A study using observations from Chandrayaan 1 mission has found how plasma particles from the solar wind make their way into the Moon’s night side, filling up the wake region, long thought to be devoid of plasma particles. This has significance in understanding bodies like the Moon which do not have global magnetic fields.
In recent times, there has been a huge interest in understanding the plasma environment of the Moon, which is generated mainly by its interaction with the solar plasma wind flowing towards it from the Sun. This plasma wind consists of charged particles such as protons and is partly absorbed by the side of the Moon facing the sun. The rest of the solar plasma wind incident on the Moon flows around it, but leaves a wake (a void) on the side not facing the sun (the night-side of the Moon).
Recent studies
Earlier, it was believed that this wake was devoid of any particles. But recent Moon missions such as Chandrayaan-1, Kaguya, Chang’e-1 and Artemis have found evidence of refilling of near lunar wake (heights of 100 km to 200 km above the lunar surface on the night side) with solar wind protons.
Unlike the Earth, the Moon has no global magnetic field originating from a magnetized core. It has weak crustal fields that are too small to shield it globally from charged solar plasma particles incident on it. At some regions the crustal fields are quite strong and these are known as magnetic anomalies. The plasma particles scatter off these anomalous crustal fields.
The present work shows that solar wind protons scattered from the magnetic anomaly located at the South Pole Aitken basin on the Moon can enter the near wake region. “This is the first observational evidence for such a process,” says Anil Bhardwaj of Physical Research Laboratory, Ahmedabad, who is one of the principal investigators in this international collaboration. The work is published in Geophysical Research Letters.
The group also characterised the energy and flux of the proton population in the near wake region. They find that the flux, or intensity, of the protons is approximately 0.0005 times the solar wind proton flux. “Such a magnitude is comparable to the proton population in near wake due to other known processes and can significantly affect the electromagnetic environment in near wake region,” says Prof. Bhardwaj.
Chandrayaan 1 data
The data for the calculation was collected using the Chandrayaan-1 observations. “We used the observations from the Sub-keV Atom Reflecting Analyser (SARA) experiment on Chandrayaan-1,” says M. B. Dhanya, the first author of the paper.
Small scale crustal magnetic fields on the Moon can also cause scattering of impinging solar wind protons back into space. “This paper shows that such particles scattered from the lunar magnetic regions on the day side can get transported to the night side of the Moon, thereby contributing to plasma refilling in the near wake region,” says Dr Dhanya.
The interaction between the Moon and the solar plasma is a topic of interest now because understanding it can help us study any celestial body which has no atmosphere or global magnetic field, such as asteroids and some planetary satellites.
