  Planetary plasma and magnetic field environments can be studied by in situ
measurements or by remote sensing. While the former provide precise information
about plasma behaviour, instabilities and dynamics on local scales, the latter
offers the global view necessary to understand the overall interaction of the
magnetospheric plasma with the solar wind. Here we propose a novel and more
elegant approach employing remote X-ray imaging techniques, which are now
possible thanks to the relatively recent discovery of solar wind charge
exchange X-ray emissions in the vicinity of the Earth's magnetosphere. We
describe how an appropriately designed and located X-ray telescope, supported
by simultaneous in situ measurements of the solar wind, can be used to image
the dayside magnetosphere, magnetosheath and bow shock, with a temporal and
spatial resolution sufficient to address several key outstanding questions
concerning how the solar wind interacts with the Earth's magnetosphere on a
global level. Our studies have led us to propose 'AXIOM: Advanced X-ray Imaging
Of the Magnetosphere', a concept mission using a Vega launcher with a LISA
Pathfinder-type Propulsion Module to place the spacecraft in a Lissajous orbit
around the Earth - Moon L1 point. The model payload consists of an X-ray Wide
Field Imager and an in situ plasma and magnetic field measurement package. This
package comprises sensors designed to measure the bulk properties of the solar
wind and to characterise its minor ion populations which cause charge exchange
emission, and a magnetometer designed to measure the strength and direction of
the solar wind magnetic field. We show simulations that demonstrate how the
proposed X-ray telescope design is capable of imaging the predicted emission
from the dayside magnetosphere with the sensitivity and cadence required to
achieve the science goals of the mission.
