  The ac spin pumping of noncollinear antiferromagnets is theoretically
investigated. Starting from an effective action description of the spin system,
we derive the Onsager coefficients connecting the spin pumping and
spin-transfer torque associated with the dynamics of the SO(3)-valued
antiferromagnetic order parameter. Our theory is applied to a kagome
antiferromagnet resonantly driven by a uniform external magnetic field. We
demonstrate that the reactive (dissipative) spin-transfer torque parameter can
be extracted from the pumped ac spin current in-phase (in quadrature) with the
driving field. Furthermore, we find that the three spin-wave bands of the
kagome AF generate spin currents with mutually orthogonal polarization
directions. This offers a unique way of controlling the spin orientation of the
pumped spin current by exciting different spin-wave modes.
