Automatic selection of Dst storms and their seasonal variations in two versions of Dst in 50\xa0years

N Balan,Y Kamide,P K Rajesh,S Tulasiram,J R Souza,K Shiokawa,N J Victor,I S Batista

doi:10.1186/s40623-017-0642-2

Abstract

A computer program is developed to automatically identify the geomagnetic storms in Dst index by applying four selection criteria that minimize non-storm-like fluctuations. The program is used to identify the storms in Kyoto Dst and USGS Dst in 50 years (1958–2007). The identified storms (DstMin ≤ −50 nT) are used to investigate their seasonal variations. It is found that the overall seasonal variations of the storm parameters such as occurrence, average intensity (average DstMin) and average strength (average ⟨DstMP⟩) in both versions of Dst exhibit clear semiannual variations with equinoctial maxima and solstice minima; and the maxima and minima in intensity and strength (~±17% each) are less than those in occurrence (~±28%). Wavelet spectra of the storms reveal the existence of distinct semiannual component in four solar cycles (SCs 20–23) and weak longer and shorter-period components in some SCs. The semiannual variation observed also in the mean energy input during the main phase (MP) of the storms estimated from Dst is interpreted in terms of the (1) equinoctial mechanism based on the varying angle between the Earth–Sun line and Earth’s dipole axis and (2) Russell–Mcpherron effect based on the varying angle between the GSM Z-axis and GSE Y-axis; and the yearly range of the dipole tilt angle µ (23.2°) involved in the equinoctial mechanism is found larger than the title angle θ (16.3°) involved in the RM effect.Graphical .

Highlights

Geomagnetic storms are disturbances in Earth’s magnetic field lasting from several hours to several days, and are produced by enhanced solar wind–magnetosphere coupling (e.g., Svalgaard 1977; Akasofu 1981; Gonzalez et al 1994; Kamide et al 1998) and ionosphere–magnetosphere plasma coupling (e.g., Daglis 1997; Khazanov et al 2003; Ebihara et al 2005)
The storms are analyzed for their important characteristics including the mean value of Dst during main phase (MP)
The program identified 761/585 storms in Kyoto Dst/USGS Dst, which include 34/33 super storms (DstMin ≤ −250 nT), 296/210 intense storms (−250 < DstMin ≤ −100 nT) and 431/342 moderate storms (−100 < DstMin ≤ −50 nT)

Summary

Introduction

Geomagnetic storms are disturbances in Earth’s magnetic field lasting from several hours to several days, and are produced by enhanced solar wind–magnetosphere coupling (e.g., Svalgaard 1977; Akasofu 1981; Gonzalez et al 1994; Kamide et al 1998) and ionosphere–magnetosphere plasma coupling (e.g., Daglis 1997; Khazanov et al 2003; Ebihara et al 2005). 1980; Takalo et al 1995; Siscoe and Crooker 1996; Daglis 1997; Kamide et al 1998; Gonzalez et al 1999; Cliver et al 2000; Fok et al 2001; Liemohn et al 2001; O’Brien and McPherron 2002; Vijaya Lekshmi et al 2011; Yakovchouk et al 2012; Zhao and Zong 2012; Katus and Liemohn 2013; Kumar et al 2015; Lakhina and Tsuritani 2016) From these studies, it is known that the occurrence and intensity of the storms vary with solar activity, time of year and time of day.

Results

Discussion

Conclusion