Chandrayaan-2 orbiter detects high density plasma in the Lunar wake region.

 DFRS payload onboard Chandrayaan-2 orbiter detects high density plasma in the Lunar wake region.

Chandrayaan 2 orbital 

The Moon is believed to have a very tenuous atmosphere. Since the ionosphere derives its origin from the atmosphere the plasma density at the Moon is considered to be only a few hundred ions per cubic centimeter. Measurements made using the Dual Frequency Radio Science (DFRS) experiment onboard Chandrayaan-2 orbiter, however, have shown that Moon’s ionosphere has a plasma density of the order of 104 cm-3, in the wake region which is at least one order of magnitude more than that is present in the day side.
Chandrayaan-2, the second Indian Lunar exploration mission launched by the Indian Space Research Organization (ISRO) on 22 July 2019, carried several payloads, among which the DFRS was designed to study the lunar ionosphere. It uses two coherent signals at the S-band (2240 MHz) and X-band (8496 MHz) of radio frequencies, transmitted from the Chandrayaan-2 orbiter and received at the ground station at Byalalu, Bangalore to explore the lunar plasma ambiance using the radio occultation (RO) technique. Simultaneous measurements by two coherent radio signals help to mitigate the effect of the Earth’s atmosphere and any uncertainties due to various sources during the experiments. The DFRS payload was conceptualized and jointly developed by Space Physics Laboratory (SPL) of Vikram Sarabhai Space Centre, Trivandrum; UR Rao Space Centre (URSC), Bangalore; and ISRO Telemetry, Tracking, and Command Network (ISTRAC), Bangalore. For the RO observations, an algorithm to estimate the integrated electron density profile was developed at SPL and used to study the Moon’s ionosphere in the lunar wake region, a region of the Lunar ionosphere that does not directly interact with the solar wind.
A total of 12 radio occultation experiments have been conducted in campaign mode on four different occasions based on carefully selected geometry suitable for the RO measurements. Detailed analysis shows that the total electron content along the ray path in the Lunar ionosphere can be as large as 1.5 TECU (1 TECU = 1016 m-2) with the uncertainty of 0.15 TECU, in the Lunar wake region. Large electron content is also seen near lunar polar regions during solar twilight conditions. These findings are unique and first of its kind as they show substantial post-sunset enhancement in plasma density compared to the dayside values reported so far by earlier missions.
The observed large enhancements in electron density in the Lunar wake region open new dimensions in understanding the lunar dark side plasma environment. In the wake region, neither the solar radiation nor the solar wind interacts directly with the available neutral particles, but still, the plasma is getting generated. Numerical simulations of the dark side of plasma environment using a 3-dimensional Lunar Ionospheric Model (3D-LIM) developed at SPL suggest that the production of ions by charge exchange reactions may play a pivotal role in producing a significantly large plasma density in the Lunar wake region, which can sustain for a longer period.  The model suggests that the dominant ions in the wake region are Ar+, and Ne+ which have a comparatively longer lifetime than the molecular ions (CO
These path-breaking scientific results on the Moon's ionosphere using DFRS payload onboard the Chandrayaan-2 orbiter and modeling of the physical mechanism are published in the Monthly Notices of the Royal Astronomical Society- Letter; "A study on the characteristic features of the Lunar ionosphere using dual-frequency radio science (DFRS) experiment onboard Chandrayaan-2 orbiter", Keshav R. Tripathi, R. K. Choudhary, K. M. Ambili, K. R. Bindu, R. Manikantan and Umang Parikh, DOI: 10.1093/mnrasl/slac058 (
 

Chandrayaan-2 DFRS

Figure 1: Top left panel:  Altitude/latitude/longitude variations of electron density along the ray path as simulated by 3D-LIM. The color bar represents the electron density at different points along the radio path.