  Polars (AM Herculis binaries) are a prominent class of bright soft X-ray
sources, many of which were discovered with ROSAT. We present a homogenous
analysis of all the pointed ROSAT PSPC observations of polars subdivided into
two papers that discuss the prototype polar AM Her in detail and summarize the
class properties of all other polars. We derive the high-state soft X-ray flux
and short-term spectral variability of AM Her using a new detector response
matrix and a confirmed flux calibration of the ROSAT PSPC below 0.28 keV. The
best-fit mean single-blackbody temperature and integrated bright-phase energy
flux of AM Her in its April 1991 high state are 27.2 +/- 1.0 eV and (2.6 +/-
0.6) x 10^-9 erg cm^-2s^-1, respectively. The total blackbody flux of a
multi-temperature model that fits both the soft X-ray and the fluctuating
far-ultraviolet components is Fbb = (4.5 +/- 1.5) x 10^-9 erg cm^-2s^-1. The
total accretion luminosity at a distance of 80 pc, Lbb = (2.1 +/- 0.7) x 10^33
erg s-1, implies an accretion rate of Mdot = (2.4 +/- 0.8) x 10^-10 Msun yr^-1
for an 0.78 Msun white dwarf. The soft X-ray flux displays significant
variability on time scales down to 200 ms. Correlated spectral and count-rate
variations are seen in flares on time scales down to 1 s, demonstrating the
heating and cooling associated with individual accretion events. Our spectral
and temporal analysis provides direct evidence for the blobby accretion model
and suggests a connection between the soft X-ray and the fluctuating
far-ultraviolet components.
