The investigations on-board SOHO (Table 1.1) can be divided into three main groups, according to their area of research : helioseismology, solar atmospheric remote sensing, and ``in situ'' particle measurements.
Helioseismology
The helioseismology investigations primarily aim at the study
of those parts of the solar oscillations spectrum
that cannot be obtained from the ground.
The required sensitivity for observing the very low modes () and the
high modes () is difficult to achieve from the ground
because of
noise effects introduced by the Earth's diurnal rotation for the low modes,
and the transparency and seeing fluctuations of the Earth's
atmosphere for the high modes.
Solar atmospheric remote sensing
The solar atmosphere remote sensing investigations are carried out
with a set of telescopes and spectrometers
that will produce the data necessary to study the dynamic phenomena
that take place in the solar atmosphere
at and above the chromosphere. The plasma will be studied by
spectroscopic measurements and high resolution images at different
levels of the solar atmosphere. Plasma diagnostics obtained
with these instruments will provide temperature, density, and
velocity measurements of the material in the outer solar atmosphere.
``In situ'' measurements
The instruments to measure ``in situ'' the composition of the
solar wind and energetic particles will determine
the elemental and isotopic abundances, the ionic charge states
and velocity distributions of ions originating in
the solar atmosphere. The energy ranges covered will allow the
study of the processes of ion acceleration and fractionation under
the various conditions.