What is a \textbf{"Dipole mode"}? There are many ways in which a sphere (and therefore a star) can oscillate. Mathematically any possible oscillation can be described as a sum of functions called spherical harmonics. The simplest one represent a radial oscillation (harmonic degree $\ell =0$), in which the whole sphere expands and contracts around its center. Then we have a dipole ($\ell =1$), a quadrupole ($\ell =2$), an octupole ($\ell =3$) etc. Check here movies showing a number of \href{http://mcnamara.asu.edu/sphere_harmonics/}{spherical harmonics}.\\
  
  
  
  