Solution (source code)

= Solution

For $a>0$, both coordinates are nonnegative exactly on the loop branch $t\geq0$. It leaves the origin at $t=0$ and returns there in the limit $t\to+\infty$. Thus the loop is traced once by
$$
\boxed{0\leq t<\infty},
$$
with the limiting endpoint identified with the starting point. The same parameter range traces the rotated loop when $a<0$.