13 Concluding Remarks

The purpose of this review is to detail the development of reconstructions of solar and heliospheric magnetic fields from geomagnetic activity data and to make some initial comparisons with the results from cosmogenic isotopes and models based on sunspot number observations. One important aspect of this work on centennial-scale solar variability is that it fills the timescale gap between decadal-scale variations (almost 5 full cycles are now covered by in-situ space measurements) and millennia (covered by cosmogenic isotope data) and so allows modern space-age understanding to be applied to the cosmogenic isotope data in a more precise, insightful and quantitative manner.

A fundamental insight that has accrued is that although the sunspot number returns to an (almost) constant baselevel state at every solar minimum, this disguises the fact that the Sun does not return to the same state at each sunspot minimum: there are long-term variations on coronal and heliospheric fields which mean that one solar minimum is not the same as the next. The cycle-to-cycle variation of the Sun at solar minimum is the basis of the “precursor” method for predicting the peak of the sunspot cycle (Schatten et al., 1978) and using the polar fields Svalgaard and Cliver (2005) made predictions for cycle 24 which are proving exceptionally accurate. This long-term change influences many space-weather phenomena, such as the galactic cosmic ray flux reaching Earth, (“gradual”) solar energetic particles generated ahead of solar coronal mass ejections, and solar-wind interactions that drive solar-terrestrial activity giving phenomena such as Geomagnetically Induced Currents in power grids. Hence, we need to allow for “space climate change” as well as space weather.

Another important realisation has been that information has been lost by grouping the variations of various observation series and indices into a general, catch-all “solar activity” classification. Specifically, although the variations of the various geomagnetic activity indices have a great many similarities (for example, all reflect the decadal-scale sunspot cycle), they are measures of different parts of the currents systems in near-Earth space and they have different dependencies on solar wind parameters. These differences can be exploited to derive new information. The example discussed most here is how combinations of range and interdiurnal variation geomagnetic indices allow us to separate the effects of solar wind speed and the interplanetary magnetic field and so reconstruct both back in time.

The review has covered many pitfalls of the reconstructions from knowing the full provenance of historic data, to limitations of statistical methods. Nevertheless considerable consensus now exists between the various reconstructions for between about 1880 and the present day. Thus, using geomagnetic activity allows us to extend the 50-year sequence of in-situ spacecraft data a further 80 years back in time with great confidence. Before 1880 there are increasing differences. There were fewer stations and they were frequently in increasingly noisy environments and eventually had to be moved as cities expanded around them. Equipment was less accurate and more prone to calibration drifts. Thus, although there is, in theory, information available for a further 75 years back into the past there is necessarily greater uncertainty. This is not to say that the full 185-year record cannot be recovered, but the earliest data will always have greater associated uncertainties.

The most important realisation that these reconstructions have allowed, when combined with cosmogenic isotope data, is that the modern space age has been an unusually active period for the Sun, compared to most of the last millennium. Just how unusual is a matter of on-going debate, but even this is now converging to a consensus view that it has been, until recent years at least, very unusual indeed. That being the case, we should not be surprised that average solar activity levels are now declining and that cycle 24 is weak compared to the others in the space age. The cosmogenic isotope data tell us that this decline is more likely to continue than not. This has great implications for solar physics, solar-terrestrial science and space weather. Some effects of a continuation of the current decline are well known, for example, the cosmic ray flux incident on Earth will rise. However, others are not. It is possible that although large space weather “events” may become fewer in number, the largest could become more severe in their terrestrial effects because the CME is ejected into a lower-field heliosphere making the Alfvén Mach number of the event greater and potentially reducing Sun-to-Earth transit times. The long, low minimum between cycles 23 and 24 was only “exceptional” in the context of the space age and may give pointers to other changes that we should now expect. The author’s personal view is that this offers great scientific possibilities and the modern observation techniques applied to a quieter Sun will teach us much more than a continuation of the high activity levels seen during cycles 21, 22, and 23. All the evidence is that cycle 24 has just passed its peak, and that peak is a weak one, the development of solar activity into the next minimum will be very interesting to monitor.

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