4.5 Related implications

Reconstructions of long-term solar activity have different implications in related areas of science. The results, discussed in this overview, can be used in such diverse research disciplines as theoretical astrophysics, solar-terrestrial studies, paleo-climatology, and even archeology and geology. We will not discuss all possible implications of long-term solar activity in great detail but only briefly mention them here.

4.5.1 Theoretical constrains

The basic principles of the occurrence of the 11-year Schwabe cycle are more-or-less understood in terms of the solar dynamo, which acts, in its classical form (e.g., Parker, 1955), as follows. Differential rotation Ω produces a toroidal magnetic field from a poloidal one, while the “α-effect” associated with the helicity of the velocity field produces a poloidal magnetic field from a toroidal one. This classical model results in a periodic process in the form of propagation of a toroidal field pattern in the latitudinal direction (the “butterfly diagram”). As evident from observation, the solar cycle is far from being a strictly periodic phenomenon, with essential variations in the cycle length and especially in the amplitude, varying dramatically between nearly spotless grand minima and very large values during grand maxima. The mere fact of such great variability, known from sunspot data, forced solar physicists to develop dynamo models further. Simple deterministic numerical dynamo models, developed on the basis of Parker’s migratory dynamo, can simulate events, which are seemingly comparable with grand minima/maxima occurrence (e.g., Brandenburg et al., 1989). However, since variations in the solar-activity level, as deduced from cosmogenic isotopes, appear essentially nonperiodic and irregular, appropriate models have been developed to reproduce irregularly-occurring grand minima (e.g., Jennings and Weiss, 1991Tobias et al., 1995Covas et al., 1998). Models, including an ad hoc stochastic driver (Choudhuri, 1992Schmitt et al., 1996Ossendrijver, 2000Weiss and Tobias, 2000Mininni et al., 2001Charbonneau, 2001Charbonneau et al., 2004), are able to reproduce the great variability and intermittency found in the solar cycle (see the review by Charbonneau, 2005). A recent statistical result of grand minima occurrence (Usoskin et al., 2007) shows disagreement between observational data, depicting a degree of self-organization or “memory”, and the above dynamo model, which predicts a pure Poisson occurrence rate for grand minima (see Section 4.3). This poses a new constraint on the dynamo theory, responsible for long-term solar-activity variations.

In general, the following additional constraints can be posed on dynamo models aiming to describe the long-term (during the past 11,000 years) evolution of solar magnetic activity.

4.5.2 Solar-terrestrial relations

The sun ultimately defines the climate on Earth supplying it with energy via radiation received by the terrestrial system, but the role of solar variability in climate variations is far from being clear. Solar variability can affect the Earth’s environment and climate in different ways (see, e.g., a review by Haigh, 2007). Variability of total solar irradiance (TSI) measured during recent decades is known to be too small to explain observed climate variations (e.g., Foukal et al., 2006Fröhlich, 2006). On the other hand, there are other ways solar variability may affect the climate, e.g., an unknown long-term trend in TSI (Solanki and Krivova, 2004Wang et al., 2005) or a terrestrial amplifier of spectral irradiance variations (Shindell et al., 1999). Alternatively, an indirect mechanism also driven by solar activity, such as ionization of the atmosphere by CR (Usoskin and Kovaltsov, 2006Jump To The Next Citation Point) or the global terrestrial current system (Tinsley and Zhou, 2006) can modify atmospheric properties, in particular cloud cover (Ney, 1959Svensmark, 1998). Even a small change in cloud cover modifies the transparency/absorption/reflectance of the atmosphere and affects the amount of absorbed solar radiation, even without changes in the solar irradiance. Since the CR flux at Earth is modulated not only by solar activity, but also by the slowly changing geomagnetic field, the two CR modulation mechanisms are independent and act on different timescales, thus giving one the opportunity to study the CR effect on Earth separately from solar irradiance (de Jager and Usoskin, 2006Usoskin et al., 2005b).

Accordingly, improved knowledge of the solar driver’s variability may help in disentangling various effects in the very complicated system that is the terrestrial climate (e.g., de Jager, 2005Versteegh, 2005Jump To The Next Citation Point). It is of particular importance to know the driving forces in the pre-industrial era, when all climate changes were natural. Knowledge of the natural variability can lead to an improved understanding of anthropogenic effects upon the Earth’s climate.

Studies of the long-term solar-terrestrial terrestrial are mostly phenomenological, lacking a clear quantitative physical mechanism. Even phenomenological and empirical studies suffer from large uncertainties, related to the quantitative interpretation of proxy data, temporal and spatial resolution (Versteegh, 2005). Therefore, more precise knowledge of past solar activity, especially since it is accompanied by continuous efforts of the paleo-climatic community on improving climatic data sets, is crucial for improved understanding of the natural (including solar) variability of the terrestrial environment.

4.5.3 Other issues

The proxy method of solar-activity reconstruction, based on cosmogenic isotopes, was developed from the radiocarbon dating method, when it was recognized that the production rate of 14C is not constant and may vary in time due to solar variability and geomagnetic field changes. Neglect of these effects can lead to inaccurate radiocarbon (or more generally, cosmogenic nuclide) dating, which is a key for, e.g., archeology and Quaternary geology. Thus, knowledge of past solar activity and geomagnetic changes allows for the improvement of the quality of calibration curves, such as the IntCal (Stuiver et al., 1998Reimer et al., 2004) for radiocarbon, eventually leading to more precise dating.

Long-term variations in the geomagnetic field are often evaluated using cosmogenic isotope data. Knowledge of source variability due to solar modulation is important for better results.


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