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PC index

Polar Cap Activity index

By V.A. Sergeev

PC-index is an empirical magnetic activity index based on data from single near-pole station (Thule or Vostok for N or S hemispheres, respectively), c.f. Troshichev et al., 1988 and Vennerstrom et al., 1994 for description of algorithm and examples. Its derivation procedure is optimized to achieve the best correlation of the PC-index with the solar wind electric field (SWEF) magnitude ( v*B*sin(teta/2)**2 ). The averaged horizontal magnetic disturbance vector (quiet value subtracted) is projected onto the optimal direction (defined empirically for each UT hour and each season based on the best correlation with the SWEF) and, after normalization to the equivalent value of SWEF, it gives the PC-index (expressed in mV/m). Although PC-index is formally expressed in mV/m, it actually represents the measure of magnetic activity, the normalization procedure (to SWEF) helps to reduce the seasonal/diurnal effects and facilitate intercomparison.

The major contribution to the PC-index was found from the twin-vortice DP2-like current system related to the SWEF, but some other current systems existing in the polar cap (related to IMF By or positive IMF Bz during summer or distant effects of auroral zone FACs in winter) may contribute also. The high correlation of the PC-index with SWEF and magnetic AE index have been demonstrated. The positive values of the PC index (especially when they are similar in both hemispheres) may be used as a measure of magnetospheric activity whereas the negative values have to be disregarded (see more about the interpretation of the PC index in the references below).

The averaged PC index values from Thule (PCN) and Vostok (PCS) are provided by Danish Meteorological Institute (S.Vennerstrom) and Arctic-Antarctic Research Institute (V.G.Andresen, O.A.Troshichev). They are checked, converted to flat files, supplied with flag info and sent to IKI by Physical Inst., S.Petersburg Univ.

The resolution of the northern cap PC-index is 5 min and of the southern one is 15 min. One time scale with the 5 min step is used for both indices and each (15 min) value of southern index is repeated three times.

Separate Status Flags are given for the PC-index data from each station. Their values are: 0/2/5 - valid/absent/delayed data.

Troshichev, O.A., V.G.Andrezen, S.Vennerstrom, E.Friis-Christensen, Magnetic activity in the polar cap - a new index, Planet.Space Sci., 36, 1095, 1988.

Vennerstrom,S., E.Friis-Christensen, O.A.Troshichev, V.G.Andresen, Geomagnetic Polar Cap (PC) Index, 1975-1993. Report UAG-103, World Data Center A for STP, Boulder, April 1994

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