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.
- Nosé, M., T. Iyemori, L. Wang, A. Hitchman, J. Matzka, M. Feller, S. Egdorf, S. Gilder, N. Kumasaka, K. Koga, H. Matsumoto, H. Koshiishi, G. Cifuentes-Nava, J. J. Curto, A. Segarra, and C. Çelik, Wp index: A new substorm index derived from high-resolution geomagnetic field data at low latitude, Space Weather, 10, S08002, doi:10.1029/2012SW000785, 2012.
- Nosé M., T. Iyemori, M. Takeda, H. Toh, T. Ookawa, G. Cifuentes-Nava, J. Matzka, J. J. Love, H. McCreadie, M. K. Tuncer, and J. J. Curto, New substorm index derived from high-resolution geomagnetic field data at low latitude and its comparison with AE and ASY indices, in Proceedings of XIIIth IAGA Workshop on Geomagnetic Observatory Instruments, Data Acquisition, and Processing, U.S. Geological Survey Open-File Report 2009-1226, edited by J. J. Love, pp. 202-207, 2009.