The science objective of the Integrated Science Investigation of the Sun, ISOIS,
experiment is to explore the physical mechanisms that produce, accelerate, and transport energetic
particles in the inner heliosphere. This will be achieved by observing energetic electrons, protons,
and heavy ions in the Sun's atmosphere and inner heliosphere to learn about the origin and seed population
of solar energetic particles, how these and other particles are accelerated, and what mechanisms are
responsible for transporting the different particle populations into the heliosphere. It will also
contribute to tracing the flow of energy that heats and accelerates the solar corona and solar wind,
and determining the structure and dynamics of the plasma and magnetic fields at the sources of the solar wind.
The Integrated Science Investigation of the Sun consists of two instruments, EPI-Lo and EPI-Hi, mounted on
the side of the spacecraft near the back ram-facing side, protected by the heat shield.
The EPI-Hi instrument system is based on ion-implanted solid-state detectors. It comprises three detector stacks
(particle telescopes). One is designated a High Energy Telescope (HET) and two are designated Low-Energy Telescopes (LET1 and LET2).