The constant, omnidirectional flux of cosmic rays traversing the Solar System, originating from beyond the galaxy, might not be as uniform as previously assumed.
Data collected by China’s Chang’e-4 lander, positioned on the far side of the Moon, suggests the presence of a peculiar “void” within the cosmic ray stream between Earth and the Moon. This void seemingly materializes when these two celestial bodies align in a specific configuration. The research findings detailing this phenomenon were published in the journal Science Advances.
This revelation implies that galactic cosmic rays are not distributed with the evenness we once believed, potentially unlocking avenues for space exploration that could help mitigate the radiation hazards posed by these charged particles.
Outer space can be an incredibly dynamic region, abundant with all sorts of spectacular events that pepper the cosmos with energetic particles—supernova explosions and their remnants, for instance, haphazardly eject cosmic rays at tremendous velocities. These rays are predominantly protons, accompanied by a lesser amount of helium nuclei and a minor fraction of heavy atomic nuclei, and they are generally considered to be widely prevalent throughout space.
Furthermore, they emit ionizing radiation—you know, the kind capable of knocking electrons off atoms within your body, leading to DNA damage and an increased susceptibility to mutations that might result in cancer—so, it’s decidedly not benign.
Galactic Cosmic Rays (GCRs) are largely intercepted by Earth’s atmosphere before reaching the surface. Nevertheless, they constitute a substantial radiation threat to astronauts and high-altitude pilots, a risk acknowledged as part of their operational duties and factored into the design of missions and the technologies supporting them.
The intensity of the galactic cosmic ray (GCR) background flux can fluctuate depending on solar activity. During a solar maximum, this flux decreases considerably, as the heightened solar wind and magnetic activity deflect a significant portion of the particles away.
According to a new analysis by an international team of scientists, the Sun isn’t the sole entity capable of blocking galactic ray emissions; Earth’s magnetic field can also perform this function, though the Sun remains an indirect participant in this process.
This evidence was gathered by the Chang’e-4 spacecraft, which has been situated on the Moon’s far side since 2019, utilizing its Lunar Lander Neutron Detector (LND) instrument to monitor protons. This monitoring is restricted to lunar daytime, when its location receives sunlight, as the Moon becomes too frigid for the lander to operate after sunset.
However, this daytime operation provides an excellent opportunity to gauge the impact of Earth’s magnetic field on the galactic cosmic ray flux. The researchers analyzed data spanning 31 lunar cycles, specifically looking for systematic variations in the proton flux as the Moon orbited Earth.
They uncovered that during one segment of its orbit—the pre-noon sector, prior to reaching local noon relative to the Sun—the Moon passes through a region where the proton flux is approximately 20 percent lower compared to the rest of its orbital path.
The researchers surmise that this localized reduction is likely correlated with the alignment of the Interplanetary Magnetic Field (IMF), which is the component of the Sun’s magnetic field that extends far into the Solar System.
As the Sun rotates, its magnetic field becomes twisted into a spiral shape, known as the Parker Spiral. When this spiral aligns with the Earth-Moon system in a particular configuration, it gives rise to a cavity where galactic cosmic rays are comparatively absent.
“Generally, the motion of charged particles in a magnetic field is characterized by gyrating helically along the magnetic field lines,” the researchers state. “When the Moon is in the pre-noon sector under the condition of a Parker Spiral, the local interplanetary magnetic field lines can align in such a way as to connect the Moon to a region of strong terrestrial magnetic field. Consequently, the motion of particles along these field lines, particularly the protons we report here, is susceptible to the influence of the Earth’s strong magnetic field.”
Effectively, the curved lines of the IMF bend in space, tilting towards Earth in a specific orientation and intersecting the planet’s magnetic field, thereby creating a kind of “shadow” that shields against galactic cosmic rays. As the Moon traverses this shadow—a process taking roughly two days—the Chang’e-4 satellite registers a dip in the proton flux originating from galactic cosmic rays.
According to the investigators, this finding could present a viable means for minimizing astronaut radiation exposure.
“This discovery offers a prospective strategy for mission planning, especially for crewed lunar missions and extravehicular activities, as operations could be scheduled to coincide with periods of lower radiation levels to reduce exposure risks,” the researchers write. “Future investigations utilizing expanded datasets could further delineate the spatial dimensions and behavior of this void, leading to a deeper comprehension of potential radiation shielding strategies not only for the Earth-Moon system but possibly for missions near other magnetized bodies within the Solar System.”
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