Solar-terrestrial physics is one of the oldest fields of space science. By solar-terrestrial physics we mean the mechanisms by which the earth , including its atmosphere and magnetosphere, responds to solar activity. Experimental research in this field involves observations of the near-earth environment, and it has enjoyed a longer period of attention from space vehicles than have more distant targets for the simple reason that early space vehicles could only reach the near-earth environment. On this account it has been stated that all the major features of the near-earth environment have been adequately explored and that focus of space efforts should shift to more distant objectives to the near exclusion of further investigation of the near-earth environment. However, this is not true, since much remains to be learned about solar-terrestrial physics; the solutions to many important problems are within sight, and new physical phenomena that affect the earth and its interaction with the interplanetary environment and the sun are still being discovered. Questions can now be precisely formulated, permitting the design of more definitive experiments than was possible in the earlier exploratory investigations; now measurements of related quantities from a variety of sources must be carefully coordinated.