The nature of the Earth's interior, in terms of the dynamics of the crust, mantle, and core, can be investigated through extended ground-based and space observations (Bell, 1982; Hayakawa and Molchanov, 2002; Pulinets and Boyarchuk, 2004; De Santis et al., 2015). In the same way, it is possible to inquest the natural or anthropogenic origin of the electromagnetic emissions (EMEs) using measurements in the near-Earth space (Parrot and Zaslavski, 1996; Buzzi, 2006). In addition, several works (Parrot, 1995; Parrot and Zaslavski, 1996; Buzzi, 2007) remarked that EMEs of both anthropogenic (such as HF broadcasting stations and VLF transmitters) and natural (i.e. Earth's surface) origin can influence the dynamics and the composition of the ionosphere–magnetosphere region (Parrot and Molchanov, 1995; Parrot and Zaslavski, 1996). Taking into account that extreme reliability is needed to call for preseismic phenomena, a characteristic background for the regions on Earth where we want to detect the effects of earthquake-related EMEs should be available. In the interim, comprehensive study of both magnetospheric and ionospheric disturbances driven by ground preseismic EME waves has to be carried out. In this context, aseismic fault creep and EME waves are expected to be the principal mechanical and electromagnetic earthquake precursors, respectively (Buzzi, 2007, and references therein). Parrot and Molchanov (1995) and Parrot and Zaslavski (1996) achieved the first promising result analysing rare EME wave observations over the ionosphere–magnetosphere region. In their studies, they first distinguished between internal and external components of the geomagnetic field and, then, gave an efficient measure of the electric field, the plasma temperature, and the density of the ionosphere. Despite several theoretical models having been developed, the physical mechanisms leading to the observation of these effects both at ground and in space are as yet largely unexplained. That is, the following remains to be understood: the genesis of EME waves over the focal area (especially soon before a seismic event, if any); its propagation through lithospheric layers characterized by fixed vertical conductivity; its access into both the neutral atmosphere and ionosphere; and its arrival in the magnetosphere and its relative interaction. It is worth noting that, when a EME wave propagates through both the ionosphere and magnetosphere, the medium has to be considered dispersive too (Chen, 1977). Recently, Perrone et al. (2018) analysed ionosonde data and crustal earthquakes with magnitude M≥6.0 observed in Greece during 2003–2015 to check whether the relationships obtained between precursory ionospheric anomalies and earthquakes in Japan and central Italy are also valid for Greek earthquakes. They identified the ionospheric anomalies as observed variations of the sporadic E-layer parameters (h′Es, foEs) and foF2 at the ionospheric station of Athens and found similar corresponding empirical relationships between the seismoionospheric disturbances and the earthquake magnitude and the epicentral distance. Moreover, the large lead times found for the ionospheric anomalies' occurrence seem to confirm a rather long earthquake preparation period.

The search for ionospheric disturbances associated with earthquakes relies on thorough statistical studies to disentangle seismic effects from the variations induced by the physical processes that control the ionosphere dynamic and natural emissions. Several studies have been performed, in the framework of the DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) mission, and one of them has shown a decrease in the extremely low-frequency (ELF) wave intensity in the frequency range between 1 and 2 kHz a few hours before the shock (Parrot et al., 2006; Píša et al., 2012 and 2013; Zhang et al., 2012; Walker et al., 2013). Nĕmec et al. (2008) built a statistical map of electromagnetic wave intensity obtained from DEMETER satellite ICE and IMSC data available at that time (2004–2007). Then, they estimated the probability of occurrence during a seismic event of signals with higher intensity with respect to the background level defined by the map. Their study was the first attempt to generate a background map in the electromagnetic emission above seismic regions for the determination of the statistical distribution of the wave energy in the absence of seismic activity.

The present paper uses a completely new method to determine both the environmental and instrumental backgrounds in the electromagnetic emission above seismic regions by using the DEMETER satellite electric and magnetic field observations. This algorithm is based on a new data analysis technique called ALIF (adaptive local iterative filtering, Cicone et al., 2016, 2017; Piersanti et al., 2017b) and through a multiscale statistical analysis of the electromagnetic observations. The results obtained with this technique allowed the construction of an electromagnetic energy background map over the L'Aquila seismic region from 2004 to 2011. In addition, on 4 April 2009, 2 days before the 6.3 Mw earthquake (USGS Earthquake catalogue), when DEMETER flew exactly over L'Aquila at UT = 20:29, an anomalous signal with respect to the background was observed.

Figure 5 shows the characteristics of the DEMETER orbit (upper panel) that occurred on 4 April 2009 (2 days before a 6.3 magnitude earthquake), in terms of latitude and longitude position, and the relative electric (left panels) and magnetic (right panels) field observations. This orbit was identified as anomalous by our technique. In fact, Fig. 6, exhibiting ϵrel for both the electric and magnetic field components, shows an anomalous signal (s∗) at frequency f*=333 Hz, which is not present in quiet conditions (see Fig. 4). It is worth noting that the time of f∗ onset corresponds exactly to the DEMETER passage through the L'Aquila geographic footprint. s∗ has a peculiar electromagnetic (e.m.) polarization, characterized by a magnetic field oscillating principally in the y-z plane and an electric field (less clear situation) oscillating principally along the x-y plane (in the satellite reference frame).

Since ALIF extracts both the electric and magnetic field wave forms at each frequency, we were able to calculate the instantaneous phase difference between the two signals, resulting in ∼90∘. This condition allowed the evaluation of the Poynting vector S=E×B, showing the following characteristic angles with respect to the satellite coordinate system: ϑ1=167.1∘ and φ1=15.4∘ (ϑ and φ being the angles between S and x, and S and z, respectively). The direction of S confirms that s∗ is directed toward the satellite, coming from the ground.

Interestingly, the same peculiar frequency, f∗, was found on 11 February 2009, with lower (∼60 %) ϵrel (see Fig. 2) and comparable polarization in both magnetic and electric fields (not shown). Also for this case event, the evaluated direction of S confirms a signal coming from the ground (ϑ2=154.6∘ and φ2=6.4∘).

The correct identification of a background in the e.m. emission over seismic regions has a crucial role for the detection of possible signals related to earthquake or pre-earthquake activity. The algorithm presented here represents a new and very efficient technique to distinguish between instrumental, environmental, and external source signals from satellite observations. The efficiency of ALIF for both non-linear and non-stationary signal analysis, and peculiar frequency onset identification, has been proved in several works (i.e. Piersanti and Villante, 2016; Alberti et al., 2016, and references therein). In any event, its possible application to identify correctly the instrumental origin noise has never been presented before. Here, we showed that the coupling between ALIF and MSA represents a powerful tool to identify and remove noise from a signal. In fact, our method was able to determine all the noise frequencies declared in electric and magnetic field experiments of the DEMETER satellite (Lagoutte et al., 2005), such as 1 Hz in the E field (see Fig. 4). This signal is an effect of the instrumental disturbance, i.e. the sweeping voltage of the Langmuir probe (Lagoutte et al., 2005). Concerning the continuous 20 Hz signal detected in the magnetic field observations, we speculated that it can be attributed to one of the peaks of Shumann resonance in the ELF portion of the Earth's electromagnetic field spectrum generated and excited by lightning discharges in the cavity formed by the Earth's surface and the ionosphere (Barr et al., 2000, and references therein). In any event, Lagoutte et al. (2005) in their DEMETER satellite user guide manual certificated ∼20 Hz as a BANT (Boîtier Analogique et Numérique de Traitement) noise.

On the other hand, this paper presents a useful method for the correct selection of anomalous signals with respect to the evaluated background. The choice of using the ratio Rϵ(ℓ) was to take into account possible anomalous energy enhancements as well as a new signal onset. In addition, the choice of a threshold equal to 3σ makes the anomaly selection as strong as possible and should exclude possible false positives. In any event, at this stage, a visual inspection of each anomalous signal detected is needed. Last but not least, an analysis of the geomagnetic index behaviour associated with a possible e.m. anomaly detected by our method is crucial. In fact, it is worth remarking that the external origin perturbations, in terms of solar activity, represent the principal disturbance of both the Earth's ionospheric electric and magnetospheric fields (Vellante et al., 2014; Piersanti et al., 2017a, and references therein).

In this context, the 333 Hz component, appearing when the DEMETER flew exactly over L'Aquila (Fig. 6), is not visible in the corresponding background (in terms of Kp and M indices – Fig. 4) and then may be an interesting anomaly. In fact, 4 April 2009 was characterized by very low geomagnetic activity, since the Sym-H index was between 8 and 10 nT, and the AE index was less than 95 nT. This confirms that 4 April 2009 was a solar quiet (Sq) day (Matsushida and Maeda, 1965; Chulliat et al., 2005). Sq is caused by the concurring contribution of a current system flowing in the so-called ionospheric dynamo region and of the induced telluric currents in the Earth's upper mantle. Briefly, their interaction gives rise to two pairs of vortices: two in the sunlit hemisphere and the other two in the dark one (Richmond et al., 1976; Shinbori et al., 2014). This is confirmed by the behaviour of the geomagnetic field observation at L'Aquila ground station (Fig. 7), which presents the typical Sq daily variation at middle/low latitude in April (De Michelis et al., 2010). In addition, the inspection of the solar wind conditions coupled with a map of auroral oval emission from the DMSP satellite taken at 20:34 UT (not shown) confirmed the absence of any disturbance of solar origin, such as substorm activity, that could affect the low–middle latitudes' magnetic and electric fields. Indeed, L'Aquila geomagnetic trace (Fig. 7) did not show any Pi2 wave activity for the entire day (Olson, 1999; Piersanti et al., 2017a). Moreover, no ELF perturbations are observed between 20:30 and 20:40 UT. As a consequence, we can reasonably assert that s∗ cannot be related to any solar perturbation.

As a matter of fact, the relative Poynting vector S indicates a wave propagating from the ground to the ionosphere. The s∗ peculiar polarization might be associated with a horizontal current system flowing at the ground, switched on by an anomalous ground impedance generated by the fault break. It is thought that the low-frequency components (ULF/ELF) of seismo-electromagnetic emission (SEME) waves generated by pre-seismic sources (such as local deformation of fields, rock dislocation and micro-fracturing, gas emission, fluid diffusion, charged particle generation and motion, electro-kinetic, piezo-magnetic and piezoelectric effects, and fair weather currents) are transmitted into the near-Earth space (Dobrovolsky, 1989; Teisseyre, 1997; Pulinets and Kirill, 2004; Sorokin, 2001). During their propagation through the solid crust, the SEME waves characterized by lower periods are attenuated. As a consequence, only low-frequency waves (in the ULF/ELF band) can go over the Earth's crust and propagate through the ionosphere–magnetosphere system with moderate attenuation (Bortnik and Bleier, 2004). Observations from the low-Earth orbit (LEO) satellite seem to confirm this scenario. In fact, pre-seismic variations of electric and magnetic fields and of ionospheric plasma temperature and density (Parrot, 1993; Chmyrev, 1997, Buzzi, 2007) have been observed from a few minutes to several hours (2–6 h) prior to earthquakes of moderate or strong magnitude (M>4.0). Unfortunately, no magnetotelluric measurements that could confirm or contradict our hypothesis were available for the event under investigation. In any event, it is interesting to emphasize that, repeating the same analysis for cells further north and south than the L'Aquila cell (not shown), no anomalous signal centred at 333 Hz was found. So, we are confident that what is seen on 4 April only occurred above L'Aquila and not elsewhere.

Interestingly, on 11 February 2009, a similar signal, characterized by lower (∼60 %) ϵrel and comparable polarization, was observed on both electric and magnetic field components. Despite the direction of S confirming that this signal also comes from the ground (ϑ2=154.6∘ and φ2=6.4∘), nothing can be speculated as to its physical causes in this case. In fact, first of all, it is characterized by different solar activity conditions, with Sym-H between 40 and 50 nT, and AE between 150 and 200 nT. Last but not least, the satellite orbit was diurnal. Hence, to be consistent with our cell division method, 11 February 2009 cannot be compared to our quiet background or to the 4 April 2009 case event. In any event, its peculiar characteristics need to be investigated in a companion paper containing a statistical approach.

The analysis of the 4 April 2009 event showed that only through a multi-instrumental and multi-disciplinary approach can a reliable disentanglement of the earthquake effects from changes due to the physical processes that govern the ionosphere dynamic and natural EME be obtained.

This work could be considered as a suggested analysis approach for the forthcoming scientific phase of the first CSES mission (launched in February 2018, and still in the commissioning phase) aiming to reduce the lack of knowledge of lithosphere–ionosphere coupling. As soon as further applications, performed on different seismic events, reach the expected reliability, the proposed method could be used to compute the global background level (with 1 squared degree of resolution) for a direct real-time comparison of CSES in-flight data.