EPONA is an energetic particle detector system incorporating totally depleted silicon surface barrier layer detectors. Active and passive background shielding will be employed and, by applying various techniques, particles of different species, including electrons, protons, alpha particles and pick-up ions of cometary origin may be detected over a wide spectrum of energies extending from the tens of KeV into the MeV range. The instrument can operate in two modes namely (a) in a cruise phase or

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... torage mode and (b) in a real time mode During the real time mode observations at high spatial (octosectoTing) and temporal (O.Ss) resolution in the cometary environment permit studies to be made of accelerated particles at the bow shock and/or in the tail of the comet. In conjunction with magnetic field measurements on board Giotto, observations of energetic electrons and their anasotropies can determine whether the magnetic field lines in the cometary tail are open or closed Further, the absorption of low energy solar particles in the cometary atmosphere can be measured and such data would provide an integral value of the pertaining gas and dust distribution. Solar particle background measurements during encounter may also be used to correct the measurements of other spacecraft borne instruments potentially vulnerable to such radiation. Solar particle flux measurements, obtained during the cruise phase will, when combined with simultaneous observations made by other spacecraft at different heliographic longitudes, provide information concerning solar particle propagation in the corona and in interplanetary space.