Earth is surrounded by its constantly changing magnetic field that shields our habitat against cosmic rays and solar wind. Investigating past geomagnetic field changes relies on detailed and time-consuming laboratory experiments on paleo- and archeomagnetic materials. A recent article in Reviews of Geophysics summarizes current knowledge about global geomagnetic field evolution over the past hundred thousand years. Here, the authors outline the main characteristics found in global paleomagnetic field compilations and reconstructions, and their applications and challenges.
Why does Earth’s magnetic field change and how are these variations observed and recorded?
Earth possesses a magnetic field that is generated by a self-sustaining dynamo in the planet’s liquid outer core. Magnetohydrodynamic processes in the core generate continuous temporal and spatial variations in the geomagnetic field, and fluctuations in the geomagnetic field on time scales of years to millennia are referred to as geomagnetic secular variation.
Spatial and temporal variations of the geomagnetic field have been observed directly via satellites, geomagnetic observatories, and historical measurements over the past 500 years. Further study of such variations over periods of hundreds to millions of years requires very long records. Paleomagnetic studies provide indirect measurements, based on remanent magnetization in lake and marine sediments, volcanic rocks, and archeological artifacts, and generate information on these timescales.
What are some of the applications of knowing about Earth’s magnetic field and its variations over time?
The basic scientific interest is to understand the “geodynamo”, the source of the magnetic field, and a fundamental property of our planet whose change over time reflects the structure and evolution of Earth’s deep interior.
Global reconstructions over long time intervals are essential to validate numerical simulations which, through comparisons with real field behavior, reveal the underlying physics.
The geomagnetic field provides an energy barrier that shields the planet from cosmic rays, may have contributed to the formation and preservation of the atmosphere, and continues to affect past and current space climate and weather.
Models provide the basis for navigation by magnetic compass and on recent time scales support the ability to navigate with mobile phones. A variety of animals (whales, birds, fish, turtles, insects, bacteria) sense the geomagnetic field and use it for orientation.
Geomagnetic variations also affect cosmogenic isotope production, and provide age constraints for a variety of climate, sedimentary, and archeological studies.
What has recent research revealed about the frequency and global picture of geomagnetic field changes over different timescales?
For the first time, we have a global view of the geomagnetic field over an extended period, as shown in the figure at the top. This includes geomagnetic “excursions”, events when geomagnetic field intensity decreases dramatically, and directions lie far from the basic dipole approximation.
The hundred thousand-year model exhibits a broad range of variations in geomagnetic axial dipole strength, as shown in the figure below, with the lowest value during the Laschamp excursion (approximately 41 thousand years ago).
Except for the Laschamp excursion, which is seen globally in the model, other apparent excursions appear in limited locations and are likely regional in nature. Transitional or reversed directions during excursions do not occur simultaneously all over the globe. And, the regional duration of the excursions varies from a few centuries to about 3.5 thousand years.
Recent models suggest that the Laschamp excursion, which occurred about 41 thousand years ago, mainly manifests as a decay in axial dipole strength and subsequent recovery, with normal amplitude secular variation persisting throughout in the non-axial-dipole contributions.
An interesting observation is the similarity of the present-day geomagnetic field to some other instances over the past hundred thousand years, as seen in the figure at the top. It has been suggested that the current dipole decrease and association with the South Atlantic Anomaly where field intensity is unusually weak might indicate an imminent field reversal or excursion.
However, none of these similar cases progressed to transitional events: thus the present field morphology cannot be taken as any clear indication of an upcoming reversal or excursion.
What are some of the unresolved questions where additional research, data or modeling is needed?
The spatial and temporal data distribution, and the temporal resolution of paleomagnetic records decrease considerably with age. All paleomagnetic models therefore provide a rather blurred picture of the past geomagnetic field, and generally have large uncertainties.
Thus continued effort is required to improve data coverage with high quality records that can enhance understanding of the paleomagnetic field. A strict assessment of the records is necessary to identify robust features and assess regionally inconsistent data.
Chronological information is another key area for improvement: the quality of independent and up to date age control is of paramount importance to characterize field variations, especially during transitional events.
Regarding the modeling approaches, proper estimation and treatment of chronological and data uncertainties is needed to derive reliable field models. Furthermore, investigations of even longer periods and other geomagnetic excursions and reversals are required to identify the mechanisms behind these extreme geomagnetic field variations.
—Sanja Panovska ([email protected]; 0000-0003-4340-5668), GFZ Potsdam, Germany; Monika Korte ( 0000-0003-2970-9075), GFZ Potsdam, Germany; and Catherine G. Constable ( 0000-0003-4534-4977), University of California San Diego, USA