Jacobian matrix structure for a N_ℓ-level atom problem. J_F is a (N_ℓN_z) × (N_ℓN_z) matrix that contains the derivatives of the residual vector components with respect to the population densities. This matrix can be considered as a N_z × N_z block matrix, where each block J^kℓ is a N_ℓ × N_ℓ matrix. J^kℓ stores the derivatives of F at depth index k with respect to the population densities at depth index ℓ.