On the periodic variations of geomagnetic activity indices Ap and ap

H Schreiber

doi:10.1007/s00585-998-0510-2

Abstract

Abstract. Yearly averages of geomagnetic activity indices Ap for the years 1967–1984 are compared to the respective averages of ν2·Bs, where v is the solar wind velocity and Bs is the southward interplanetary magnetic field (IMF) component. The correlation of both quantities is known to be rather good. Comparing the averages of Ap with ν2 and Bs separately we find that, during the declining phase of the solar cycle, ν2 and during the ascending phase Bs have more influence on Ap. According to this observation (using Fourier spectral analysis) the semiannual and 27 days, Ap variations for the years 1932–1993 were analysed separately for years before and after sunspot minima. Only those time-intervals before sunspot minima with a significant 27-day recurrent period of the IMF sector structure and those intervals after sunspot minima with a significant 28-28.5-day recurrent period of the sector structure were used. The averaged spectra of the two Ap data sets clearly show a period of 27 days before and a period of 28–29 days after sunspot minimum. Moreover, the phase of the average semiannual wave of Ap is significantly different for the two groups of data: the Ap variation maximizes near the equinoxes during the declining phase of the sunspot cycle and near the beginning of April and October during the ascending phase of the sunspot cycle, as predicted by the Russell-McPherron (R-M) mechanism. Analysing the daily variation of ap in an analogue manner, the same equinoctial and R-M mechanisms are seen, suggesting that during phases of the solar cycle, when ap depends more on the IMF-Bs component, the R-M mechanism is predominant, whereas during phases when ap increases as v increases the equinoctial mechanism is more likely to be effective.Key words. Interplanetary physics · Magnetic fields · Solar wind plasma · Solar wind · magnetosphere interaction

Highlights

Geomagnetic activity is generated by the interaction of the solar wind with the magnetosphere in connection with energy and mass transfer
Sources of high solar wind speed and associated strong southward IMF, generating increased geomagnetic activity, are found in coronal holes, fast coronal mass ejections (CMEs), and interaction regions between slow and fast solar wind (e.g. Crooker and Cliver, 1994, and references therein; Lindsay et al, 1995). Studies relating these solar wind characteristics with geomagnetic activity may reveal the physics of the interaction of solar and magnetospheric plasmas
We may have found time-intervals during the solar cycle when geomagnetic activity is more inuenced by a southward IMF, generating a semiannual wave according to the RM model and intervals dominated by the solar wind velocity and resulting in semiannual waves according to the equinoctial mechanism

Summary

Introduction

Geomagnetic activity is generated by the interaction of the solar wind with the magnetosphere in connection with energy and mass transfer. Crooker and Cliver, 1994, and references therein; Lindsay et al, 1995) Studies relating these solar wind characteristics with geomagnetic activity may reveal the physics of the interaction of solar and magnetospheric plasmas. Most observations suggest that the solar wind energy is transferred to the magnetosphere by the reconnection process at the dayside magnetopause, where oppositely directed southward IMF and northward magnetospheric magnetic ®eld lines merge (Dungey, 1961). Petschek, 1964; Vasilyunas, 1975); not all details of the physics are yet understood (Dungey, 1994), and still other energy transfer mechanisms such as viscous interaction and Kelvin-Helmholtz wave instabilities at theanks of the magnetosphere are discussed [see the comprehensive overview of Crooker and Siscoe (1986) and references therein] Theoretical models of the merging process have been described (e.g. Petschek, 1964; Vasilyunas, 1975); not all details of the physics are yet understood (Dungey, 1994), and still other energy transfer mechanisms such as viscous interaction and Kelvin-Helmholtz wave instabilities at theanks of the magnetosphere are discussed [see the comprehensive overview of Crooker and Siscoe (1986) and references therein]

Schreiber

The solar cycle periodic variations of Ap

The semiannual variation

The 27-day variation

The universal time variation of ap

Discussion and conclusion