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Polar Cap Index

Copyright WDC C1 for STP at RAL.

The Polar Cap Index (PCI) is a newly introduced index for magnetic activity (Troshichev et al., 1988) based on an idea by Troshichev et al. (1979). Indices are available for both the north (Thule) and south (Vostok) poles.

Many statistical investigations (Papitashvili et al., 1981; Troshichev, 1982; Friis-Christensen and Wilhjelm, 1975) have shown that a nearly linear relationship exists between the southward component of the interplanetary magnetic field, IMF Bz, and the amplitude of the magnetic disturbance associated with the well known two-cell convection pattern at high latitudes, named the DP2 disturbance. The DP2 disturbance at a station located very near to the pole will correspond to the antisunward part of the two-cell convection pattern at all local times. Troshichev et al. (1979) therefore proposed to use a single near pole station to derive a magnetic activity index PC, which could be regarded as a signature of the magnetic activity driven by the IMF Bz-component.

The PC index may be considered as a ground analogue of various interplanetary parameters, especially the southward component of the IMF, and it may be used for diagnosing these parameters, in particular the electric field defining the polar cap convection. In this respect it would be complementary to the auroral zone indices such as AE and AL. These indices are typical substorm indices, while the PC index would be more related to the process of the solar wind-magnetosphere interaction. The conductivity in the polar cap ionosphere as its main source in solar u.v.-radiation, and the DP2 disturbances must therefore be expected to be very different during summer and winter. To overcome this problem it is necessary to use two stations, one near the south pole (Vostok) and one near the north pole (Thule).

The data currently covers 1975 to the present for Thule, and 1978-1991 for Vostok. All the Vostok values are final; the Thule values are final to the end of 1996, but preliminary values up to yesterday are supplied to us daily by WDC-C1 for Geomagnetism, Copenhagen.

If either PC index is used in publications, please acknowledge the relevant institute: the Danish Meteorological Institute for the Thule index, and the Arctic and Antarctic Research Institute, St Petersburg for the Vostok index.

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