  GRS 1915+105 being one of the brightest transient black hole binary (BHB) in
the X-rays, offers a unique test-bed for the study of the connection between
accretion and ejection mechanisms in BHBs. In particular, this source can be
used to study the accretion disc wind and how it depends on the state changes
in BHBs. Our aim is to investigate the origin and geometry of the accretion
disc wind in GRS 1915+105. We analysed the spectra of GRS 1915+105 in the soft
$\phi$ and hard $\chi$ classes, using the high resolution spectroscopy offered
by Chandra HETGS. In the soft state, we find a series of wind absorption lines
that follow a non linear dependence of velocity width, velocity shift and
equivalent width with respect to ionisation, indicating a multiple component or
stratified outflow. In the hard state we find only a faint Fe XXVI absorption
line. We model the absorption lines in both the states using a dedicated MHD
wind model to investigate a magnetic origin of the wind and to probe the cause
of variability in the observed lines flux between the two states. The MHD disc
wind model provides a good fit for both states, indicating the possibility of a
magnetic origin of the wind. The multiple ionisation components of the wind are
well characterised as a stratification of the same magnetic outflow. We find
that the observed variability in the lines flux between soft and hard states
cannot be explained by photo-ionisation alone but it is most likely due to a
large (three orders of magnitude) increase in the wind density. We find the
mass outflow rate of the wind to be comparable to the accretion rate,
suggesting a intimate link between accretion and ejection processes that lead
to state changes in BHBs.
