{B}_j= \sum_{n (\ne n_0)} \frac{2\hbar \ {\rm Im}[\mathcal{I}_{n_0n} \ \mathcal{F}^j_{nn_0}]}{(E_n-E_{n_0})^2} .
Accordingly, one can argue that the "magnetic charge" that generates (so to say) the B field consists of quantized "Dirac monopoles".