A large earthquake of 7.8 magnitude occurred on 25 April 2015, 06:26 UTC, with the epicenter in Nepal. Here, taking advantage of measurements provided by the Swarm magnetic satellites, we investigate the possibility to detect some series of pre-earthquake magnetic anomalous signals, likely due to a lithosphere-atmosphere-ionosphere coupling, that can be a potential earthquake precursory pattern. Different techniques have been applied to Swarm data available during two months around earthquake occurrence. From the detected magnetic anomalies series (during night and magnetically quiet times or with an automatic detection algorithm), we show that the cumulative number of anomalies follows the same typical power-law behavior of a critical system approaching its critical time, and hence recovers as the typical recovery phase after a large event. The similarity of this behavior with the one obtained from seismic data analysis and the application of the analyses also to another period without significant seismicity do support a lithospheric-linked origin of the observed magnetic anomalies. We suggest that they might be connected to the preparation phase of the Nepal earthquake. Confidential manuscript submitted to Earth and Planetary Science Letters • earthquake [e.g. Occhipinti et al., 2013, and references therein]. This offers the possibility to retrieve seismic information from ionospheric observations. An important and debated question arises about the possibility that, during the phase of EQ preparation, electromagnetic waves and/or particles could be transferred from the solid Earth (in particular the lithosphere) to the atmosphere, with a particular effect in the ionosphere, above around 50 km [e.g. Pulinets and Boyarchuk, 2004; Freund, 2011; Pulinets and Ouzounov, 2011; De Santis et al., 2015]. One of the most general models of coupling is based on the emission of a radioactive gas [Pulinets and Boyarchuk, 2004] or metallic ions [Freund, 2011] before a large earthquake, which may change the distribution of electric potential above the surface of the Earth and then up to the ionosphere [e.g., Pulinets and Boyarchuk, 2004; Sorokin et al., 2001]. Penetration of the electric field to the ionosphere could produce ionospheric plasma density and/or conductivity anomalies, which are observed above seismic zones [e.g., Liu et al., 2006; Kon et al., 2011]. An alternative explanation is that the radon emitted before an earthquake would increase the conductivity of air at ground level and that the ensuing increase of current in the fair weather global circuit would lower the ionosphere [Harrison et al. 2010]. Therefore, it is expected that low Earth orbiting (LEO) satellites could be the best possible dedicated platforms of sensors to detect any electromagnetic, acoustic or infrared seismic-linked precursors. Certainly, space observations have to be investigated together with ground (and near-surface) seismic and other geophysical observations, in order to have a more complete picture of the possible involved phenomena.
