  Filament eruptions occurring at different places within a relatively short
time internal, but with a certain physical causal connection are usually known
as sympathetic eruption. Studies on sympathetic eruptions are not uncommon.
However, in the existed reports, the causal links between sympathetic eruptions
remain rather speculative. In this work, we present detailed observations of a
sympathetic filament eruption event, where an identifiable causal link between
two eruptive filaments is observed. On 2015 November 15, two filaments (F1 in
the north and F2 in the south) were located at the southwestern quadrant of
solar disk. The main axes of them were almost parallel to each other. Around
22:20 UT, F1 began to erupt, forming two flare ribbons. The southwestern ribbon
apparently moved to southwest and intruded southeast part of F2. This
continuous intrusion caused F2's eventual eruption. Accompanying the eruption
of F2, flare ribbons and post-flare loops appeared in northwest region of F2.
Meanwhile, neither flare ribbons nor post-flare loops could be observed in
southeastern area of F2. In addition, the nonlinear force-free field (NLFFF)
extrapolations show that the magnetic fields above F2 in the southeast region
are much weaker than that in the northwest region. These results imply that the
overlying magnetic fields of F2 were not uniform. So we propose that the
southwest ribbon formed by eruptive F1 invaded F2 from its southeast region
with relatively weaker overlying magnetic fields in comparison with its
northwest region, disturbing F2 and leading F2 to erupt eventually.
