Study of geomagnetic field shielding patterns over the past 100,000 years

By | June 6, 2023

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Dipole power (a) and non-dipole power (b) evaluated at the Earth’s surface over the last 100 ka, as predicted by five continuous spherical harmonic geomagnetic field models: IMOLEe (Leonhardt et al., 2009), CALS10k.2 (Constable et al ., 2016), GGF100k (Panovska et al., 2018b), LSMOD.2 (Korte et al., 2019b) and GGFSS70 (Panovska et al., 2021). Five global and regional excursions are labeled on the top panel at: PB: Post Blake, NGS: NorwegianGreenland Sea, La: Laschamps, ML: Mono Lake/Auckland, and HP: Hilina Pali. The gray areas indicate the three excursions studied in more detail here: NGS, La and ML. The model time intervals and the models used to calculate Rc100k are indicated by the colored bars below. Credit: Journal of Space Weather and Space Climate (2022). DOI: 10.1051/swsc/2022027

New models of how the geomagnetic field that protects Earth’s atmosphere from cosmic rays has changed over tens of thousands of years can help us understand how climate has changed over a similar timescale.

Earth’s atmosphere is protected from the impact of cosmic rays and other energetic particles by a magnetic field, the geomagnetic field, which extends into space from our planet’s molten outer core. The strength of the geomagnetic field is not constant but varies over time scales of thousands and tens of thousands of years.

Now, a team of Chinese and German scientists, led by Jiawei Gao of the Institute of Geology and Geophysical Sciences of the Chinese Academy of Sciences in Beijing, has modeled fluctuations in the field over the past 100,000 years. Their research is published in Journal of Space Weather and Space Climate.

The geomagnetic field is a natural but very beneficial phenomenon as it protects the earth’s atmosphere from the impact of cosmic rays and other energetic particles, which produce long-lived radionuclides such as carbon-14. As it weakens, the flux of cosmic rays reaching the Earth increases. We know, for example, that every few tens of thousands of years the field experiences an “excursion” or “reversal” that significantly decreases its strength, and weaker and more rapid fluctuations superimpose such long-term changes.

“The global flux of cosmic rays reaching Earth’s atmosphere was up to three times higher at the midpoint of the so-called Laschamps excursion about 34,000 years ago than it is today, and about twice as high in another excursion about 65,000 years ago. years ago,” says Gao.

Understanding these changes over time can help us understand long-term patterns of solar activity and non-anthropogenic climate changes, ie those not caused by human activity, that occurred during prehistory. It is possible to measure the strength of the geomagnetic field shielding using a parameter of momentum per unit charge known as “stiffness”.

Charged particles with equal stiffness move in the same way. If one examines all charged particles moving towards the atmosphere at a given position and angle of incidence, only those with stiffness above a certain value will be able to penetrate it. This value, or the “geomagnetic shear stiffness” is a direct measure of the strength of the geomagnetic field and, therefore, of the degree of shielding.

Gao and his collaborators estimated the global shear stiffness using models of the geomagnetic field at individual time points during the last 100,000 years, comparing and combining four different models of the field.

“Early models were based on dipolar field assumptions. All advanced geomagnetic field models also include non-dipolar components, which are more accurate than those involving only dipolar components,” he says. ‘Using these models, we found that, during excursions (i.e. when the field strength is low) the flux of energetic cosmic rays in the atmosphere was high and, moreover, it was almost independent of latitude.’

These ‘best available models’ developed by the research team allow scientists to estimate the rate of radionuclide production and thus the cosmic radiation dose rate and solar activity during this period. This will help them explore how the climate changed throughout prehistory, which should provide useful insights into the mechanisms and effects of anthropogenic climate change today. Although high-energy particles from outside the solar system can affect Earth’s climate, there is scientific consensus that these factors are not responsible for the warming trend we have seen in recent decades. Recent global warming and associated climate change are attributed to human activities, especially greenhouse gas emissions.

More information:
Jiawei Gao et al, Shielding of the geomagnetic field over the past hundred thousand years, Journal of Space Weather and Space Climate (2022). DOI: 10.1051/swsc/2022027

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