8. Let $a$ and $b$ be constants, and $b<0$. The graph of the parabola $y=a x^{2}+b x+a^{2}+\sqrt{2} a-4$ is one of the four graphs in Figure 1. Then $a=$ $\qquad$ .
8. $a=\sqrt{2}$.

From $b0$, we get $a$ $>0$. Therefore, $a=\sqrt{2}$.