Electromagnetic Waves of frequencies higher than $9\sqrt{2}$ MHz are found to be reflected by the ionosphere on a particular day at a place. The maximum electron density in the ionosphere is

$2 × 10^{12} m^{-3}$

The frequency of electromagnetic waves

$\nu = 9\sqrt{\text{ maximum electron density}} = 9 \sqrt{d_{max}}$

$\Rightarrow d_{max} = \frac{\nu^2}{81} = \frac{162\times 10^{12}}{81} = 2\times 10^{12}$