Analysis of the background signal in Tianwen-1 MINPA

Since November 2021, Tianwen-1 started its scientific instrument Mars Ion and Neutral Particle Analyzer (MINPA) to detect the particles in the Martian space. To evaluate the reliability of the plasma parameters from the MINPA measurements, in this study, we analyze and reduce the background signal (or noise) appearing in the MINPA data, and then calculate the plasma moments based on the noise-reduced data. It is found that the velocity from MINPA is highly correlated with that from the Solar Wind Ion Analyzer (SWIA) onboard the MAVEN spacecraft, indicating good reliability, and the temperature is also correlated with the SWIA data, although it is underestimated and has more scatter. However, due to the limited $2\pi$ field of view (FOV), it's impossible for MINPA to observe the ions in all directions, which makes the number density and the thermal pressure highly underestimated compared to the SWIA data. For these moments, a more complicated procedure that fully takes into account the limited FOV is required to obtain their reliable values. In addition, we perform a detailed analysis of the noise source and find that the noise comes from the electronic noise in the circuits of MINPA. Based on this study, we may conclude that MINPA is in normal operating condition and could provide reliable plasma parameters by taking some further procedures. The analysis of the noise source can also provide a reference for future instrument design.