Triviality of the Hecke action on ordinary Drinfeld cuspforms of level ${\mathrm{\Gamma }}_1(t^n)$

Let $k\ge 2$ and $n\ge 1$ be any integers. In this paper, we prove that all Hecke operators act trivially on the space of ordinary Drinfeld cuspforms of level ${\mathrm{\Gamma }}_1(t^n)$ and weight $k$.