  We report on a 120 ks Chandra/HETG spectrum of the black hole GRS 1915+105.
The observation was made during an extended and bright soft state in June,
2015. An extremely rich disk wind absorption spectrum is detected, similar to
that observed at lower sensitivity in 2007. The very high resolution of the
third-order spectrum reveals four components to the disk wind in the Fe K band
alone; the fastest has a blue-shift of v = 0.03c. Broadened re-emission from
the wind is also detected in the first-order spectrum, giving rise to clear
accretion disk P Cygni profiles. Dynamical modeling of the re-emission spectrum
gives wind launching radii of r ~ 10^(2-4) GM/c^2. Wind density values of n ~
10^(13-16) cm^-3 are then required by the ionization parameter formalism. The
small launching radii, high density values, and inferred high mass outflow
rates signal a role for magnetic driving. With simple, reasonable assumptions,
the wind properties constrain the magnitude of the emergent magnetic field to B
~ 10^(3-4) Gauss if the wind is driven via magnetohydrodynamic (MHD) pressure
from within the disk, and B ~ 10^(4-5) Gauss if the wind is driven by
magnetocentrifugal acceleration. The MHD estimates are below upper limits
predicted by the canonical alpha-disk model (Shakura & Sunyaev 1973). We
discuss these results in terms of fundamental disk physics and black hole
accretion modes.
