Having considered two major applications of space plasma activity, it seems appropriate to close this part with a discussion of their differences and similarities. We first address the role of magnetic reconnection and then suggest a general eruption scheme that covers both magnetospheric and solar activity.

The reconnection problem

We begin by considering reconnection in solar activity and then bring in magnetospheric reconnection for comparison.

As we have seen, models of solar activity involve magnetic reconnection in one way or the other. In some models reconnection is involved in the eruption process itself. In addition, reconnection is considered generally as a potential release process for a field configuration with a thin current layer below a fast rising object. A CME-associated flare would be the consequence (e.g., Amari et al., 2000).

Here, and more generally in the context of solar activity, the difficult question arises of what are the quantitative criteria for reconnection to occur. Unfortunately, we can only narrow down the problem, a clear-cut answer is not yet available. We limit the discussion to fast reconnection (Chapter 11).

As magnetic reconnection cannot take place under ideal MHD conditions, the first point to address is whether the necessary nonideal process is collisional or collisionless. Using the values of the middle column of Table 9.1 as an example for coronal plasma conditions, we see that the plasma parameter is of the order of 108, which is a first indication that collisions are extremely rare.

Here we will give a qualitative overview on major activity processes in the solar system. Since our main aim is to concentrate on basic aspects and on theoretical results, a full account of the observational background is outside our present scope. However, in the following sections we will summarize the main observational facts that are relevant for our later discussion. For details the reader is referred to the literature. Note that in the present chapter we will largely refrain from giving physical interpretations. They will be discussed in Part IV using the tools provided in Parts II and III.

Geospace

Magnetospheric activity comprises the major global dynamical phenomena of the Earth's magnetosphere including ionospheric processes. It results from the interaction of the solar wind with the Earth's magnetosphere (Fig. 2.1).

The solar wind is characterized by a fast (supersonic) plasma flow from the Sun into interplanetary space. The magnetosphere is the region above the ionosphere that is dominated by the geomagnetic field. The solar wind compresses the Earth's magnetic field on the day-side and stretches it out to a long tail (magnetotail) on the night-side of the Earth (Fig. 2.1). A bow shock wave stands in front of the magnetosphere, which has a rather thin boundary, the magnetopause. Its thickness, which varies considerably, can become as small as a few hundred km.

In the Solar System the most spectacular manifestations of space plasma activity are the large-scale solar eruptions, such as coronal mass ejections (CMEs), solar flares and prominence eruptions, as briefly described in Section 2.2. In this chapter we attempt to address the underlying physical processes. The approach leaves aside many details, although they would be exciting from a more morphological point of view. Instead, we are interested in the basic physical mechanisms and concentrate on the models and numerical simulations, which provide an excellent frame for our discussion. Naturally, as in the previous chapter on magnetospheric activity, the focus is on loading and release processes.

As we will see, the building blocks, such as ideal dynamics, magnetic reconnection, formation of thin current layers, plasmoid or flux rope formation are relevant elements also in current modelling of solar activity. However, in most solar activity models their role is different from their magnetospheric role. In other words, the building blocks are put together in a different way.

General aspects

Observations strongly suggest that solar eruption processes are of the loading/release type. The energy flux into the corona from below is considerably smaller than the energy flux that would be required if the eruptions were directly driven by the subphotospheric dynamics. In fact, it has been argued that models based on direct driving have been shown to be grossly inconsistent with observations (e.g., Forbes, 2000a).

In this chapter we will address the activity of the Earth's magnetosphere, with emphasis placed on the substorm, which is regarded as the dominant dynamical process of the magnetosphere. What can be expected from our theory-oriented approach? Certainly not deterministic predictions, which are excluded not only by the limitations of the present state of the theory, but also by more fundamental properties such as the chaotic nature of large particle ensembles. Rather, the realistic question is this: Equipped with the tools of Parts II and III, how far do we get? Can the tools help us to make the step from a mere phenomenological picture to a description which allows insights and interpretations in terms of physical processes? Where this goal is not reached, can we at least identify realistic possibilities?

To pursue this line, substantial observational input is required, which means that we abandon the strict theoretical point of view, which was appropriate in Parts II and III to generate a set of tools. (Even there, the selection of problem areas, the choice of parameter regimes or of simplifications were influenced by observations.)

A full discussion of all processes observed to be related to magnetospheric activity is far beyond the present scope. In particular, this applies to the wealth of ionospheric phenomena. The aim is to understand the physics of the large-scale magnetospheric phenomena.

Space plasma phenomena have attracted particular interest since the beginning of the exploration of space about half a century ago. Already a first set of pioneering observations (e.g., Ness, 1969) discovered that matter and electromagnetic fields in space have a complex structure, which was largely unpredicted. Terrestrial and, particularly, spacecraft observations of solar plasmas and fields point in the same direction. In fact, our present picture of the plasma and the electromagnetic fields in space throughout the solar system (and beyond) is that of an extremely complex medium with spatial and temporal variations on large ranges of scales. The wealth of dynamical phenomena observed in space plasmas has steadily increased as more and more refined observational techniques have become available, and it can be expected that important processes still await their detection.

An outstanding class of space plasma phenomena is addressed here under the notion of space plasma activity. Quite generally, in the area of space and astrophysical plasmas the term activity is used for a set of particular magnetospheric, stellar or galactic phenomena, which, although vastly different regarding their space and time scales and their dominant physical processes, have an important characteristic property in common. In all cases they show sudden transitions from relatively quiet states with less pronounced time-dependence to dynamic states in a strongly time-dependent evolution. (Note that this property by no means is restricted to plasma phenomena, volcanic activity being a prominent example from another discipline.)

As outlined in Chapter 1, it is widely understood that an essential aspect of space plasma activity is that quiescent plasma configurations suddenly turn into a state of fast dynamic evolution. Examples for activity-relevant quiescent structures are the magnetotail during the growth phase of a magnetospheric substorm and a preflare configuration in the solar atmosphere.

Fundamental questions arise about the conditions under which such transitions take place. Since they start out from a quiescent state it is important to obtain detailed knowledge about those states, before one can tackle the transitions themselves. The present part is devoted to that task.

Even during quiescence a system often undergoes significant changes due to external driving forces. Planetary magnetospheres are driven by the solar wind, solar chromospheric or coronal structures by subphotospheric convective motions. But any snapshot that one would take during a sufficiently slow evolution would approximately satisfy the equations of a steady state. Thus, quiescence is an asymptotic concept, applying to situations where the time scale of external driving is large compared with any relevant dynamical time scale of the system considered.

We illustrate these aspects more precisely for fluid models. Let the characteristic time of external forces be tc and the largest dynamical time scale be td. Here, td typically is the time it takes for the slowest wave mode to travel across the system.

By plasma model we denote a set of equations governing the temporal evolution of a plasma under a given set of boundary and initial conditions. Ideally, plasma models should be based on first principles. Unfortunately, these are not yet available, at least not from a strict point of view. In any event, simplifications are necessary to keep applications feasible.

For describing space plasma dynamics, it is largely appropriate to ignore quantum effects. The condition for this assumption to be satisfied is that typical values of the action, such as momentum × length or energy × time are much larger than Planck's constant h = 6.63 ċ 10–34 Js. This condition is well satisfied for typical space dynamical processes. Therefore, we will discuss models based on classical elementary or statistical mechanics and electromagnetism. Radiation reaction is ignored.

After the foundations were laid by Isaac Newton, James Clerk Maxwell, Ludwig Boltzmann, Albert Einstein and others, these theories have been extremely successful within their ranges of applicability and form the classical basis of modern technology. Thus, the difficulties that we will be facing, to a large extent, do not lie in uncertainties about the foundations but rather in the complexity of the interactions, which require further simplifying assumptions. Therefore, a number of different plasma models exist, each of which has its characteristic range of applicability. It is the aim of this chapter to summarize the plasma models that are relevant for our purposes.

A major motivation for writing this book is the strong fascination that visible signatures of plasma activity are able to generate. This goes along with considerable professional research interest in this area. Also, those who have admired spectacular pictures or video presentations on the internet displaying spacecraft observations of auroral activity or of solar eruptions, are often motivated to learn more about their physical background.

In the early days of spacecraft observations, the understanding of dynamical phenomena such as geomagnetic storms and solar flares was considered as poor and high up on the list of particularly challenging problems. Remarkably, this is still true today. The observational database has increased dramatically and important new phenomena were discovered, such as coronal mass ejections and manifestations of the global nature of magnetospheric substorms involving large regions of the magnetosphere. There are many more aspects than envisaged originally, and today we have good reasons to use the comprehensive notions of solar and magnetospheric activity, which in this book are combined under the working term space plasma activity. The desire to understand these complex phenomena has mobilized considerable research efforts, but due to the overwhelming complexity that one encounters, our present understanding is still far from being satisfactory.

One might ask, whether in this situation it is appropriate to write a book that concentrates on space plasma activity.