 Solution

Use the preceding electron density as the line-of-sight mean, $L/c=2000\mathrm{yr}$, $\omega =2\pi (1.4\times 10^9){\mathrm{s}}^{-1}$ and $\Delta \omega =2\pi (4.0\times 10^7){\mathrm{s}}^{-1}$. ThenThis estimate corresponds to dispersion measure about $69.5\mathrm{pc}\mathrm{c}{\mathrm{m}}^{-3}$. A cloud density inferred from absorption need not equal the actual path average, so a real pulsar may differ even at the same distance.