Magnetospheric fields and their induced counterparts are the largest source of error in models representing the geomagnetic field. Of particular concern is the current practice of coupling the internal induced field for convenience to the external field by a real constant, independent of the frequency content of the external inducing source. The error introduced into field models by this simplified representation is of the order of 5 nT on average. Here, we propose an accurate representation of the symmetric part of the disturbance field which is easy to implement. Using a 1D conductivity model of the Earth, we split the disturbance D _st index into two new indices, E _st and I _st, which track the transient evolution of the symmetric part of the external and induced disturbance field. The ensuing D _st-based transient correction for geomagnetic field models is in remarkable agreement with the transient effect observed in CHAMP, Ørsted and SAC-C satellite magnetic measurements.