, Volume 120, Issue 3-4, pp 197-241

Solar Forcing of Climate. 1: Solar Variability

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Abstract

We describe the properties of the Sun, those of its Active Regions (Centres of Activity, ARs or CAs) and the 11 and 22-year cycles as observed via the variable numbers of sunspots. We describe the variations with time of the solar irradiance and of the flux of ejected magnetised plasma. We discuss the probable cause of solar variability. Planetary influences are ruled out; the variability is intrinsic and is described by the solar dynamo. The dynamo is characterised by internal toroidal and more superficial poloidal fields, interchanging and alternating in a 22-year periodicity. From these two components in the solar magnetic fields emanate two possible scenarios for the Sun-climate interaction.

Solar irradiance variations are related to those in the solar toroidal magnetic fields. The fraction of the solar irradiance that reaches the Earth's ground level and low troposphere is emitted by the solar photosphere. That fraction does not significantly vary since the quiet photosphere does not significantly vary during the cycle. The variable part of the solar radiation flux is mainly emitted by the chromospheric parts of the CAs. That radiation component does not reach the Earth's troposphere since it is absorbed in the higher, stratospheric terrestrial layers. Tropospheric solar-driven variations should therefore be due to stratosphere–troposphere coupling. The Group Sunspot number R Gs is a proxy for the variable irradiance component and for the toroidal field variations.

Ejected solar plasma clouds such as the Coronal Mass Ejections (CMEs) and plasma ejected from Ephemeral Solar Regions and from the polar facular regions are related to variations in the poloidal magnetic fields. On the average they have their maximum intensity about a year after the maximum number of spots: we call this interval the Energetic Emissions Delay. These gas clouds fill the heliosphere with magnetised plasma. Thus, by emitting magnetised plasma, the Sun influences the Earth's atmosphere indirectly, by heliospheric modulation of the component of the galactic cosmic radiation (CR) that reaches tropospheric levels. Modulation is only important for cosmic ray particles with energies below about 50 GeV. Cosmic ray ionisation plays a minor role at ground level but it is the predominant ionising agent in higher atmospheric layers, already above a few kilometres. The amplitudes of the CR variations depend on those of the solar cycle. The atmospheric rate of ionisation varies with CR-intensity. A current hypothesis is that the variable ionisation may affect the degree of cloudiness. Cosmogenic radionuclides such as 10Be are proxies for this influence and for the poloidal field variations.

The R G and cosmogenic radionuclide proxies, although loosely correlated, refer to the two different aspects of the solar dynamo with their different terrestrial effects; they do not reach maximum intensity simultaneously and should therefore neither be confused nor be interchanged. Cases have occurred in which the one varied strongly while the other did hardly or not at all. The explanation must be intrinsic in dynamo theory.

There is a significant solar signal in the troposphere; it depends on latitude and longitude. A physical investigation of the cause of the Sun-climate relationship based on one unique ΔT(time)-curve, assumed valid for the whole Earth's surface, is therefore basically incorrect. Never during the past 10,000 years has the Sun been as active in ejecting magnetised plasma as during the past few decades. Estimates suggest that the level of solar activity may recently have passed its maximum and that it may decrease in coming decades. Solar activity is variable with six well-determined quasi-periodicities. Attempts to theoretically describe the solar dynamo have so far succeeded only in explaining the qualitative aspects. They fail in a numerical description and notably in one that would permit one to forecast solar activity with acceptable precision. This is so because the solar dynamo is a non-linear system that occasionally shows phase catastrophes. It is a quasi-periodic engine with the properties of deterministic chaos. “The future of such a chaotic system is intrinsically unpredictable”.