ISRO's Chandrayaan-2 Detects High Density Plasma In Moon's Ionosphere
The DFRS experiment onboard Chandrayaan-2 orbiter has made some measurements which show that the plasma density of the ionosphere is of the order of 10^4 ions per cubic centimetre in the wake region.
The Indian Space Research Organisation's (ISRO's) Chandrayaan-2 has discovered high density plasma in the Moon's ionosphere. When the atmosphere of the Moon meets the vacuum of space, the Sun's ultraviolet rays break apart atoms of air, creating a layer of ionised gas called the ionosphere.
The findings were recently published in the Monthly Notices of the Royal Astronomical Society.
What Is The Plasma Density Of The Ionosphere?
The ionosphere originates from the atmosphere, and hence, the plasma density of the layer is considered to be only a few hundred ions per cubic centimetre. The Dual Frequency Radio Science (DFRS) experiment onboard Chandrayaan-2 orbiter has made some measurements which show that the plasma density of the ionosphere is of the order of 104 ions per cubic centimetre in the wake region, according to ISRO. The lunar wake region is a region in the ionosphere of the Moon that does not directly interact with the solar wind.
The plasma density of the ionosphere in the lunar wake region is at least one order of magnitude more than that on the day side.
Chandrayaan-2 is India's second lunar exploration mission launched by ISRO on July 22, 2019. The mission carried several payloads, including the DFRS, designed to study the lunar ionosphere.
How DFRS Explores Lunar Plasma Density
The payload uses two coherent signals of the S-band and X-band, at frequencies of 2240 MHz and 8496 MHz, respectively. These signals are transmitted from the Chandrayaan-2 orbiter, and received at the ground station at Byalalu, Bangalore.
According to ISRO, the signals are used to explore the lunar plasma density using the radio occultation technique, which involves a low-Earth orbit satellite receiving a signal from a GPS satellite. The signal passes through the atmosphere, and gets refracted along the way, as a result of which the temperature and the water vapour concentration in the atmosphere determines the magnitude of the refraction.
The effect of the Earth's atmosphere and any uncertainties due to various sources when the experiments are conducted are mitigated using simultaneous measurements made by two coherent radio signals.
Who Developed DFRS?
The Space Physics Laboratory (SPL) of Vikram Sarabhai Space Centre in Trivandrum, UR Rao Space Centre (URSC) in Bangalore and ISRO Telemetry, Tracking, and Command Network (ISTRAC) in Bangalore jointly conceptualised and developed the DFRS payload.
An algorithm was developed at SPL to study the ionosphere of the Moon in the lunar wake region.
Why Is There A Large Plasma Density In The Lunar Wake Region?
On four different occasions, a total of 12 radio occultation experiments were conducted. Large electron content is observed along the ray path in the lunar ionosphere, and near lunar polar regions during twilight conditions. The findings are unique and the first of their kind. This is because they show post-sunset enhancement in plasma density compared to the day side values.
Neither the solar radiation nor the solar wind interacts directly with the available neutral particles in the lunar wake region. Despite this, plasma is generated.
A three-dimensional lunar ionospheric model developed at SPL was used to conduct numerical simulations of the dark side of the plasma environment. The simulations suggest that the production of ions by charge exchange reactions may play an important role in producing a large plasma density in the lunar wake region. This plasma density can sustain for a long period.
Ar+ and Ne+ (cations of Argon and Neon) are the dominant ions in the wake region. They have a comparatively longer lifetime than molecular ions such as CO2+ and H2O+, that are dominant at other regions. Solar wind sweeps away some ions produced by solar radiation during the daytime. This leads to reduced daytime plasma density.
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