Open New Horizon with L4 Mission

The Sun-Earth Lagrange point L4 is considered as one of the unique places where the solar activity and heliospheric environment can be observed in a continuous and comprehensive manner. The L4 mission affords a clear and wide-angle view of the Sun-Earth line for the study of the Sun-Earth and Sun-Moon connections from remote-sensing observations. In-situ measurements of the solar radiation, solar wind, and heliospheric magnetic field are critical to monitor and forecast the radiation environment for safe human exploration of the Moon and Mars. The L4 mission will significantly contribute advancing heliophysics science, improving the capability of space weather forecasting, and extending space weather studies far beyond near-Earth space. This white paper outlines the importance of L4 observations and advocates comprehensive and coordinated observations of the heliosphere at multi-points including other planned L1 and L5 missions.