Plasma measurements options for a Solar Probe and fire missions

Measurements of velocity distributions of coronal and solar wind plasma are a very important part of in-situ diagnostics of coronal heating and solar wind acceleration mechanisms. Coronal heating mechanisms have signatures in the velocity distributions of ions and electrons. Understanding these mechanisms requires measurements of 3-dimensional velocity distributions of electrons, protons and alpha-particles as two important dynamic components, and, at least, one heavier ion component as tracer element. Measurements of mass composition and ion ionization state would be very desirable in solving the problem of solar wind sources. Existing experimental techniques give a reasonable experimental basis for development of plasma instrument for Solar Probe, provided sufficient (within several kg) mass allocation. We review some of flown and proposed instruments and discuss their merits in terms of proposed Solar Probe investigations.