T. Iyemori | Geology Reports

USGS2014

Automated detection of Pi 2 pulsations using wavelet analysis: 1. Method and an application for substorm monitoring

Wavelet analysis is suitable for investigating waves, such as Pi 2 pulsations, which are limited in both time and frequency. We have developed an algorithm to detect Pi 2 pulsations by wavelet analysis. We tested the algorithm and found that the results of Pi 2 detection are consistent with those obtained by visual inspection. The algorithm is applied in a project which aims at the nowcasting of substorm onsets. In this project we use real-time geomagnetic field data, with a sampling rate of 1 second, obtained at mid- and low-latitude stations (Mineyama in Japan, the York SAMNET station in the U.K., and Boulder in the U.S.). These stations are each separated by about 120° in longitude, so at least one station is on the nightside at all times. We plan to analyze the real-time data at each station using the Pi 2 detection algorithm, and to exchange the detection results among these stations via the Internet. Therefore we can obtain information about substorm onsets in real-time, even if we are on the dayside. We have constructed a system to detect Pi 2 pulsations automatically at Mineyama observatory. The detection results for the period of February to August 1996 showed that the rate of successful detection of Pi 2 pulsations was 83.4% for the nightside (18-06MLT) and 26.5% for the dayside (06-18MLT). The detection results near local midnight (20-02MLT) give the rate of successful detection of 93.2%.

Earth, Planets and Space↗

USGS2009

New substorm index derived from high-resolution geomagnetic field data at low latitude and its comparison with AE and ASY indices

High-resolution geomagnetic field data (i.e., ≤5 seconds) have recently become more commonly used by space physicists. The data permit the identification of Pi2 pulsations, having periods of 40-150 seconds and irregular waveforms. Pulsations of this type appear clearly in time series from mid- and low-latitude ground stations on the nightside at substorm onset. Therefore, with data from multiple observatories, substorm genesis and evolution can be monitored. Here we propose a new substorm index, the Wp index (Wavelet and planetary), which measures Pi2 spectral power at low-latitude. This index is derived from geomagnetic field data obtained from observatories arranged in longitude around the Earth’s circumference. Presently, data from 5 ground stations (Fürstenfeldbruck, Iznik, Urumqi, Kakioka, and Teoloyucan) are used, but future work will include data from other sites as well (Honolulu, Tucson, San Juan, Tristan da Cunha, and Ebro). Here we compare substorm occurrence estimated from the Wp index and those from the AE and ASY indices. We show that Wp index is a good indicator of substorm onset.

Conference Paper↗