Introduction to recycling

All the long baseline interferometers for the detection of gravitational radiation which are presently being studied are based on the construction of a large, Michelson-like interferometer with an armlength of 1 to 4 km, containing some kind of gravito-optic transducer in each arm. In order to improve the shot-noise limited sensitivity, all these interferometers will use high-power lasers, in conjunction with so-called light recycling techniques. The basic idea of recycling was proposed by R. W. P. Drever (1983): it consists in building a resonant optical cavity which contains the interferometer, so that, if the losses are low and if the cavity is kept on resonance with the incoming monochromatic light, there is a power build-up which results in a reduction of the shot noise. This can be realized in different ways, depending on the geometry of the gravito-optic transducer (delay line or Fabry-Perot).

A general theory of recycling interferometers was recently developed and published (Vinet et al, 1988) and the Garching (Schnupp, 1987) and Orsay (Man, 1987) groups have obtained the first experimental verifications of the efficiency of this technique. In this chapter, we first remind the reader of the main ideas and results of the theory, which is fully expressed in Vinet et al. (1988). We then describe today's experimental achievements, and we end up with a short discussion of possible future improvements.

Introduction

The preceding chapter has discussed the structure and heating of single plasma loops in the solar corona. We turn now to consider systems of loops, exploring the questions of the origin, evolution and global topology of plasma loops and their associated magnetic fields in the corona of the Sun and in the coronae of other stars.

Two major physical processes determine the structure and development of the plasma loops that are so frequently delineated in the solar corona. First, the coronal magnetic field in regions occupied by loops must conform to an elongated or tubular topology compatible with the presence of potential or force-free conditions in the corona (Sections 5.2.2, 5.2.3) and with the existence of spatially isolated sources with complex polarities intermingled in the photosphere. Secondly, the injection, transport and dissipation of matter and energy fluxes in the corona must be localized in and guided by this tubular magnetic topology. The anisotropic character of energy and mass transport coefficients in the presence of a magnetic field helps to explain this behaviour. However, anisotropic transport coefficients by themselves do not explain why only some magnetic tubes are delineated at any time. Spatially localized conversion and concentration of the mass and energy fluxes must also occur and, as we shall see, this aspect of the phenomenon is mediated by processes occurring beneath the visible layers of the Sun.

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.