Step 1: Use the field ratio.
For any point in a plane electromagnetic wave in free space, the ratio of the electric field magnitude to the magnetic field magnitude is constant and equals the speed of light: \(\dfrac{E_0}{B_0} = c\).
Step 2: Rearrange for the electric field amplitude.
\(E_0 = c\,B_0\).
Step 3: Convert the magnetic amplitude to tesla.
\(B_0 = 500\ \text{nT} = 5 \times 10^{-7}\ \text{T}\) (since \(1\ \text{nT} = 10^{-9}\ \text{T}\)).
Step 4: Multiply.
\(E_0 = (3 \times 10^{8}\ \text{m/s})(5 \times 10^{-7}\ \text{T}) = 150\ \text{V/m}\).
So the electric field oscillates with a peak value of 150 volts per metre.
\[\boxed{E_0 = 150\ \text{V/m}}\]