In the $x y$-plane, the region $y>0$ has a uniform magnetic field $B_1 \hat{k}$ and the region $y<0$ has another uniform magnetic field $B_2 \hat{k}$. A positively charged particle is projected from the origin along the positive $y$-axis with speed $v_0=\pi m s^{-1}$ at $t=0$, as shown in the figure. Neglect gravity in this problem. Let $t=T$ be the time when the particle crosses the $x$-axis from below for the first time. If $\mathrm{B}_2=4 B_1$, the average speed of the particle, in $m s^{-1}$, along the $x$-axis in the time interval $T$ is $\_\_\_\_$ .