Exploring strong lunar ionosphere successfully with the service module of Chinese circumlunar return and reentry spacecraft

Abstract

The existence of lunar ionosphere has been under debate for a long time. In Apollo 14 mission, the electron density detected by the charged particle lunar environment experiment (CPLEE) was 104 el/cm3 at several hundred meters high during lunar day time. In Luna-19/22 mission, the electron density profiles were detect and the peak densities were about 103 el/cm3. In the last decade, European mission SMART-1 and Japanese mission SELENE also performed radio occultation experiment for lunar ionosphere. The results of these missions don’t well-matched. In order to explore the lunar ionosphere, a very low frequency radio astronomical payload has been suggested to be sent to the surface of lunar far-side by the Chinese Chang’E-4 lunar lander mission in 2019. The payload will record the Type II solar burst, which may cover the frequency of electro-magnetic wave as low as several dozen kilo-Herz. The possible lunar ionosphere above the payload with certain electron density may truncate or block the solar burst signal as corresponding plasma frequency. To estimate the possible truncate frequencies for these observations by the new kind of payload, and to determine the lunar ionospheric distribution, an lunar radio occultation experiment with the service module of Chinese circumlunar return and reentry spacecraft has been performing. The circumlunar return and reentry spacecraft is a Chinese precursor mission for the Chinese lunar sample return mission. It was launched on 23 October 2014. After the return and reentry experiments, the service module went back into a Lunar Orbit on 11 January 2015 to image the target landing zone for the Chinese lunar sample return mission which has not yet been disclosed. During this period, the radio occultation experiments have been performed to detect the lunar ionosphere. The service module provides a stable and reliable frequency source, whose short-term stability is n ´ 10–9, for both X-band and S-band signal. The signals transmitted from the spacecraft in S and X band passed through lunar ionosphere, interplanetary plasma, Earth ionosphere and atmosphere, finally received by the ground tracking stations. According to the coherent ratio of the S/X signal, we convert the phase information of S-band signal to the frequency of X-band signal and calculate the difference of these two signal. Then, the extrapolation algorithm was used here to deduct the interference error of the earth ionosphere and the interplanetary plasma. Based on the above work, the electron column concentrations of lunar ionosphere was explored preliminary. The maximums of electron column concentrations are between 0.4 ´ 1016–0.5 ´ 1016 el/m2, are two times of the maximum result from Luna 19/22, are 1–2 orders higher than the SELENE result, but well-matched with the result from CPLEE. These results show that the lunar ionosphere is clearly exist and much stronger than we expected. The result here gives a positive support and some dynamical constrains for the scientific objective of the very low frequency radio astronomical payload onboard the Chang’E-4 lander mission. But it also raises a new question that the characteristics and formation mechanism of a stronger lunar ionosphere is remain unknown. More observations will be performed for further scientific targets.