  Superhorizon perturbations induce large-scale temperature anisotropies in the
cosmic microwave background (CMB) via the Grishchuk-Zel'dovich effect. We
analyze the CMB temperature anisotropies generated by a single-mode adiabatic
superhorizon perturbation. We show that an adiabatic superhorizon perturbation
in a LCDM universe does not generate a CMB temperature dipole, and we derive
constraints to the amplitude and wavelength of a superhorizon potential
perturbation from measurements of the CMB quadrupole and octupole. We also
consider constraints to a superhorizon fluctuation in the curvaton field, which
was recently proposed as a source of the hemispherical power asymmetry in the
CMB.
