Solar magnetism and Earths climate


The hypotheses proposing that solar magnetism affects Earth's climate are often based on a physical model, but long, reliable data records are not always available for testing the hypotheses. Satellite data for cloud observations start in the early 1980s. The quality of these data furthermore may not be sufficiently good for testing some of these hypotheses. The time span of space-borne measurements of the total solar irradiation (TSI) is also short compared to the climatic and sunspot cycle timescales, and even the TSI measurements suffer from errors. The problem can be illustrated by the findings of Toma and White (2000) who found an apparent non-stationary relationship between the TSI and the solar activity proxies between cycles 22 and 23. Thus, one of the obstacles for these hypotheses is the lack of long records with high-quality direct measurements. There are nevertheless some long data records which may bear the imprint of a solar connection.

One way to start looking for a connection between solar activity and the clouds is to search for sunspot signals in precipitation records which tend to be longer (and with perhaps higher-quality measurements) than the cloud observations themselves. A 27-day variability in the rainfall may be a solar signal, but it is important not to confuse this timescale with the lunar orbit of 27.3 days.

There are also some long records of proxy data that can be used for assessing the conceptual models. Lockwood et al. (1999) used the aa-index (Mayaud, 1972) as a proxy for the long-term change in solar magnetism, but this record may be susceptible to contamination from internally generated geomagnetic variations. In addition to actual observations, some of these hypotheses may be tested in a laboratory.


The northern lights (the aurora borealis) have been observed by humans for much longer than the sunspots, although people in the high latitudes have been more frequently witness to this fascinating phenomenon than people at lower latitudes. The aurora has entered folklore in Scotland, Finland, Canada, North America, Greenland, Sweden and Norway (Brekke and Egeland, 1994). There are also the southern lights in the southern hemisphere which can be seen over New Zealand during active periods.

The northern lights are related to the Sun's activity but are also influenced by Earth's magnetic field. In the past, there have been various hypotheses proposed about the northern lights influencing the weather, but this notion is not taken seriously by the wider community today. Nevertheless, the northern lights activity may be taken as an indicator of the solar activity level. The controversial hypothesis today is whether magnetic fields from the Sun can affect the cloud formation on Earth indirectly by modulating the stream of cosmic galactic rays entering Earth's atmosphere.

7.3 EARTH'S MAGNETIC AND ELECTRIC FIELDS 7.3.1 Geomagnetic storms

Variations in the geomagnetic field, such as those due to electromagnetic disturbances during sunspot maximum, may generate currents in Earth's surface. Some examples include electric currents induced in telegraph lines and powergrids, and enhanced corrosion of pipelines. Geomagnetic storms occur when clouds of ions, protons and electrons (the cloud may be neutral although the particles themselves are charged) blown out from the Sun enter Earth's magnetic field. The electric charge of the individual particles will induce an electric current in the presence of the geomagnetic field in accordance with classical electromagnetics (Maxwell's equations). The force which the magnetic field exerts on these particles together with the collisions with molecules and atoms in Earth's atmosphere prevent most of them from reaching Earth's surface, and in doing so, part of the energy associated with these particles will be converted to the compression of the geomagnetic field. Chapman and Ferraro suggested in 1931 that the energy drives an electric ring-current that flows around the Earth (Kuiper, 1953, p. 442). Despite this, the correlation between the daily measurements of the geomagnetic activity is not correlated with the daily sunspot activity. However, there may still be a correlation between a certain size of sunspots, the types of flare, and the morphology (physical character and the evolution) of geomagnetic storms. Furthermore, at sunspot minimum, there may be geomagnetic storms despite the fact that almost no flares are seen.

Figure 7.1. A schematic diagram showing how the magnetic field on the day-side is compressed by the solar wind whereas the night-side field is dragged out into a tail shape.

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