Solar causes of the long‐term increase in geomagnetic activity

Abstract

Abstract. We analyze the causes of the century-long increase in geomagnetic activity, quantified by annual means of the aa index, using observations of interplanetary space, galactic cosmic rays, the ionosphere, and the auroral electrojet, made during the last three solar cycles. The effects of changes in ionospheric conductivity, the Earth's dipole tilt, and magnetic moment are shown to be small; only changes in near-Earth interplanetary space make a significant contribution to the long-term increase in activity. We study the effects of the interplanetary medium by applying dimensional analysis to generate the optimum solar windmagnetosphere energy coupling function, having an unprecedentedly high correlation coefficient of 0.97. Analysis of the terms of the coupling function shows that the largest contributions to the drift in activity over solar cycles 20-22 originate from rises in the average interplanetary magnetic field (IMF) strength, solar wind concentration, and speed; average IMF orientation has grown somewhat less propitious for causing geomagnetic activity. The combination of these factors explains almost all of the 39% rise in aa observed over the last three solar cycles. Whereas the IMF strength varies approximately in phase with sunspot numbers, neither its orientation nor the solar wind density shows any coherent solar cycle variation. The solar wind speed peaks strongly in the declining phase of even-numbered cycles and can be identified as the chief cause of the phase shift between the sunspot numbers and the aa index. The rise in the IMF magnitude, the largest single contributor to the drift in geomagnetic activity, is shown to be caused by a rise in the solar coronal magnetic field, consistent with a rise in the coronal source field, modeled from photospheric observations, and an observed decay in cosmic ray fluxes.

An evaluation of the correlation between open solar flux and total solar irradiance

“…Parker spiral theory predicts the heliospheric field components in heliocentric polar (r s , φ, ψ) coordinates will be: Stamper et al (1999) and Gazis (1996) have shown that this theory correctly predicts B ψ = 0 and the garden hose angle γ in annual means. In addition, the value of γ is almost constant in annual mean data from near Earth.…”

The importance of geomagnetic field changes versus rising CO2levels for long-term change in the upper atmosphere

“…Still, it illustrates the possibility that magnetic perturbations at high latitudes could also be affected by main magnetic field changes, which should be investigated further with a more appropriate model. If long-term changes in the geomagnetic field affect the magnetic perturbations associated with geomagnetic storms, this could have important implications for the interpretation of reported trends in geomagnetic activity (e.g., Stamper et al 1999;Clilverd et al 2002).…”

Geomagnetic activity during the rising phase of solar cycle 24

“…One parameterization of the solar wind energy input into the magnetosphere is the e parameter (Perrealt & Akasofu 1978) given by e ¼ l 2 0 V sw B 2 sin 4 ðh=2Þ, where l 2 0 is the area of the magnetopause through which the energy enters, V SW is the solar wind speed, B is the magnetic field intensity, and h is the ''clock angle'' of the IMF relative to the Sun-Earth line; e has the form of the Poynting flux in the upstream solar wind. Thus, geomagnetic activity reflects conditions in the solar wind that encounters the Earth which in turn are influenced by both long-term changes and transient activity at the Sun (e.g., Russell 1975;Feynman & Crooker 1978;Stamper et al 1999;Richardson et al 2000Richardson et al , 2002aRichardson et al , 2002bSvalgaard & Cliver 2010;Richardson & Cane 2012a;and references therein).…”