adopted a similar
scheme as the above-mentioned works but assumed,
in order to locate the cobpoint, that the distance of
the cobpoint to the observer along the IMF line connecting
with the observer decreases linearly with time.
They also used a complex parametric function to describe,
Q, including energetic, temporal and spatial
dependences. Differences among the above models have
been described in Sanahuja and Lario (1998) and
Kallenrode (2001).