Double layers and ion-acoustic waves are investigated by using a one-dimensional electrostatic particle-in-cell simulation code. Our results show that double layers can be formed even when the drift velocity between electrons and ions is less than the electron thermal velocity. Electron and ion density depressions were clearly seen. Electrons gradually developed a distribution comprising both background and beam components. In fact, as the initial electron-ion drift velocity was less than the electron thermal velocity, intense ion-acoustic waves could be found only at the places where the electron beam was located, suggesting that they are excited by the self-consistently developed electron beam. Besides the Langmuir waves and ion-acoustic waves, the beam mode excited by electron beams produced in our simulation has been clearly found.

The Sino-Russian Joint Mars-Phobos Exploration program provides unique opportunity to investigate the global structure of the solar wind/Mars interaction region. The Mars orbiter YH-1 will carry a plasma package consisting of two identical ion detectors, and one electron detector, which is aimed at monitoring the plasma with energy up to a few keV in the environment of the planet, provides a mass analysis of the ions in the mass range form 1 to 44 au. Together with the magnetic field experiment on board YH-1, the plasma package will be the first to give full insight into the local plasma characteristics near the planet, as well as to determine the possible the total ion escape (particles/s) for the major ion species.