Answer
2.2 amps per square meter, north.
Work Step by Step
Positive helium ions moving north have the same direction of current as negative oxygen ions moving south, so we can add the current densities.
We know the net charge, number per unit volume, and a speed. Use equation 18–10 to determine the current per unit area.
$$\frac{I}{A}=n_1q_1v_1+n_2q_2v_2$$
$$=(2.4\times10^{12}\frac{ions}{m^3})(2(1.6\times10^{-19}C/ion))(2.0\times10^6\frac{m}{s})+(7.0\times10^{11}\frac{ions}{m^3})(-1.6\times10^{-19}C/ion)(-6.2\times10^6\frac{m}{s})$$
$$=2.2\frac{A}{m^2},\; north$$