Introduction

As we have seen in the preceding chapter, observations made in cool visible and EUV lines have provided extensive information on the large-scale systems of loops which dominate the structure of the lower corona above active regions. The loops are believed to trace out closed lines of force of the magnetic field which protrude up from beneath the photosphere and expand to fill the whole of the coronal volume above an active region. Hence a picture of the loop systems gives us some insight into the three-dimensional configuration of the magnetic field. But the picture obtained from observations of cool loops is far from complete. For more detail we have to turn to observations of hot loops, filled with material at coronal temperatures of a million degrees or more; these form the subject of the present chapter.

Emission from material at coronal temperatures dominates the EUV and soft X-ray regions of the solar spectrum. The lack of any appreciable photospheric or chromospheric emission at these wavelengths enables the corona to be viewed directly against the disk and, in fact, most of the available information on hot loops has been obtained from such (space) observations. In addition, other important contributions to our knowledge have come from the visible and radio regions of the spectrum. The line and continuum radiation emitted by the corona in the visible region is many orders of magnitude too faint to be detected on the disk against the glare of the underlying photosphere.

Introduction

Solar flares are remarkably diverse and complicated phenomena involving the transient heating of localized regions of the corona and underlying chromosphere within an active region. The sudden release of energy is accompanied by the emission of electromagnetic radiation over a very wide span of the spectrum, ranging – in extreme cases – from γ-rays to kilometric radio waves. In almost all cases flares seen in the chromospheric Hα line also produce an increase in the flux of soft X-rays. Moreover, the variation in the soft X-ray flux with time roughly follows that of the Hα intensity at the brightest point of the flare (Svestka, 1981, p. 74). Both curves are characterized by a rapid rise to a comparatively short-lived maximum, followed by a much slower decay. The intensity of the soft Xray emission generally increases with the optical importance of the flare, although individual flares may show marked deviations from this rule. For this reason it has become customary to assign both an optical (Hα) and an X-ray importance to each flare. Both the Hα and soft X-ray emission pertain to what is conventionally called the ‘thermaly’ or ‘quasi-thermal’ component of the flare, i.e. they originate in plasmas where the distribution of electron velocities is believed to be Maxwellian.

Flares are classified optically according to their area at the time of maximum Hα brightness into the four categories listed in Table 4.1, adapted from Svestka (1981).

The discovery that a significant part of the energy emission from the solar corona is concentrated along well-defined curved paths – called loops – represents a major advance in our understanding of the Sun. Such plasma loops are the basic structural elements of the corona, particularly in and over active regions. Moreover, they play a decisive role in the origin and physics of solar flares. Our new insight is due largely to the wealth of space observations of the Sun obtained, in particular, from the manned satellite Skylab (1973–4) and the unmanned satellites Solar Maximum Mission and Hinotori which followed. Ground-based observations in the visible and microwave regions of the electromagnetic spectrum have also played a vital role. The literature on coronal plasma loops is vast and includes not only hundreds of research papers but also the proceedings of numerous symposia and workshops. This book presents for the first time a comprehensive, unified and well-illustrated account of the properties of coronal loops based on the best space and ground-based observations currently available (Chapters 2–4). A magnetohydrodynamic analysis of the stability and dynamics of loops is presented in Chapter 5, while the final chapter (Chapter 6) explores the wider implications of the loop regime on our understanding of both the solar corona and stellar coronae.

Introduction

In the preceding chapters we have summarized the observed properties of coronal loop structures; we now turn to the interpretation of these properties and provide an account of the physics of coronal loops. In this chapter we shall describe the models of individual loops and in the following chapter we shall consider systems of loops in the context of global models of the solar corona and of stellar coronae in general.

As a preliminary, though, we need to define more precisely than before what is meant by a coronal loop. So far we have employed the intuitive definition of a plasma loop as a continuous structure traceable from a point near the photospheric surface along an arc in the corona to another point at which it returns to the surface. Generally the limited spatial resolution offered by the observations precludes us from saying whether a loop is simple, with a more or less uniform curvature, or whether it possesses a more complicated topology with knots or braids. We have also adopted the view that the visible structures of the corona – the plasma loops – act as tracers of an underlying magnetic field structure, although we have seen that there is little direct evidence of the connection. Thus the first question which we must address in this chapter is the relation between the observed loops and magnetic loops. At this point, the meaning of the term loop becomes a matter of some subtlety.

Eclipse observations of prominences: Middle Ages to 1868

Historically, visual observations of prominences were the first to reveal the existence of well-defined loop structures arching upwards from the surface of the Sun high into the overlying corona. Regular visual observations of prominences obtained during total eclipses of the Sun date from 1842, but sporadic reports go back to the Middle Ages. Most of the early descriptions of the eclipses refer only to the corona but there are specific references to prominences in medieval Russian chronicles. For example, in the first volume of his well-known book Le Soleil (1875, p. 330) the Italian astronomer Father Angelo Secchi (1818–78) cited the description of a prominence observed at the eclipse of 1239. According to Secchi, this was the most ancient eclipse for which a detailed description of the corona was then extant. Early eclipses were extensively described by the historian R. Grant in his History of Physical Astronomy (1852). Grant concluded, pessimistically but realistically, that ‘Down to the beginning of the eighteenth century, the accounts respecting total eclipses of the sun, contain very few remarks which are of advantage in forming the basis of any physical enquiry’.

By contrast, the arrival of the eighteenth century saw a growth in the spirit of exact enquiry in many branches of science.

Introduction

Coronal loops are a phenomenon of active regions (Chapter 1) and there is growing evidence that they are in fact the dominant structures in the higher levels (inner corona) of the Sun's atmosphere. Our knowledge of loops has greatly expanded in recent years as a result of space observations in the far ultraviolet and X-ray regions of the spectrum. However, the success of the space work should not be allowed to obscure the fact that a considerable amount of quantitative information on the morphological, dynamical, and physical properties of coronal loops has been derived from ground-based observations in the visible and near-visible regions. In fact, observations at these wavelengths have achieved significantly higher spatial resolution (better than 1″ of arc) than almost all of the space observations so far obtained. Our aim in this and the following chapter is to bring together all the available data and thus present an integrated and consistent picture of the properties of non-flare coronal loops.

Observations show that coronal loops, depending on their temperature, can be divided into two distinct categories. The properties of the two types differ radically. Loops formed at temperatures in excess of ∼ 1 × 106 K are conventionally referred to as ‘hot’ loops, while those formed at lower temperatures are termed ‘cool’ loops. It is convenient to deal with the two types separately, cool loops in the present chapter and hot loops in Chapter 3.

Introduction

The actual initiation of jets is a subject that remains extremely difficult to discuss in a detailed and convincing manner despite the obvious importance of this fundamental topic. There are two main reasons for this. The first is a lack of unequivocal observations. Although VLBI measurements have provided structural information on scales corresponding to ≲ 0.1 pc in extragalactic sources, the phenomena that govern the beginnings of jets almost certainly occur on scales at least two to three orders of magnitude smaller. Other observations, especially of X-ray and optical variability, are indubitably important and provide useful constraints on models; however, they do not yield information that is clearly interpretable in a model-independent fashion.

The second generic difficulty has to do with the certainty that the physical processes involved in producing jets are extraordinarily complex. The core of the picture, that accretion onto a super-massive black hole (SMBH) of somewhere between 106 and 1010M≲ is at the heart of beam generation as well as the other properties of active galactic nuclei (AGN), has been commonly accepted for about a decade. However, in attempting to add details to this picture, astrophysicists find that general relativity, hydrodynamics, plasma physics and radiation transport all form thick blobs on their palettes, and the portraits which emerge from combining them in different proportions are, not surprisingly, rather different.

Introduction

The attractive features of the ‘beam’ model for galactic and extragalactic radio sources are predicated on the contention that astrophysical processes permit the formation of high-power collimated flows, and that such flows survive their passage from the generating engine to the outer parts of the source without losing most of their energy. In other words, astrophysical plasma beams must be capable of exceptional stability, although there are sources for which less stability is necessary. This may be seen in the ‘P-D’ diagram of Baldwin (1982) (cf. Chapter 2) – for a given radio power (say 1027 W Hz −1 sr −1), radio sources spanning a wide range of linear sizes (about 1 to 1000 kpc) are found, and hence the beams driving these sources must be stable over distances exceeding 1 Mpc in the largest objects, but need be stable for only 1 kpc in the smallest. The purpose of this chapter is to discuss the physical mechanisms that are effective in stabilising and destabilising beam flows, and to calculate the stability properties of some beams that might be components of extragalactic radio sources. Much of the physics discussed here can be applied to Galactic jets (Chapter 10), with some modification for the difference in physical parameters.

Introduction

Our interferometric images of radio sources reflect the synchrotron emissivity arising from their relativistic electrons and magnetic fields. These trace the underlying plasma flow, albeit imperfectly. The local dynamical evolution of the particles and fields is determined by their transport from the nuclear source, and by their in situ dynamics. This chapter presents the physics necessary for an understanding of current theories of particle acceleration and magnetic field evolution. It describes these theories and attempts to assess whether or not they provide an adequate account of the inferred particle spectra, energetics and magnetic field geometry of extragalactic jets.

It was shown in Chapter 3 that some sources have severe lifetime problems, in that the time for the electrons to be carried out to the lobes (even with a jet speed ~ c) is longer than their radiation lifetime (the upper limit of which is the lifetime to Compton losses on the 3 K background) and that the surface brightness and spectral index distributions do not decay as fast as would be expected in a constant velocity, expanding flow. These problems may be overcome by the local reacceleration of the radiating particles. Further, simple estimates of convection of flux-frozen magnetic field out from the core predict that the convected field decays significantly; however, this is probably offset by in situ amplification of the magnetic field by some dynamo process.

Introduction

The interpretation of the diverse forms of observed radio source structure has always been problematical since this normally involves the use of some form of classification scheme. With the benefit of hindsight, it is clear that this exercise has not always proved to be a total success. Every astronomical object is the product of a unique set of physical circumstances which must, at some level, ultimately preclude the imposition of a generalised classification scheme covering many objects. It remains, however, a necessary basic stage in the process of scientific investigation. Any classification scheme is based upon gross structural features derived from observation. Such observations are of an inhomogeneous set of objects and are limited by sensitivity and imaging techniques. Schemes are therefore subject to strong selection effects and their subdivisions are arbitrary. A scheme can, however, prove useful provided the subdivisions broadly map out differing segments in the parameter space of the physical conditions of radio sources. The problem is, of course, that it is those very same physical conditions that are as yet unknown and that one is attempting to investigate. Thus, any current classification scheme is dominated by the characteristics of the telescopes available to observers at the time, and incorporates the ‘conventional wisdom’ derived from the interpretation of previous work. Such circumstances are profoundly inelegant but probably unavoidable.