We study hemispherical power asymmetry ( 5 ; 9 ; 10 ) in the WMAP 9-year data .
We analyse the combined V- and W-band sky maps , after application of the KQ85 mask , and find that the asymmetry is statistically significant at the 3.4 \sigma confidence level for \ell = 2 –600 , where the data is signal dominated , with a preferred asymmetry direction ( l,b ) = ( 227 , -27 ) .
Individual asymmetry axes estimated from six independent multipole ranges are all consistent with this direction .
Subsequently , we estimate cosmological parameters on different parts of the sky and show that the parameters A _ { s } , n _ { s } and \Omega _ { b } are the most sensitive to this power asymmetry .
In particular , for the two opposite hemispheres aligned with the preferred asymmetry axis , we find n _ { s } = 0.959 \pm 0.022 and n _ { s } = 0.989 \pm 0.024 , respectively .