In a major finding, scientists from Space Physics Laboratory, VSSC, analysing radio signals from India’s Chandrayaan-2 (CH-2) orbiter – which is in good health and providing data – have revealed that the Moon’s ionosphere exhibits unexpectedly high electron densities when it enters the Earth’s geomagnetic tail. This finding sheds new light on how plasma behaves in the lunar environment and suggests a stronger influence of the Moon’s remnant magnetic fields than previously thought.
The scientists have used an innovative method to study the plasma distribution around moon. In this method they conducted experiments using the S-band Telemetry and Telecommand (TTC) radio signals in a two-way radio occultation experiment, tracking CH-2’s radio transmissions through the Moon’s plasma layer. These signals were received at the Indian Deep Space Network (IDSN), Byallalu, Bangalore. The results revealed a surprisingly high electron density of approximately 23,000 electrons per cubic centimetre in the lunar environment, comparable to densities observed in the Moon’s wake region (previously discovered by the same team) and nearly 100 times higher than those on the sunlit side of the Moon.
The Moon passes through Earth’s extended magnetic field, or “geotail,” for nearly 4 days in each orbit. During this period, the moon is shielded from direct solar wind, and was thought to have lower plasma densities due to free diffusion along Earth’s magnetic field lines. However, the Chandrayaan-2 observations challenge this assumption. Scientists have proposed that the presence of remnant lunar crustal magnetic fields could be trapping plasma, preventing its diffusion, and leading to localized enhancements in electron density. To confirm this, they used in-house Three-Dimensional Lunar Ionospheric Model (3D-LIM) developed at SPL/VSSC, which simulated plasma dynamics under different conditions. The simulations showed that to sustain such high plasma densities, the ionosphere must be in photochemical equilibrium, a condition only achievable in the geotail when crustal magnetic fields are present. The model also suggested a localized reduction in neutral Argon (Ar) and Neon (Ne) densities near the Moon’s poles, aligning with previous spacecraft observations.
High plasma densities can influence radio communications, surface charging effects, and interactions with lunar dust, all of which are important for the upcoming robotic and crewed missions near lunar orbital magnetic field region. Understanding how the lunar ionosphere behaves in different space environments will also improve planning for lunar habitats, particularly in regions influenced by crustal magnetic fields.
The study marks a significant step in unravelling the complex plasma environment around the Moon and highlights the continued impact of Chandrayaan-2’s science mission in advancing lunar research. As more nations gear up for Moon exploration, findings like these will play a crucial role in shaping the future of lunar science and technology.
