Natural Hazards induced by tsunamis, earthquakes and volcano eruptions occurring mostly around the areas with large human population have caused tragedies resulting in death of many people during and after these violent events, as well as inevitable environmental devastation. For instance, the death toll from the tsunami following the Sumatra earthquake in 2004 reached at least 225,000 (Encyclopædia Britannica, 2018). Destructive tsunamis with a high energy density and moderate occurrence rate make them part of a very challenging scientific task enforcing us to develop a well-functioning early warning system. However, we are committed to better understand the entire mechanism governed by many coupled systems ranging from deeper regions of the Earth’s interior to the upper atmosphere.
The observations and modelling of the ionospheric parameters have confirmed the existence and detectability of tsunami-triggered gravity waves in the ionosphere. Physically, a displacement induced by tsunamis at the sea surface is transmitted into the atmosphere, where it produces internal gravity waves (IGWs) penetrating upward. The normal ocean surface variability has a typical high frequency – compared to tsunami waves – and does not transfer detectable energy into the atmosphere. In contrast to the typical ocean surface variability, a tsunami produces propagating waves in the atmosphere. During the upward propagation, these waves are strongly amplified by the double effects of the conservation of kinetic energy and the decrease of atmospheric density, resulting in a local displacement of several tens of meters per second at 300 km altitude in the atmosphere. This displacement can reach a few hundred meters per second for the largest events.
At an altitude of about 300 km, the neutral atmosphere is strongly coupled with the ionospheric plasma producing perturbations in the electron density (ED). These perturbations are visible in the TEC parameter, i.e. the ED integral along the line-of-sight, calculated from the data acquired from the dual-frequency GNSS receivers, and in the ionograms and resulting ED profiles. In the range approximately between 5 and 15 minutes, the waves generated at the sea surface associated with tsunami can reach ionospheric altitudes, creating measurable fluctuations in the ionospheric plasma and consequently in Total Electron Content (TEC).
The ionospheric-based tsunami detection method is much more accurate when based on the availability of dense networks of GNSS receivers and/or ionospheric sounders. These networks are sufficiently dense in the land but there is a sparsity of observation points in the oceans. The use of Swarm data can improve basically the detection capability, especially over the oceans where the tsunami occurrence is most probable. The three Swarm satellites are equipped with instruments to measure the direction and the strength of the magnetic and electric field as well as ionospheric plasma parameters, such as the ED or the ion drift. Additionally, dual-frequency on-board GNSS receivers allow for precise orbit determination as well as monitoring of the Earth’s gravity field and TEC. From the latter, TEC values and TID signatures can be derived. Our focus in this project is to use both, the ED measurements from the Langmuir Probe (LP) as well as the information provided from the SWARM on-board GNSS receivers.