What is the history of the Earth global magnetic field and how will it evolve? Will we lose our magnetic shield against the solar wind, one day? Being able to answer these questions is of prime importance for humankind.
The Earth possesses a global magnetic field of internal origin which interacts with the solar wind, producing currents of charged particles around the globe producing an external magnetic field. A dynamo, convection movements deep inside the planet’s liquid metallic outer core, generates the Earth's internal magnetic field. The dynamo varies in time and has a temporal drift known as the secular variation, as well as sudden changes called jerks. Finally, the Earth crust contributes to the internal field, depending on when and how rocks got magnetized in the presence of a global magnetic field during a given geological process. The external field is sourced by the ionospheric and magnetospheric electrical currents of the plasma ionized particles, resulting from the interaction between the solar wind and Earth’s internal magnetic field. For decades, the global magnetic field (internal and external) has been carefully monitored by networks of geomagnetic observatories, low altitude aircraft magnetic surveys, and satellite measurements from space, in order to determine the field morphology and its temporal variation. In our Solar System, the Earth is a unique case where so many different technological approaches have been used to unveil the complex characteristics of the magnetic field.
Global/regional measurements, coupled with theoretical model developments, and paleomagnetic analysis of collected samples, are being employed to constrain the, not yet fully understood, evolution of our planet’s magnetic field. In order to understand how the Earth dynamo mechanisms work, it is required to understand how a planetary dynamo operates as a whole, from its birth to its demise. For this purpose, our inner Solar System provides a natural laboratory. Today, amongst all our companion telluric planets, only Mercury possesses a core magnetic field. Venus has no observable internal magnetic field, which is enigmatic. Crustal magnetic fields are observed at the surface of Mars and the Moon, which is indicative that these bodies likely had a dynamo in their history, but is no longer active. More importantly, these crustal fields hold fundamental information about the ancient core field, such as its morphology, intensity and temporal variation. By studying the crustal magnetic fields of other planetary bodies, such as Mercury, the Moon or Mars, where different dynamos might have operated, the dynamo processes themselves can be better understood.
The planetary crustal magnetic fields of the Moon, Mercury, and Mars have been further investigated in the context of the SIGMA project. A method has been established, for the first time, to retrieve the location and geometry of regional magnetic sources (or magnetized material) by using spacecraft magnetic field data only. This work gives the opportunity to study further the magnetic anomalies detected on planetary surfaces using spacecraft, which allows to relate the origin of their magnetized sources to both dynamo and geological histories. Besides the use of spacecraft data, terrestrial analogs of other planet’s volcanic structures have been used as a natural laboratory to understand the magnetic signals measured from the ground to low altitude and their relationship with the rock capability to hold magnetization (content in magnetic carriers). The SIGMA project also contributed in assessing the benefits for future Martian or Lunar missions to include instrumentation measuring rock magnetic properties. In detail, this project contributed to proving that a susceptometer is an instrument that can be used to distinguish geological structures based on their magnetic properties.
