The correlation between solar and geomagnetic activity – Part 1: Two-term decomposition of geomagnetic activity

Z L Du

doi:10.5194/angeo-29-1331-2011

Abstract

Abstract. By analyzing the logarithmic relationship between geomagnetic activity as represented by the annual aa index and solar magnetic field activity as represented by the annual sunspot number (Rz) during the period 1844–2010, aa is shown to lie in between two lines defined solely by Rz. Two ways can be used to decompose the aa index into two components. One is decomposing aa into the sum of the baseline (aab) and the remainder (aau) with a null correlation. Another is dividing the top-line (aat) into the sum of aa and the remainder (aad) with a null correlation. The first decomposition is similar to the traditional one. The second decomposition implies a nonlinear relationship of aa with Rz (aat) and a decay process (aad). Therefore, aat=aa+aad=aab+aau+aad: (i) aat is related to the solar energy potential of generating geomagnetic activity (associated with Rz); (ii) aab is related to transient phenomena; (iii) aau is related to recurrent phenomena; and (iv) aad is related to the energy loss in the transmission from solar surface to the magnetosphere and ionosphere that failed to generate geomagnetic activity.

Highlights

Since Mayaud (1972) designed the geomagnetic activity aa index from the 3-hourly K indices at two near-antipodal midlatitude stations, numerous authors have used it to analyze the global geomagnetic activity and its correlation with solar activity (Schatten et al, 1978; Feynman, 1982; Legrand and Simon, 1989a; Nevanlinna and Kataja, 1993; Lukianova et al, 2009; Du et al, 2009; Du, 2011a)
It is believed that the geomagnetic activity is well associated with the solar wind speed (V ), the southward component (Bz) of the interplanetary magnetic field (IMF) and their product (Snyder et al, 1963; Russell and McPherron, 1973; Garrett et al, 1974; Crooker et al, 1977; Svalgaard, 1977; Feynman, 1980; Wang and Sheeley, 2009)
The two components of aaR and aaI have been explained by two main solar sources of geomagnetic activity (Legrand and Simon, 1981; Venkatesan et al, 1982; Legrand and Simon, 1989a; Gonzalez et al, 1990; Venkatesan et al, 1991; Legrand and Simon, 1989a,b; Echer et al, 2004): (i) aaR is related to solar flares and coronal mass ejections (CMEs); and (ii) aaI is related to high-speed solar wind streams and is out of phase with the sunspot cycle (Feynman, 1982)

Summary

Introduction

Since Mayaud (1972) designed the geomagnetic activity aa index from the 3-hourly K indices at two near-antipodal midlatitude stations, numerous authors have used it to analyze the global geomagnetic activity and its correlation with solar activity The two components of aaR and aaI have been explained by two main solar sources of geomagnetic activity (Legrand and Simon, 1981; Venkatesan et al, 1982; Legrand and Simon, 1989a; Gonzalez et al, 1990; Venkatesan et al, 1991; Legrand and Simon, 1989a,b; Echer et al, 2004): (i) aaR is related to solar flares and coronal mass ejections (CMEs); and (ii) aaI is related to high-speed solar wind streams and is out of phase with the sunspot cycle (Feynman, 1982).

Two-term decomposition of aa based on the baseline

Two-term decomposition of aa based on the top-line

A nonlinear model

A decay process

Discussions and conclusions