Ar-Rehman

Earth's strong magnetic field protects us from radiation’s damaging effects

Barreling through the universe with incredible power and speed, galactic cosmic rays are a major source of radiation in space. But thanks to Earth’s strong magnetic field, these charged particles don’t usually make it directly to our planet, so we are protected from the radiation’s damaging effects. This field may be doing much more: new data collected by China’s Chang’e 4 lunar lander shows that our magnetic field’s influence is so powerful that it extends farther into space than previously believed, stretching even beyond the moon. A major defense against everything space throws at us, Earth’s magnetic field may even protect the moon from damaging galactic cosmic rays. Though it is far from Earth’s magnetic core, the moon feels even more of the core’s effects than scientists previously thought. High-energy particles called galactic cosmic rays (GCRs) bombard unprotected objects in space, often causing damage. Earth magnetic field creates a protective shell around the planet which can deflect dangerous charged particles, like GCRs. The moon is known to pass through the tail-like part of Earth's magnetosphere, but a new study, suggests the moon might experience additional protection at another point in its orbit. Although this pocket of protection exists when the moon is outside of the magnetosphere, researchers believe the effects are still due to Earth's magnetic field.

Illustration of the formation of the GCR cavity in the ecliptic plane. white lines from the Sun show the typical pattern of magnetic field lines in interplanetary space, referred to as the Parker spiral IMF. The magenta segment of the lunar orbit (dashed white circle) indicates the operational periods of LND, specifically from LP = 7.5 to 16.5 hM. The cylindrical spirals in two colors indicate two opposing directions of motion for GCR protons along the magnetic field lines. Shielded by Earth's magnetic field, two regions of reduced GCRs in the near-Earth space are expected to exist, as marked by the shadowed areas. When the Moon moves to the afternoon sector, the angle (φ) between the IMF and the Earth-Moon vector reaches 90◦ at LP = ∼16 hM, allowing the propagation of GCRs to remain unobstructed in the afternoon sector.

When the research team analyzed data taken from the Lunar Lander Neutron and Dosimetry (LND), onboard China's Chang'E-4 lander, they were surprised to find that the LND experienced a 20% dip in GCR particles hitting detectors while the lander was on the moon's far side. This occurred at a specific time during the lunar "morning" and only for about 2 days each lunar cycle. Since the LND took data over 31 cycles, the team could see that this was not just a one-off occurrence. This was unexpected because it was previously assumed that GCRs are evenly distributed in the space between Earth and the moon, outside Earth's magnetosphere. Researchers describe a “cavity” in space between Earth and the moon where cosmic rays are deflected by Earth’s magnetic field. This suggests that the effects of our planet’s magnetism are present much farther from us than anyone could have expected. GCRs consist of different types of charged particles with varying energies. Most (about 85%) are protons, while about 12% are helium atoms, and only around 1% are heavier nuclei. The data showed that the reduction in particle count was most pronounced for lower-energy protons. Higher energy particle counts were also reduced, but to a lesser degree.

Launched in 2018, Chang’e 4 was the first spacecraft to land on the far side of the moon. Among the many scientific instruments onboard was the Lunar Lander Neutron and Dosimetry experiment, which was designed to measure the radiation future astronauts might experience if they were to land there. Scientists had long assumed most of the moon lay beyond the protection of Earth’s magnetic field, but in 2019 scientists began noticing something odd about the experiment’s data which suggested the moon was somewhat protected from galactic cosmic rays. Magnetic fields don't simply stop existing at a certain point. Instead, their influence just decreases more and more with distance away from the source. The magnetosphere is the region where Earth's magnetic field prevails over the solar wind magnetic field. So, although the moon was outside of Earth's magnetosphere during these points of lower particle counts, its magnetic field was still exerting some degree of magnetic force, enough to influence the GCR particles. The team says that the particles were deflected due to the gyroradii of the particles, which is the radius of the circular motion they follow in the presence of a uniform magnetic field. This is also dependent on the mass of the particle, its velocity and its charge. "The size of Earth's magnetosphere on the dayside extends about 6 to 10 Earth radii, which is comparable to the gyroradii of lower-energy protons. Therefore, the lower-energy particles are easily affected by Earth's magnetic field because of their smaller gyroradii compared to the higher-energy particles," the study authors write.

The finding came as “a surprise,” says Robert Wimmer-Schweingruber, a co-author of the study and a physicist at Kiel University in Germany. “Personally, I didn’t believe it for a long, long time. I thought it was an artifact in the data until we did a lot of statistical tests.” Galactic cosmic rays originate from a variety of sources in space, such as stars, supernovae and black holes. These diverse origins mean that by the time the rays get near Earth, they don’t all carry the same level of energy. The highest-energy particles move quickly through the solar system, while some of the weaker particles linger, and their radiation could affect astronauts, Wimmer-Schweingruber says. “These low-energy particles weren’t that interesting to us until we saw this effect, and then we realized this is actually important for the skin dose of astronauts,” he says. Although the full spatial extent of the cavity is not precisely determined yet, the team believes these findings are valuable for future space missions. Knowing where areas of reduced radiation are can help to keep astronauts and equipment safer on future missions, since GCR particles are seriously detrimental to human health and can damage equipment. This finding provides a potential strategy for mission planning, especially for manned lunar missions and extravehicular activities, as operations could be timed to coincide with these lower radiation periods to reduce exposure risk. Future studies with extended datasets could further clarify the spatial extent and behavior of this cavity, offering deeper insights into potential radiation protection strategies, not only for the Earth-Moon system but potentially for missions near other magnetized bodies within the solar system," the study authors write.

Shielding astronauts from the dangers of radiation is critical to ensuring a human presence in space. This means creating materials which are light enough to bring into space but protective enough to keep radiation at bay, says Philip Metzger, a professor of planetary science and space technology at the University of Central Florida. Knowing more about the distribution of radiation in space, and especially between the moon and Earth, could help scientists plan safer missions. To further validate these findings, the team conducted particle simulations to model the effect of Earth's magnetic field on GCR propagation. These simulations, along with previous additional spacecraft data confirmed the observed reduction in GCRs at these locations. If NASA’s plan to put humans on the moon in a semipermanent capacity comes to pass, then it may make sense for astronauts to schedule activities outside any sheltered habitats while the moon is within the influence of Earth’s magnetic field. “It is brilliant research, and it just shows us that the more we study phenomena outside of our planet, the more we discover we don’t know,” Metzger says. “That’s why we need to do space missions.”