The second variation of a harmonic map

Question

For maps $ u :\mathbb{B}^n \rightarrow \mathbb{S}^{n-1}$ the second variation of the dirichlet energy \begin{align} \mathbb{E}[u] = \int_{\mathbb{B}^n}|\nabla u|^2 \,dx \end{align} is given by \begin{align} D^2\mathbb{E}[u][\phi,\phi] = 2\int_{\mathbb{B}^n} |\nabla {\phi}|^2 - |\nabla u|^2 |{\phi}|^2 \,dx \end{align} for $\phi \in C^\infty_0(\mathbb{B}^n,\mathbb{R}^n)$ such that $\langle u ,\phi\rangle =0$. The function $\frac{x}{|x|}$ is a global minimizer of $\mathbb{E}$ and so i should be able to directly show that the second variation is positive at $\frac{x}{|x|}$ but i am struggling to do so. Can anyone offer any hints?