- tosphere generate magneto-hydro-dynamic (MHD) ground EM noise. In this review paper, we illustrate EM waves of natu- waves and other relevant phenomena, especially in ral origin and discuss their characterization in order to try discriminate the polar regions (e.g., auroral displays, magnetic sub- those of lithospheric origin detectable at or near the Earth’s surface. storms, Polar Caps Absorption) and also wave-parti- cle interactions (whistlers, chorus, hiss). According to 1. Introduction Lan-zerotti et al. [1990], PSD varies of about 16 or- The electromagnetic (EM) spectrum is subdivided ders of magnitude in the frequency band 10-5 - 105 into regions, according to wavelength (Table 1). The Hz, with values ranging from ≈10-1 to 10-17 W m-2 Earth’s atmosphere prevents most of the electromag- Hz-1. For frequencies up to the GHz, the PSD falls to netic radiation coming from outer space from reaching ≈10-21 W m-2 Hz-1. The spectrum of the EM pertur- the Earth’s surface; in fact, only portions of radio and bations inside the ionosphere and magnetosphere visible light reach the ground. cavities is mostly studied for the ULF (Ultra Low Fre- The natural electromagnetic background noise in quencies) up to the HF (High Frequency) bands. the Earth’s environment originates from different phe- Highest frequencies will not be considered in this nomena that involve both Earth’s external and internal paper since, starting from HF onwards, i.e. corre- sources and their possible interaction with the ionos- sponding to waves able to escape the ionosphere, the pheric and magnetospheric cavities. External sources background noise decreases accordingly. In Table 1 bring EM radiations inside the magnetospheric cavity acronyms, frequency, wavelength and PSD approxi- G0330 MELONI ET AL. Sources of natural EM radio noise Frequency Wavelength Spectral power Physical (m) (W m-2 Hz-1) environment resonance in the magnetospheric cavity, ULF interaction with particles of solar origin and 3·1011 ÷ 108 10-1 ÷ 10-10 magnetosphere 0.1 ÷ 3,000 mHz radiative pressure with the magnetosphere ELF resonance in the ionospheric cavity 108 ÷ 105 10-10 ÷ 10-12 ionosphere 3 ÷ 3,000 Hz propagation in the ionospheric cavity VLF 105 ÷ 104 10-12 ÷ 10-14 ionosphere of the radiating atmospheric discharge 3 ÷ 30 kHz LF-MF-HF atmospheric noise 104 ÷ 102 10-14 ÷ 10-17 ionosphere 30 kHz ÷ 30 MHz Table 1. Main physical characteristics of the natural EM radio noise sources in the magnetosphere and ionosphere. mate values of the natural radio noise sources in the spectral density decreases considerably from lower to magnetosphere and ionosphere, are reported. As higher frequency, since contributions to background briefly reported above, the background EM noise be- noise from sources internal to the cavities are less im- comes less important at higher frequency while the portant and cosmic noise becomes prevalent up to the cosmic noise is prevalent starting from VHF-UHF fre- millimetric wavelength. The f -1 decay is a natural quency band. At lower frequency a primary source of trend, being a constant when considering a confined noise is the interaction between particles and waves environment [Füllekrug and Fraser-Smith 2011]. coming from outer space and the magnetosphere, The purpose of this work is to describe the EM while a secondary source originates inside the ionos- background noise in the Earth’s environment and to pheric cavity: in this case atmospheric lightning dis- compare the natural external signals with the internal charges produce several remarkable phenomena such Earth’s crust, or more generally lithospheric, sources. as sferics, tweeks and so on [e.g., Wait 1982, Helliwell These are supposed to be weaker and local and to have 2006, Bianchi and Meloni 2007, and references peculiar characteristics that can be used to discriminate therein]. Over the range ≈10-4 to ≈104 Hz the ampli- the internal from the external contributions (see a re- tude spectra (fT/√Hz) fall between the two straight view by Hayakawa and Hobara [2010]). EM emissions lines f -1.5 and f -1 with an average half line slope that radiated from the lithosphere were at times related to rolls off as f -1.25 (Figure 1). As a natural trend, the earthquakes occurrence. In particular, many authors have hypothesized that EM waves can be generated at the hypocenter during the earthquake or its preparation phase [e.g., Hayakawa et al. 1996, Johnston 2002, Teys- seire and Ernst 2002, Uyeda et al. 2009]; reported EM phenomena take place in a wide frequency range [see for example: Hayakawa and Fujinawa 1994, Hayakawa and Molchanov 2002]. Some reported ionospheric phe- nomena, were related to earthquake occurrence [Pu- linets et al. 2007], but were criticized. In fact Thomas et al. [2012] had reproduced the same time series re- sults, in which the authors identified an anomalous sig- nal, and found it was normal solar and geomagnetic activity that were, in their opinion, unrelated to the earthquake. In the following, the most relevant sources of nat- ural EM waves, in the various frequency ranges, will be treated separately. 2. Background noise in the ULF range Geomagnetic pulsations, also called micropulsa- Figure 1. EM natural noise PSD. In the frequency interval from 10-4 to 104 Hz, the noise falls between the two straight lines trending as tions (or simply pulsations) are the ground signature of f -1 and f -1.5 (redrawn after Lanzerotti et al. [1990]). ultra low frequency (ULF) hydromagnetic waves prop- 2 EARTH’S BACKGROUND EM NOISE CHARACTERIZATION agating in the Earth’s magnetosphere [Anderson 1993, the magnetospheric plasma, in particular drift of pro- 1994]. Their period varies from less than 1 s to 10 min, tons and heavier ions. while their amplitude varies from tens to hundreds nT, Pulsations amplitude is normally not larger than increasing with increasing period and magnetic latitude. a few nT in the Pc1 and Pc2 range and a few tens of Traditionally, micropulsations are classified as continuous nT at longer periods; in exceptional cases pulsations (Pc) and irregular (Pi) as reported in Table 2. Most of the can reach very large amplitudes (100 nT and more). ULF waves detected at the Earth’s surface originate Short period pulsations Pc1, Pc2 and Pi1 are travel- outside the magnetosphere [Yumoto 1988]. The solar ling waves, while long period pulsations are mainly wind, the magnetospheric foreshock, the magnetos- standing waves inside the magnetospheric cavity. The pheric bow shock, and the magnetopause, are sources same mechanism can be observed in the ionospheric of ULF waves. Many of these waves pass through the cavity for low frequency longitudinal and transversal magnetopause and propagate through the magnetos- standing waves. The short