Montag 07.03.2022 (15:00 - 16:00 Uhr)

The geomagnetic field: A signal for probing processes from the Earth’s core to outer space

Hermann Lühr

(Deutsches GeForschungsZentrum GFZ, Sektion 2.3, Geomagnetismus, Potsdam)

The geomagnetic field is one of the few physical phenomena that can fully penetrate the Earth, and it extends out into deep space. A lot can be learned from it about the Earth interior and electric currents in near-Earth space.
Early magnetic field measurements were motivated primarily by its use for navigation purposes. Meanwhile, magnetic fields serve more as a tool for remote sensing. For extending the dataset into the past, magnetic fields stored in rocks, sediments, and artefacts are retrieved. They provide valuable information about long-term characteristics of the geodynamo and about tectonic evolution.
Nowadays, high-resolution measurements from magnetic survey satellite missions like CHAMP and Swarm provide detailed temporal and spatial information of the geomagnetic field. Latest versions of the highly reliable main field models, covering two decades, are so precise that observed small deviations can be interpreted as signals from unmodeled sources. From the global distribution of crustal sediment and rock magnetization we learned about plate tectonics, crustal age and geology.
Magnetic fields also result from electric currents, e.g. the magnetospheric ring current causes a southward field deflection. During magnetic storms it is strongest in the 18h local time sector and weakest around 06h LT. Recent studies offer an alternative explanation to the traditional ‘partial ring current’ picture. The distribution of the solar quiet (Sq) current system has well been confirmed by space observation. But the interhemispheric currents, connecting the vortices in the two hemispheres, are found to be markedly different from earlier expectations. The equatorial electrojet (EEJ) is a current flowing above the magnetic equator. With the help of spacecraft its intensity distribution and variability has be resolved. It turns out, the EEJ is a sensitive detector for tracking upper atmospheric dynamics and tides, which are partly relevant for climatological changes.
Also other terrestrial processes produce small magnetic signatures, e.g. salty seawater flowing across the geomagnetic field induce electric currents. Their magnetic fields can be used to estimate ocean circulation and tides. The low-latitude ionosphere tends to become unstable after sunset, which can badly degrade or disrupt GPS navigation signals. These density fluctuations are accompanied by magnetic signatures due to their diamagnetic effect. Their global distribution can thus be mapped by satellites.