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Ann. Geophys., 26, 2645-2648, 2008
www.ann-geophys.net/26/2645/2008/
© European Geosciences Union 2008
Derivation of a planetary ionospheric storm index
T. L. Gulyaeva1,2 and I. Stanislawska2
1IZMIRAN, 142190 Troitsk, Moscow Region, Russia
2Space Research Centre PAS, Bartycka 18-A, 00-716 Warsaw, Poland
Abstract. The planetary ionospheric storm index, Wp, is deduced from the numerical global
ionospheric GPS-IONEX maps of the vertical total electron content, TEC, for
more than half a solar cycle, 1999–2008. The TEC values are extracted from the 600 grid
points of the map at latitudes 60° N to 60° S with a step of 5°
and longitudes 0° to 345° E with a step of 15° providing the data for
00:00 to 23:00 h of local time. The local effects of the solar radiant energy are
filtered out by normalizing of the TEC in terms of the solar zenith angle χ
at a particular time and the local noon value χ0. The degree of
perturbation, DTEC, is computed as log of TEC relative to quiet reference
median for 27 days prior to the day of observation. The W-index map is
generated by segmentation of DTEC with the relevant thresholds specified earlier
for foF2 so that 1 or −1 stands for the quiet state, 2 or −2 for the
moderate disturbance, 3 or −3 for the moderate ionospheric storm, and 4 or
−4 for intense ionospheric storm at each grid point of the map. The planetary
ionospheric storm Wp index is obtained from the W-index map as a latitudinal
average of the distance between maximum positive and minimum negative W-index
weighted by the latitude/longitude extent of the extreme values on the map.
The threshold Wp exceeding 4.0 index units and the peak value Wpmax≥6.0 specify
the duration and the power of the planetary ionosphere-plasmasphere storm. It is
shown that the occurrence of the Wp storms is growing with the phase of the
solar cycle being twice as much as the number of the magnetospheric storms
with Dst≤−100 nT and Ap≥100 nT.
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