The incessant, multidirectional bombardment of cosmic rays originating from beyond our Solar System might not be as uniform as previously understood.
Data relayed by China’s Chang’e 4 lunar lander, situated on the far side of the Moon, indicates the presence of a peculiar ‘void’ in the cosmic ray flow between Earth and the Moon. This phenomenon emerges when these celestial bodies achieve a specific orbital alignment.
This revelation suggests that galactic cosmic rays are not distributed with the level of homogeneity we had presumed, potentially paving the way for advancements in space exploration aimed at mitigating the radiological dangers posed by these energetic charged particles.
The cosmic environment is a dynamic arena, characterized by intense energetic particle emissions stemming from cataclysmic events like supernova explosions and their resultant remnants, which propagate cosmic rays indiscriminately at prodigious velocities. These particles are predominantly protons, accompanied by a fraction of helium nuclei and a minor component of heavier atomic nuclei, and are generally considered to be nearly omnipresent.
Furthermore, these particles constitute ionizing radiation – the kind capable of dislodging electrons from atoms within biological tissues, leading to cellular damage, DNA degradation, and an elevated predisposition to mutations that can foster oncological development, thus presenting a significant health concern.
Galactic cosmic rays (GCRs) are largely attenuated by Earth’s atmospheric shield before reaching the planet’s surface. Nevertheless, they represent a substantial radiation threat to astronauts and individuals traveling at high altitudes, a risk that is acknowledged as an inherent aspect of such endeavors and carefully factored into mission architecture and technological design.
The intensity of the GCR flux, essentially the ambient GCR radiation level, can fluctuate in response to solar activity. This flux experiences a marked decline during periods of solar maximum owing to the amplified solar wind and magnetic activity, which effectively deflect a substantial proportion of these incoming particles.
While the Sun is a primary factor influencing GCR shielding, new research from an international consortium indicates that Earth’s magnetic field also plays a role, albeit indirectly linked to solar influences.
These groundbreaking observations stem from the Chang’e 4 mission. Since 2019, its Lunar Lander Neutron and Dosimetry (LND) instrument, deployed on the lunar far side, has been diligently monitoring proton levels. This measurement capability is restricted to lunar daylight hours when the lander is illuminated by the Sun, as the Moon’s frigid temperatures render the lander inoperable during the prolonged lunar night.
However, this daytime operational window provides an exceptional vantage point for assessing the impact of Earth’s magnetosphere on the GCR flux. The research team meticulously compiled data spanning 31 lunar orbital cycles, scrutinizing variations in proton flux as the Moon traversed its trajectory around Earth.
Their findings revealed a distinct region within the Moon’s orbit – specifically, the prenoon sector, prior to reaching local solar noon – where the proton flux registered approximately 20 percent lower compared to other orbital segments.
The investigators posit that this observed reduction is likely associated with the alignment of the interplanetary magnetic field (IMF), which comprises the outward extension of the Sun’s magnetic field throughout the Solar System.
As the Sun rotates, its magnetic field assumes a spiral configuration, termed the Parker spiral. When this spiral geometry aligns optimally with the Earth-Moon system, a palpable reduction in GCRs, a GCR cavity, comes into being.
“Generally, the trajectory of charged particles within a magnetic field is characterized by a helical path following the magnetic field lines,” state the researchers in their publication.
“When the Moon occupies the prenoon sector under Parker spiral conditions, the local IMF lines may orient themselves in such a manner that they establish a connection between the Moon and Earth’s robust magnetic field. Consequently, the motion of particles, particularly the protons we have documented, along these field lines is influenced by the formidable magnetic field emanating from Earth.”
Thus, the curved trajectories of the interplanetary magnetic field extend through space, and at a specific vantage point, they bend towards Earth, intersecting the planet’s magnetic field and engendering a localized GCR ‘attenuation zone’. As the Moon traverses this region, a process that spans approximately two days, the Chang’e 4 instrument registers a discernible decrease in the proton flux originating from GCRs.
This discovery, according to the research team, holds the potential to offer strategies for minimizing astronaut exposure to harmful radiation.
“This finding presents a viable strategic approach for mission planning, particularly for crewed lunar missions and extravehicular activities, as operations could be scheduled to coincide with these periods of reduced radiation intensity, thereby diminishing the associated exposure risks,” the researchers articulate.
“Future investigations employing more extensive datasets may provide further clarification on the spatial extent and dynamic behavior of this cavity, yielding deeper insights into potential radiation mitigation strategies. These insights could be applicable not only within the Earth-Moon system but also to missions in proximity to other magnetized celestial bodies within our Solar System.”
The outcomes of this study have been disseminated in the esteemed journal Science Advances.
