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The WDC has a FTP-server to distribute the PCN index derived from the geomagnetic observatory Qaanaaq (THL) and the Kp-index data products derived at the geomagnetic observatory Niemegk (NGK). The WDC is also holding extensive archives of magnetograms and other geomagnetic observatory data products that predate the introduction of digital data recording. The material is in analogue form such as film or microfiche. The Polar Cap index (abbreviation PC index) consists of the Polar Cap North (PCN) and the Polar Cap South (PCS) index, which are derived from magnetic measurements taken at the geomagnetic observatories Qaanaaq (THL, Greenland, +85o magnetic latitude) and Vostok (VOS, Antarctica, -83o magnetic latitude), respectively. The idea behind these indices is to estimate the intensity of anti-sunward plasma convection in the polar caps. This convection is associated with electric Hall currents and consequent magnetic field variations perpendicular to the antisunward plasma flow (and related Hall current) which can be monitored at the Qaanaaq and Vostok magnetic observatories. PC aims at monitoring the energy input from solar wind to the magnetosphere (loading activity). The index is constructed in such a way that it has a linear relationship with the merging Electric Field at the magnetopause; consequently PC is given in units of mV/m as for the electric field. In August 2013, the International Association of Geomagnetism and Aeronomy (IAGA) endorsed the PC index. The endorsed PC index is accessible at pcindex.org or through WDC Copenhagen.
The International Service of Geomagnetic Indices (ISGI) is in charge of the elaboration and dissemination of geomagnetic indices, and of tables of remarkable magnetic events, based on the report of magnetic observatories distributed all over the planet, with the help of ISGI Collaborating Institutes. The interaction between the solar wind, including plasma and interplanetary magnetic field, and the Earth's magnetosphere results in a transfer of energy and particles inside the magnetosphere. Solar wind characteristics are highly variable, and they have actually a direct influence on the shape and size of the magnetosphere, on the amount of transferred energy, and on the way this energy is dissipated. It is clear that the great diversity of sources of magnetic variations give rise to a great complexity in ground magnetic signatures. Geomagnetic indices aim at describing the geomagnetic activity or some of its components. Each geomagnetic index is related to different phenomena occurring in the magnetosphere, ionosphere and deep in the Earth in its own unique way. The location of a measurement, the timing of the measurement and the way the index is calculated all affect the type of phenomenon the index relates to. The IAGA endorsed geomagnetic indices and lists of remarkable geomagnetic events constitute unique temporal and spatial coverage data series homogeneous since middle of 19th century.