Chandrayaan-2 detects high density plasma in Lunar wake region | India News
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BENGALURU: In another finding, data from India’s Chandrayaan-2 orbiter has shown that plasma density in Moon’s ionosphere in the wake region — a region of the Lunar ionosphere that does not directly interact with the solar wind — is at least one order of magnitude more than that present in the day side.
India’s second lunar mission, which was launched on July 22, 2019, carried several payloads, among which, the Dual Frequency Radio Science (DFRS) was designed to study the lunar ionosphere. It was conceptualised and jointly developed by Space Physics Laboratory (SPL) of Vikram Sarabhai Space Centre (VSSC), UR Rao Space Centre (URSC) and Isro TelemetryTracking, and Command Network (Istrac).
Pointing out that Moon’s ionosphere, which derives its origin from the atmosphere believed to be very tenuous, Isro said, the plasma density was considered to be only a few hundred ions per cubic centimetre.
“…However, measurements made using the DFRS experiment onboard Chandrayaan-2 orbiter 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,” the space agency added.
DFRS uses two coherent signals at the S-band and X-band of radio frequencies, transmitted from Chandrayaan-2 orbiter and received at the ground station in Byalalu, Bengaluru, to explore the lunar plasma ambiance using the radio occultation (RO) — a remote sensing technique used to measure physical properties of a planetary atmosphere — 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. For the RO observations, an algorithm to estimate the integrated electron density profile was developed at SPL and used to study the ionosphere in the wake region,” Isro said.
Isro said that 12 RO experiments have been conducted in campaign mode on four different occasions based on carefully selected geometry suitable for 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 = 1,016 m-2) with the uncertainty of 0.15 TECU, in the 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,” Isro said.
Arguing that the observed large enhancements in electron density in the wake region open new dimensions in understanding the lunar dark side plasma environment, researchers said that 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 3D 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,” Isro said.
It added that the model suggests that the dominant ions in the wake region are Ar+ (Argon ion), and Ne+ (Neon ion) which have a comparatively longer lifetime than the molecular ions (CO2+, and H2O+) that are dominant at other regions.
“On the other hand, a fraction of the ions produced by solar radiation during the daytime is swept by solar wind, leading to reduced daytime plasma density,” Isro added.
India’s second lunar mission, which was launched on July 22, 2019, carried several payloads, among which, the Dual Frequency Radio Science (DFRS) was designed to study the lunar ionosphere. It was conceptualised and jointly developed by Space Physics Laboratory (SPL) of Vikram Sarabhai Space Centre (VSSC), UR Rao Space Centre (URSC) and Isro TelemetryTracking, and Command Network (Istrac).
Pointing out that Moon’s ionosphere, which derives its origin from the atmosphere believed to be very tenuous, Isro said, the plasma density was considered to be only a few hundred ions per cubic centimetre.
“…However, measurements made using the DFRS experiment onboard Chandrayaan-2 orbiter 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,” the space agency added.
DFRS uses two coherent signals at the S-band and X-band of radio frequencies, transmitted from Chandrayaan-2 orbiter and received at the ground station in Byalalu, Bengaluru, to explore the lunar plasma ambiance using the radio occultation (RO) — a remote sensing technique used to measure physical properties of a planetary atmosphere — 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. For the RO observations, an algorithm to estimate the integrated electron density profile was developed at SPL and used to study the ionosphere in the wake region,” Isro said.
Isro said that 12 RO experiments have been conducted in campaign mode on four different occasions based on carefully selected geometry suitable for 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 = 1,016 m-2) with the uncertainty of 0.15 TECU, in the 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,” Isro said.
Arguing that the observed large enhancements in electron density in the wake region open new dimensions in understanding the lunar dark side plasma environment, researchers said that 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 3D 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,” Isro said.
It added that the model suggests that the dominant ions in the wake region are Ar+ (Argon ion), and Ne+ (Neon ion) which have a comparatively longer lifetime than the molecular ions (CO2+, and H2O+) that are dominant at other regions.
“On the other hand, a fraction of the ions produced by solar radiation during the daytime is swept by solar wind, leading to reduced daytime plasma density,” Isro added.
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