Electromagnetic effects (ULF/ELF waves observed on ground and at Low Earth Orbit altitudes), anomalies in ionospheric and atmospheric parameters (plasma density, TEC, VLF transmission) and bursts of electrons precipitating from the Van Allen Belts can be considered one of the main pre-seismic phenomena. Reports on observations from space of earthquake-related signals in the ELF-VLF frequency band in the ionosphere and low magnetosphere have appeared in the last years. In particular, the DEMETER mission has showed an increasing of the plasma density in vicinity of the epicenter of a future earthquake days before the main shock; the intensity of these anomalies was enhanced when the earthquake magnitude increased and reduced when the depth increased.
Relevant are the researches performed by experiments on board satellites concerning flux disturbances of Van-Allen trapped particles. Anomalous increase of low-energy electron and proton counting rate was observed several hours before the occurrence of moderate and large-magnitude earthquakes. The results of a recent analysis, carried out by data obtained with GAMMA-1, Sampex-PET and ARINA missions, are shown in Fig. 1. A spatial and temporal statistical correlation between seismic activity and the charged particle precipitation from the lower limit of the Van Allen radiation belts cleary appears. A model for this process is shown in Fig. 2. During an earthquake preparation, a lightning discharge could radiates ELF and VLF electromagnetic waves into the Earth-ionosphere waveguide which propagate away from the source, illuminating a large region of the bottomside ionosphere; a portion of the radiation incident on the bottomside ionosphere leaks through to the topside ionosphere, experiencing collisional losses along the way, and coupling into the whistler mode of propagation; as the whistler propagates, it alters the pitch angles (the angle between the particle's velocity vector and the local magnetic field) of energetic electrons, causing some electrons to precipitate into the dense upper atmosphere within one bounce period.
The particle detectors (HEPD and HEPP) on board CSES satellite are specialized to carefully study these intriguing effects.
a) b) c)
Fig. 1 - Time Difference Distribution. Zero is the current time of the particle burst. ΔT is the time difference between earthquake and the detected electron burst. a) GAMMA-1 mission, 1990-1992, Altitude 350 km, Inclination 51°, E > 50 MeV b) SAMPEX/PET mission 1992-1999, Altitude 520-740 km, Inclination 82°, 4≤E≤15 MeV c) ARINA mission 2006-in operation, 3≤E≤30 MeV
Fig. 2 - A model for pre-seismic ULF EMEs generated in the earthquake preparation area which resonantly interact with charged particles oscillating at bouncing frequency in the inner Van Allen radiation belt