Chandrayaan-2 Signal Analysis Shows Moon’s Ionosphere Contains High Electron Densities

Indian Space Research Organisation (ISRO) scientists have found that the Moon’s ionosphere exhibits high electron densities when it enters the Earth’s geomagnetic tail.

The scientists who had analysed the radio signals from India’s Chandrayaan-2 (CH-2) orbiter said such high electron densities are unexpected.

Chandrayaan-2 Returns the favour

Chandrayaan-2, India’s second Moon mission flown in 2019, included an orbiter, lander and rover. The lander crashed on the lunar surface while the orbiter was functioning well.

Announcing this on Friday ISRO said the 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 Indian space agency said the scientists from the Space Physics Laboratory at Vikram Sarabhai Space Centre (VSSC), ISRO used an innovative method to study the plasma distribution around the 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,” ISRO said.

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.

ISRO said the Moon passes through Earth’s extended magnetic field, or “geotail,” for nearly four 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, the Indian space agency said.

To confirm this, the ISRO scientists used in-house Three-Dimensional Lunar Ionospheric Model (3D-LIM), 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 localised reduction in neutral Argon (Ar) and Neon (Ne) densities near the Moon’s poles, aligning with previous spacecraft observations, ISRO added.

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 the 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.