Discovery of new suprathermal proton population around the Moon by SARA onboard Chandrayaan-1

A new group of suprathermal protons are discovered near the Moon by the Sub-keV Atom Reflecting Analyzer (SARA) experiment on Chandrayaan-1 - the first Indian lunar mission. These protons are found to exist on the sunlit side as well as the night side of the Moon.  

Sun emits not only radiation but also a stream of ionized particles known as solar wind. Sun is composed dominantly of hydrogen (H) in the ionised form (H+), which are basically protons. Solar wind is an extension of the solar corona, which is the outermost layer of the Sun dominantly of protons having typical energy of 1 kilo electron volts (keV) at the orbit of Earth. [1 eV ~14 km/s for hydrogen]. Solar wind flows through the solar system and interacts with the various ions in it

The Earth, our host planet, possesses a substantial magnetic field (~35000 nano Tesla) of its own, which effectively blocks the solar wind and shields the planet.  Contrary to this, the Moon, the only natural satellite of the Earth, does not possess a global magnetic field. This results in the continuous bombardment of solar wind on the lunar surface. Moon has localised areas where magnetic fields of few hundred nano Tesla exist and these regions are known as Lunar Magnetic Anomalies (LMA).

Interaction with the solar wind generates a variety of energetic protons (H+) around the Moon. Solar wind protons undergo scattering while interacting with the lunar surface (0.1-1%), which move around the Moon and thereby contribute to its environment. Large contribution come from the LMAs, which deflects almost half of the solar wind protons that incident on it. The Moon possesses a surface bound exosphere having neutral atoms or molecules around it including hydrogen atoms. These atoms become ionized by the ultra-violet (UV) radiation from the Sun (photoionisation), and also due to collision with solar wind protons (charge exchange process). These ions get accelerated and also add to the lunar environment.

The new population of protons that are discovered near the Moon by the Chandrayaan-1/SARA are not associated with any of these known sources or processes. The detailed analysis shows that the source is located at higher altitude (>500 km) above the dayside lunar surface and their density and velocity distribution reveals that they are neither from interplanetary nor interstellar medium and its origin is still an enigma. This indicates that our knowledge about Moon is still limited and calls for further exploration. These observations are also important to understand the environment of any non-magnetized body without atmosphere in our solar system as well as Exo-planetary systems.

The above result has been published in the international journal, Geophysical Research Letters, 2017, 44, doi:10.1002/2017GL072605.

The SARA experiment on the Chandrayaan-1 mission was an international collaborative research experiment between Space Physics Laboratory (SPL), Vikram Sarabhai Space Centre (VSSC) of Indian Space Research Organization (ISRO), and Swedish Institute of Space Physics (IRF), Kiruna, Sweden, with participation from Japan Aerospace Exploration Agency (JAXA), and University of Bern (UBe) Switzerland. SARA consisted of an ion-mass analyzer, namely SWIM (Solar WInd Monitor), to measure ions in the energy range of 10 to 15000 electron volts and an energetic neutral atom sensor namely CENA (Chandrayaan-1 Energetic Neutral Analyzer) to detect low energy neutral atoms (10 to 3000 electron volts).  More than 20 research papers have been published based on observations made by SARA in various international journals of high impact. 

Schematic of the new suprathermal population of protons observed around Moon. The Sun is on the left hand side. Orbit of Chandrayaan-1 is shown by cyan colored dotted curve. The green colored filled circles represent the newly observed population of suprathermal protons, the source of which is located at altitudes >500 km above the dayside surface. The yellow curve shows a sample trajectory of the suprathermal protons - a possible way of transportation from the source location to near the Moon where they are observed by SARA.