The introductory Chapter I Solar magnetic activity has the following contents:
1. Introduction
2. The solar cycle
2.1 The sunspot cycle
2.2 Other periodicities
3. The solar dynamo
3.1 Historical sketch
3.2 The dynamo process
3.3 Location of the dynamo
3.4 Traditional approximations
3.5 Stellar dynamos
4. The Earth's dynamo
5. Solar dynamo models
5.1 Linear theory
5.2 Non-linear models
6. Outline of the approach followed in this thesis
6.1 Critique of traditional dynamo theory
6.2 Averaging procedures
6.3 The finite magnetic energy method
References -- only those of the texts given here
Summary of the thesis.
The sections that are available here are indicated by the usual links. (Due to typographical reasons, the notation in the texts had to be adapted slightly at some points.) The remainder of this Chapter contains too many details for presentation here, hence some of the concepts in the available texts appear out of the blue. If you're interested, send me an email and I'll send you a copy of this chapter.

Chapter I


Solar magnetic activity

1. Introduction

The Sun's large-scale magnetic field shows a cyclic behaviour with a period of about 22 years. The origin of this magnetic cycle is not well known. Over the past decades many ideas and models have been developed, tested numerically, rejected, adjusted and/or improved. The principal processes involved are turbulence in a spherical shell (called the convection zone) directly underneath the solar surface and rotation in that shell. Both these velocity fields drag the magnetic field along: they stretch, twist and spread the field lines. The combination of these processes results in what is called the solar dynamo.

The reason for studying dynamo processes in the Sun is in the first place fundamental curiosity: how does it work? There is, however, certainly relevance to every day life since the Sun's magnetic activity has noticeable effects on Earth. Enhanced magnetic activity causes, among other things, more charged particles to enter the Earth's magnetosphere and ionosphere, where they create electric currents disturbing the Earth's magnetic field. These so-called geomagnetic storms result, for instance, in aurorae above the polar regions, and interference in radio traffic and (satellite) electronics.

A striking example is the geomagnetic storm of 12-14 March 1989 (see Gorney, 1990; Kappenman and Albertson, 1992). This storm was caused by increased particle streams originating from solar flares (eruptions of plasma from the solar surface which are associated with large magnetic field strengths) on 5, 9 and 10 March 1989. Geomagnetic activity remained at an extremely high level for several days; record levels were observed. During this storm aurorae were visible even below 40 degrees latitude, compass needles deviated several degrees, and a wide range of communication and navigation systems was disrupted. The worst disruption took place in eastern Canada, where first the 9500-MW Hydro-Quebec power system was tripped and shut down, which was followed by more power failures, all of which happened within a couple of minutes. In total 21,500 MW was lost, affecting 6 million customers for over 9 hours.

At times when the Sun is magnetically more active the Earth's atmosphere expands, so that lowly orbiting satellites are slowed down faster; ironically, the Solar Maximum Mission (SMM) satellite, built to study the Sun's activity, was a victim of this effect in December 1989. The composition of the upper atmosphere varies as well, because of changes in the ultraviolet radiation from the Sun. Angell (1989), for instance, found a relationship between solar activity and the atmospheric ozone concentration which seems to be significant: at maximum (minimum) activity the ozone concentration is about 2% above (below) average. There are also some indications that the weather on Earth is influenced by solar activity, as suggested by changes in the mean temperature.

Responses of the magnetosphere, the ionosphere and the atmosphere of the Earth to changes in solar activity (such as changes in the Sun's light output, its magnetic configuration, and its output of solar wind) are thus "pervasive and complex," as Gorney (1990) puts it in a comprehensive, clear review on the effects of solar activity on the Earth's environment. But, Gorney adds, "it can be extremely difficult to draw a straight line between cause and effect for individual events or measurements" since the underlying physics of the interactions is ill-understood. Furthermore, the relation between successive cycles of solar activity, in length and strength, is not well known either because only a short period of observations is available. Predictions of geomagnetic storms, for instance, are therefore difficult to give.

This introductory chapter sketches some of the features and models of the solar magnetic cycle and dynamo action in the Sun. There are other processes within the Sun that do or may affect the solar activity cycle -- such as acoustic and gravity oscillations, stellar evolution, meridional flows -- but they fall outside the scope of this text. Dynamo action such as in the Sun takes place in the Earth too, for the Earth's magnetic field may seem steady but it is not: it undergoes reversals of polarity, and its strength and direction are not constant between two reversals either.

Forward to section 2. The solar cycle

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last modified: 26 May 2001