0.3$, the characteristic damping scale is $\ell_r \approx 3$. Therefore the damping is also important at low $\ell$-modes. For homogeneous reionisation, the reionisation process damps the CMB anisotropy linearly and keeps the Gaussianity of the primordial fluctuations. However, if the reionisation is inhomogeneous, for example, due to the inhomogeneous ionisation fraction or the patchy reionisation, the CMB anisotropy of low $\ell$-modes can be partially contaminated by the inhomogeneous Thompson scattering, and becomes non-Gaussian. These low $\ell$-modes (quadrupole and octopole) happen to be aligned along the Galactic plane, make the biggest spots near the Galactic plane (Tegmark et al. 2003; de Oliveira-Costa et al. 2003), and may change the temperature distributions of the Galactic north and south hemispheres differently.

Finally, the non-Gaussian signatures that have been detected may have the primordial origin. While the simple inflation predicts that the primordial fluctuation field is Gaussian, the non-linear couplings between the inflaton and the fluctuation fields or between the quantum and the classical fluctuation fields can produce weakly non-Gaussian fluctuations (Komatsu & Spergel 2001, references therein). Multi-field inflationary models predict that the primordial metric fluctuations do not need to obey Gaussian statistics (e.g., Bernardeau & Uzan 2002). There are also many mechanisms that 