A large density irregularity structure in latitudinal extent was observed by the sun-synchronous orbiting FORMOSAT-5 satellite at 720 km topside ionosphere in the South American sector on November 24, 2018 during a magnetically quiet time. The observed density irregularity structure has a latitudinal extent of ∼30° across the dip equator, and a longitudinal width of ∼6°. Unlike previously studied equatorial plasma bubble structures with the airglow images that indicate hemispheric magnetic conjugate similarity, the gross feature of this large density irregularity structure indicates a north-south hemispheric asymmetry in density variation. We use the Hilbert-Huang Transform analysis to study the irregularity structure in different scales of density variations. It is found that the amplitude of density variations in the scale sizes from 22.5 to 450 km all indicate the hemispheric asymmetry. However, the spectral property of density variations in the two hemispheres is almost identical to each other. This implies that the density irregularity structure which originates from the same Rayleigh-Taylor (RT) instability process has preserved the spectral characteristics inside the magnetic flux tube. Therefore, the cause that fails to preserve the magnetic conjugate property in the density irregularity structure comes from the fact that the background ionospheric density distribution has a north-south hemispherical asymmetry in the beginning of the RT instability process. This hemispheric asymmetrical density distribution is caused by the seasonal variation of density distributions in the two hemispheres and the prevailing inter-hemispheric neutral wind during solstices in the South American sector.