  This paper addresses the fundamental science question: "How does solar wind
energy flow through the Earth's magnetosphere, how is it converted and
distributed?". We need to understand how the Sun creates the heliosphere, and
how the planets interact with the solar wind and its magnetic field, not just
as a matter of scientific curiosity, but to address a clear and pressing
practical problem: space weather, which can influence the performance and
reliability of our technological systems, in space and on the ground, and can
endanger human life and health.
  Much knowledge has already been acquired over the past decades, but the
infant stage of space weather forecasting demonstrates that we still have a
vast amount of learning to do. We can tackle this issue in two ways: 1) By
using multiple spacecraft measuring conditions in situ in the magnetosphere in
order to make sense of the fundamental small scale processes that enable
transport and coupling, or 2) By taking a global approach to observations of
the conditions that prevail throughout geospace in order to quantify the global
effects of external drivers.
  A global approach is now being taken by a number of space missions under
development and the first tantalising results of their exploration will be
available in the next decade. Here we propose the next step-up in the quest for
a complete understanding of how the Sun gives rise to and controls the Earth's
plasma environment: a tomographic imaging approach comprising two spacecraft
which enable global imaging of magnetopause and cusps, auroral regions,
plasmasphere and ring current, alongside in situ measurements. Such a mission
is going to be crucial on the way to achieve scientific closure on the question
of solar-terrestrial interactions.
