Simultaneous measurements of electrons and magnetic field ate used to
study the topology of the Martian magnetosphere. Halo solar wind elect
rons streaming along magnetic field lines replicate variations of the
B-x component, thus tracing the interplanetary magnetic field draping
in most parts of the Martian tail. This suggests that the magnetic fie
ld observed near the equatorial plane of Mars is mainly induced. Howev
er, there are regions where this perfect tracing is violated. This cou
ld be considered as evidence for an intrinsic planetary field. An alte
rnative interpretation involves the relationship between these regions
and ionospheric holes.