If $\tan {\theta }_1\times \tan {\theta }_2=\frac{-a^2}{b^2}$, then the chord joining $2$ points ${\theta }_1$ and ${\theta }_2$ on the ellipse $\frac{x^2}{a^2}+\frac{y^2}{b^2}=1$ will subtend a right angle at