Est and Ist index
Partial and symmetric magnetospheric ring currents, magnetopause, neutral sheet and tail currents generate a magnetic disturbance field which is monitored by the global network of magnetic observatories. The dominant axi-symmetric part of this disturbance field is estimated from the recordings at four low-latitude observatories and is distributed by the World Data Centers as the Dst index.
Further separating the disturbance magnetic field into external and induced contributions provides the Est index for the external source field and the Ist index for the internal induced field. (Ref: Maus, Stefan; Weidelt, Peter; Separating the magnetospheric disturbance magnetic field into external and transient internal contributions using a 1D conductivity model of the Earth; Geophys. Res. Lett., Vol. 31, No. 12, L12614)
The physical motivation for this separation is that the time varying magnetospheric fields induce electric currents in the Earth which in turn give rise to a secondary internal field whose strength is roughly one third of the external field. Hence, the disturbance field observed at the Earth's surface is the sum of the external source field and its induced counterpart. If the Earth were an ideal conductor then the two fields would be exactly in phase because currents would be induced in such a way as to prevent any external field from entering into the conductor.
Schematic of the Interplanetary Magnetic Field and Currents
(Click on image to see a larger version.)
Furthermore, for an ideal conductor the strength of the induced field would be such that its radial component would cancel the radial component of the inducing field everywhere on its surface. For the real Earth, however, the phase lag and amplitude relation between the induced internal and inducing external field depends on the frequency content of the external source field.
Download the Est_Ist from the ftp directory