  We have extensively investigated characters of the low-energy $E1$ strengths
in $N>Z$ nuclei, by analyzing the transition densities obtained by the HF+RPA
calculations with several effective interactions. Crossover behavior has been
confirmed, from the skin mode at low energy to the $pn$ mode at higher energy.
Decomposing the $E1$ strengths into the skin-mode, $pn$-mode and interference
fractions, we show that the ratio of the skin-mode strength to the full
strength may be regarded as a generic function of the excitation energy,
insensitive to nuclides and effective interactions, particularly beyond Ni.
