What is a “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 spherical harmonics.