The magnetopause standoff distance characterizes global magnetospheric
compression and deformation in response to changes in the solar wind
dynamic pressure and interplanetary magnetic field orientation. We
cannot derive this parameter from in-situ spacecraft measurements.
However, time-series of the magnetopause standoff distance can be
obtained in the near future using observations by soft X-ray imagers. In
two companion papers, we describe methods of finding the standoff
distance from X-ray images. In Part 1, we present the results of MHD
simulations which we use for the calculation of the X-ray emissivity in
the magnetosheath and cusps. Some MHD models predict relatively high
density in the magnetosphere, larger than observed in the data.
Correcting this, we develop magnetospheric masking methods to separate
the magnetosphere from the magnetosheath and cusps. We simulate the
X-ray emissivity in the magnetosheath for different solar wind
conditions and dipole tilts.