Wp index: A new substorm index derived from high‐resolution geomagnetic field data at low latitude

Abstract

[1] Geomagnetic field data with high time resolution (typically 1 s) have recently become more commonly acquired by ground stations. Such high time resolution data enable identifying Pi2 pulsations which have periods of 40-150 s and irregular (damped) waveforms. It is well-known that pulsations of this type are clearly observed at mid-and low-latitude ground stations on the nightside at substorm onset. Therefore, with 1-s data from multiple stations distributed in longitude around the Earth's circumference, substorm onset can be regularly monitored. In the present study we propose a new substorm index, the Wp index (Wave and planetary), which reflects Pi2 wave power at low-latitude, using geomagnetic field data from 11 ground stations. We compare the Wp index with the AE and ASY indices as well as the electron flux and magnetic field data at geosynchronous altitudes for 11 March 2010. We find that significant enhancements of the Wp index mostly coincide with those of the other data. Thus the Wp index can be considered a good indicator of substorm onset. The Wp index, other geomagnetic indices, and geosynchronous satellite data are plotted in a stack for quick and easy search of substorm onset. The stack plots and digital data of the Wp index are available at the Web site (http://s-cubed.info) for public use. These products would be useful to investigate and understand space weather events, because substorms cause injection of intense fluxes of energetic electrons into the inner magnetosphere and potentially have deleterious impacts on satellites by inducing surface charging.

Formation of the oxygen torus in the inner magnetosphere: Van Allen Probes observations

“…Figure shows the AL , W p , and D s t indices for 11–16 November 2012. The W p index reflects Pi2 wave power at low latitude on the nightside and is useful for identifying substorm onset [ Nosé et al , ]. The 7 h intervals of observations by Probe A and Probe B that are used in the present study are indicated by red and blue horizontal thick bars, respectively.…”

Low‐latitude Pi2 oscillations observed by polar Low Earth Orbiting satellite

“…To confirm Pi2 and its association with substorm activity, we further looked into the AL and Wp indices. Wp (Wave and planetary) is a substorm index developed by Nose et al [2012] which reflects the Pi2 power at low latitude. It is considered as a useful indicator of substorm onset.…”

“…AL and Wp indices together provide the evidence of substorm activity and associated Pi2 oscillations at low latitude. Wp index being sensitive to the onset of even smaller substorms in comparison to AL [Nose et al, 2009], we have chosen Wp (≥0.5) as the primary criterion for selecting Pi2 events. AL and Wp values are obtained from World Data Center (WDC), Kyoto, web page.…”

An alternative way to identify local geomagnetically quiet days: a case study using wavelet analysis

“…As an alternative to the traditional Dst index (Sugiura, 1964), new indexes were developed using DWT, such as the wavelet-based index of storm activity (WISA; Jach et al, 2006;Xu et al, 2008) and Wp (Nosé et al, 2012), among others. Motivated by the works of Mendes et al (2005), Mendes da Costa et al (2011), Hafez et al (2013) and Klausner et al (2014aKlausner et al ( , b, 2016a, which use the DWT to detect transition regions and to localize singularities in geophysical data, in this work, the geomagnetic variation associated with the events of CIRs and HSSs in 2007 will be analyzed.…”