def ddderiv_B_φ_midplane(η_H, dderiv_v_r, η_perp_sq, dderiv_v_φ, η_P, Y_2):
    """
    Compute B_φ''' around the midplane
    """
    return Y_2 - η_H * dderiv_v_r / η_perp_sq - dderiv_v_φ * η_P / η_perp_sq
