  Aims. We analyze coronagraph observations of a polar jet observed by the Sun
Earth Connection Coronal and Heliospheric Investigation (SECCHI) instrument
suite onboard the Solar TErrestrial RElations Observatory (STEREO) spacecraft.
Methods. In our analysis we compare the brightness distribution of the jet in
white-light coronagraph images with a dedicated kinetic particle model. We
obtain a consistent estimate of the time that the jet was launched from the
solar surface and an approximate initial velocity distribution in the jet
source. The method also allows us to check the consistency of the kinetic
model. In this first application, we consider only gravity as the dominant
force on the jet particles along the magnetic field. Results. We find that the
kinetic model explains the observed brightness evolution well. The derived
initiation time is consistent with the jet observations by the EUVI telescope
at various wavelengths. The initial particle velocity distribution is fitted by
Maxwellian distributions and we find deviations of the high energy tail from
the Maxwellian distributions. We estimate the jet's total electron content to
have a mass between 3.2 \times 1014 and 1.8 \times 1015 g. Mapping the
integrated particle number along the jet trajectory to its source region and
assuming a typical source region size, we obtain an initial electron density
between 8 \times 109 and 5 \times 1010 cm-3 that is characteristic for the
lower corona or the upper chromosphere. The total kinetic energy of all
particles in the jet source region amounts from 2.1 \times 1028 to 2.4 \times
1029 erg.
