We calculate the self-consistent response of an axisymmetric galactic disk perturbed by an elliptical halo potential of harmonic number m = 2 , and obtain the net disk ellipticity .
Such a potential is commonly expected to arise due to galactic tidal encounter and also during the galaxy formation process .
The self-gravitational potential corresponding to the self-consistent , non-axisymmetric density response of the disk is obtained by inversion of Poisson equation for a thin disk .
This response potential is shown to oppose the perturbation potential , because physically the disk self-gravity resists the imposed potential .
This results in a reduction in the net ellipticity of the perturbation halo potential in the disk plane .
The reduction factor denoting this decrease is independent of the strength of the perturbation potential , and has a typical minimum value of \sim 0.75 - 0.9 for a wide range of galaxy parameters .
The reduction is most important at 1.4 exponential disk scale lengths and is progressively less so at higher radii .
For the solar neighborhood region of the Galaxy , the reduction factor is 0.8 .
Beyond twice the Holmberg radius , the reduction is negligible , and there the disk asymmetry in the atomic hydrogen gas traces the true ellipticity of the halo potential .
The reduction is negligible at all radii for higher harmonics ( m \geq 3 ) of the halo potential .

On correcting for the negative disk response , the true ellipticity of the halo potential for a typical spiral galaxy is shown to be higher by \sim 20 \% than the typical halo ellipticity of \leq 0.1 deduced in the literature from observations of isophotal or kinematical asymmetry of disks .