Halliday, Resnick & Walker — Current and Resistance: Problem 8

8 Earth’s lower atmosphere contains negative and positive ions that are produced by radioactive elements in the soil and cosmic rays from space. In a certain region, the atmospheric electric field strength is 120 V/m and the field is directed vertically down. This field causes singly charged positive ions, at a density of 640 cm$^{-3}$, to drift downward and singly charged negative ions, at a density of 550 cm$^{-3}$, to drift upward (Fig. 26-17). The measured conductivity of the air in that region is $2.70 \times 10^{-14} (\Omega \cdot m)^{-1}$. Calculate (a) the magnitude of the current density and (b) the ion drift speed, assumed to be the same for positive and negative ions.