Cosmic rays passed through Earth’s atmosphere 41,000 years ago: New findings from Lashon’s travels

Earth’s magnetic field protects us from dangerous radiation from space, but it’s not as permanent as we think. Scientists at the European Geosciences Union conference presented new information about a “journey” 41,000 years ago, when our planet’s magnetic field weakened and harmful space rays bombarded the Earth.

Earth’s magnetic field envelopes our planet from the onslaught of cosmic radiation traveling through space, while also protecting us from charged particles thrown outward by the Sun. But the geomagnetic field is not static. Magnetic North not only wobbles away from True North (a geographically defined position), but it also occasionally flips. During these reversals, north becomes south and south becomes north, and in the process, the strength of the magnetic field weakens.

But there’s also a phenomenon called field excursion, a brief period of weakening magnetic field strength where the familiar dipole (or two magnetic poles) may disappear and be replaced by multiple magnetic poles. The Lachon trip, which took place approximately 41,000 years ago, is one of the best-studied. Its magnetic field is lower, which means the Earth’s surface is less protected from harmful space rays. Periods of low magnetic field strength may be associated with major upheavals in the biosphere.

To understand when cosmic rays are bombarding Earth’s surface, scientists can measure cosmic radioactive species in the cores of ice and ocean sediments. These special isotopes are produced by the interaction of cosmic rays with Earth’s atmosphere. They are produced by cosmic rays, so they are cosmogenic.

The lower paleomagnetic intensity unshielded time should be related to the higher production rate of cosmogenic radionuclide species in the atmosphere. Sanja Panovska, a researcher at GFZ Potsdam in Germany, will present her findings on the relationship between paleomagnetic field strength and cosmogenic nuclides during Lachand’s travels next week during the 2024 European Geosciences Union (EGU) Congress, focusing on space climate.

Changes in cosmogenic radionuclide species such as beryllium-10 provide an independent proxy for how Earth’s paleomagnetic intensity changes. In fact, Panowska found that the average production rate of beryllium-10 during Laschamps’ trip was twice what it is today, meaning that the magnetic field strength was very low and there were a lot of cosmic rays reaching the Earth’s atmosphere.

To gain more information from cosmogenic radionuclide and paleomagnetic data, Panowska used these two data sets to reconstruct the geomagnetic field. Her reconstructions show that when the magnetic field weakened dramatically during Lachon’s trip, the magnetosphere contracted, “thereby reducing our planet’s shielding,” she said.

“Understanding these extreme events is important for their future occurrence, space climate predictions, and assessing their impact on the environment and Earth systems.”