Wp index

The Wp index is newly proposed by Nosé in 2008. This index is related to wave power of low-latitude Pi2 pulsations. In derivation of the Wp index, wavelet analysis is adopted, which is a suitable method for investigating the power of short-lived waves, such as Pi 2 pulsations. Geomagnetic field data used for the index are the H and D components from geomagnetic latitude range of 21°-48°. Detailed explanation can be found in Nosé et al. [2009, 2012].

Pi2 pulsation is defined as geomagnetic field variations with a period of 40-150 seconds and an irregular (damping) waveform. A number of previous studies reported that one of important features of low-latitude Pi2 pulsations is their close relation to substorm onset. Thus substorm can be identified by prompt increases of the Wp index.