*Technical Note* **On the Geomagnetic Field Line Resonance Eigenfrequency Variations during Seismic Event**

**Mirko Piersanti 1,\*, William Jerome Burger 2,3, Vincenzo Carbone 4, Roberto Battiston 2,5, Roberto Iuppa 2,5 and Pietro Ubertini <sup>1</sup>**


**Abstract:** In this paper, we report high statistical evidence for a seismo–ionosphere effects occurring in conjunction with an earthquake. This finding supports a lithosphere-magnetosphere coupling mechanism producing a plasma density variation along the magnetic field lines, mechanically produced by atmospheric acoustic gravity waves (AGWs) impinging the ionosphere. We have analysed a large sample of earthquakes (EQ) using ground magnetometers data: in 28 of 42 analysed case events, we detect a temporary stepwise decrease (Δ*f*) of the magnetospheric field line resonance (FLR) eigenfrequency (*f* <sup>∗</sup>). Δ*f* decreases of ∼5–25 mHz during ∼20–35 min following the time of the EQ. We present an analytical model for *f* ∗, able to reproduce the behaviour observed during the EQ. Our work is in agreement with recent results confirming co-seismic direct coupling between lithosphere, ionosphere and magnetosphere opening the way to new remote sensing methods, from space/ground, of the earth seismic activity.

**Keywords:** earthquake; coseismic effects; field line resonance; acoustic gravity waves; lithosphere-magnetosphere coupling
