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.

Introduction

Flat spectrum nuclei are found at the centre of many types of extragalactic radio source, including the powerful classical doubles, and the ‘isolated’ compact sources (that have comparatively weak extended structure). They turn out (when examined with sufficient resolution) to be the self-absorbed bases of jets that feed the extended structure, whose continuations are often seen on much larger scales. The systematic properties of VLBI jets have been reviewed frequently and can be summarised thus:

1. The jets are nearly always seen on only one side of the nucleus. ‘Counterjets’ have only been seen in a few sources (for example 3C 236, Schilizzi et al. 1988).

2. The jets contain bright ‘knots’ of emission that are often seen to move outwards, away from the base at speeds commonly in the region of 5 to 10 times the speed of light (Ho = 100 km s−1 Mpc−1). Although some apparently stationary knots have been observed, for example the outer component in 4C 39.25 (Shaffer & Marscher 1988), none has ever been observed to move inwards (Marcaide et al. 1985).

3. The speeds and trajectories of the individual knots do not in general vary greatly as the knots move out; it is difficult to place very severe limits on acceleration because of the limited accuracy of positional measurements and the relatively short distances over which the knots are observed to move. Significant changes in speed and direction have been detected in 3C 345 (Biretta et al 1986).

4. […]

Gross morphology

A crucial morphological feature of extended extragalactic radio sources is that (see Chapter 2) there are actually two fundamentally distinct classes of object, in which the weaker, Fanaroff & Riley (FR) class I, sources are characterized by quasi-continuous luminous jets which often become distorted as they interact with the inter-galactic medium (Fig. 2.3), whereas the more powerful, FR II, sources have a simple, linear, double-lobed structure, with the brightest emission occurring in compact hotspots at the edge of each lobe (Fig. 2.8). A central challenge for any theoretical model of extragalactic radio source structure is therefore to reproduce this observed dichotomy, and to identify the factor, or factors, that determine which type of extended structure develops. Furthermore, we might hope that an improved theoretical understanding of the Fanar off & Riley classification would enable us to make improved, dynamical, estimates for the, observationally badly determined, physical parameters of jets in radio sources.

Although a complete model for radio source structure would probably have to involve variations in both the strength and direction of the central engine, and a non-uniform external medium, considerable insight into their gross morphology can be gained from axisymmetric simulations of steady jets in a constant ambient medium. This model, which we shall refer to as the basic model, has the additional virtue of being less computationally expensive than fully three-dimensional simulations, permitting a wider range of jet parameters to be investigated.

Introduction

What are extragalactic radio sources?

We believe that extragalactic radio sources are interactions between large-scale jets and the hot, diffuse gas that surrounds elliptical galaxies. The interactions not only cause the hotspots, bridges, tails etc., but also the radio emission from the jets themselves. Radio sources should be thought of as processes rather than objects: the overall radio structure must change substantially on the shortest possible dynamical time scale, the sound-crossing time. Another way to put this is that, at least in the FR II sources, there is no steady-state description. This is the main reason why radio sources are much harder to understand than, say, main-sequence stars. On the other hand, the lack of equilibrium means that the structure of radio sources reflects their past history, so that in principle it should be much easier to deduce the life-cycle of radio sources than that of stars. At present, the most successful theoretical models concentrate on regions for which a local steady-state description is likely to be appropriate, notably the bases of jets, far from any end-effects, and in the co-moving frame of the hotspots, where the jets terminate.

It is worth emphasizing that in the standard model of FR II sources, and to a lesser extent FR Is, we only see half the story in the radio. As we shall see, it is usually assumed that synchrotron radiation is only emitted by plasma which has entered the system via the jet.

Introduction

Whilst the luminous jets of radio galaxies and quasars are the most powerful examples of collimated outflow in the cosmos, there are many examples of jets and outflows to be found much closer to home, within our own Galaxy. These span a great range of luminosities and collimation factors, from the optically visible jets and “lobes” associated with low-mass young stellar objects, which are morphologically very similar to the classical radio galaxies, to the poorly collimated and much less clearly denned “jets” associated with the Galactic Centre and with various supernova remnants. Galactic jet sources also include the singular object SS 433, which is known to be emitting a two-sided jet at a quarter of the speed of light. This jet is known to be associated with a binary star system, and there is some evidence that other mass-transfer binaries may also have jets.

In many cases the jet material itself is insufficiently excited to dissociate it completely, giving us a variety of spectral lines at optical, infrared and radio frequencies with which to probe the underlying kinematics, while the mere fact that these objects are close gives us greatly enhanced linear resolution. If there is a lesson to be learnt from the wide variety of systems which exhibit collimated mass-loss it is that jets are very easily formed once symmetry is broken through rotation.

Introduction

In Chapters 1 through 4, we saw that the outer radio lobes associated with active galaxies receive their energy from a bulk hydrodynamic flow which emanates from the galactic nucleus. Bisymmetric outflow occurs on a wide range of scales in less energetic objects as well, as will be shown in Chapter 10.

Perhaps the most astonishing feature of cosmic jets is their ability to stay together over a very large range of distance scales. On their way from the black hole in the galactic nucleus to a radio lobe, cosmic jets cover a stupendous factor 109 in length scale. That is as if, exhaling forcefully at my desk in Leiden, I could blow about the papers on the desk of a colleague in Minneapolis. This should be a caution against off-the-cuff comparison between jets in radio galaxies and such comparatively easily understood items as laboratory jets, rocket exhausts, and numerical simulations.

The most natural explanation of the coherence of jets is that they are not jets at all, but gaseous cannonballs with a density that is much higher than that of their surroundings. What we perceive as jets would be a mixture of gas ablated from these “tracer bullets” and surrounding gas set aglow by their passage.

General introduction

History

In a description of an optical image of M 87 (NGC 4486), Curtis (1918) wrote “a curious straight ray … connected with the Nucleus”. By the 1950s the term ‘jet’ was being used to describe this feature which it seemed plausible to associate with ejection of material from the innermost region of the galaxy (Baade & Minkowski 1954), although the concept of a continuous flow was not then envisaged. Baade (1956) measured the optical polarization of the M 87 jet, supporting the idea that the material was a synchrotron emitting plasma akin to that of the Crab supernova remnant.

Shklovskii (1963), in an attempt to explain the double radio sources and M 87's jet, discussed many ideas that play a role in current theories – accretion of matter in the gravitational potential of a galactic nucleus; the consequent heating of a plasma that breaks out along a preferred axis; the flow of this material into intergalactic space and the re-energization of the electrons within the flow. However, the model still did not encompass the idea of a continuous flow, carrying energy in the form of bulk motion. Schmidt (1963) wrote of “a wisp or jet” on the image of the optical counterpart to 3C 273, and by about this time, the term ‘jet’ was in common usage (e.g., Greenstein & Schmidt 1965; Burbidge, Burbidge & Sandage 1965) – but still without a clear recognition that a continuous flow of matter and energy was involved.