The Physics of Fluids and Plasmas: An Introduction for Astrophysicists

This is the only book with which I am familiar that attempts to cover both fluids and plasmas. It is true that these subjects are closely related, but the logic of attempting to survey both fields, each of which is very broad, in a single volume is not really clear. References between the two subjects are not so frequent and there are excellent readily available texts in fluid mechanics. I would have preferred to see a more extensive coverage of the plasma physics of interest and usefulness to astrophysical students. The book is well written, covering quite a large number of topics in a clear and pleasant style which makes enjoyable reading. It familiarizes the student with a considerable number of interesting and important subjects. The student who reads this book will successfully gain a very good understanding of many, often referred to, astrophysical topics such as accretion discs, dynamos, astrophysical jets and magnetic reconnection. As is clear from the title, the author first treats fluids or neutral gases in this broad survey. He starts from the microscopic point of view, deriving the Boltzmann equation and the H theorem. He then arrives at the ideal and Navier-Stokes fluid equations in a systematic manner by expanding in the small mean free path, a technique also applied in plasma physics. He then discusses the basic properties of fluids that every physics student should know, such as Bernoulli's law, the conservation of circulation, the steepening of waves to form shocks and the jump conditions in a shock. He intersperses his derivations of these relations with helpful illustrations, such as aerodynamic lift and supernova blast waves. He then takes up convection in a compressible medium, and Raleigh-Bernard convection in sufficient detail to give the reader a sound appreciation of these important phenomena. At the end of the fluid section of the book he presents Kolmogoroff turbulence, and in a very useful chapter the physics of fluids in rotating systems. I found this last chapter very helpful, as well as entertaining. The first part of the book is valuable, helping students to find their way through more formal fluid dynamics texts such as those of Landau and Lifshitz, Lamb and even Shu. On the other hand, the second part of the book, on the more complicated field of plasmas, is actually shorter than the first part, 183 pages versus 200 pages devoted to fluids, and is not so well balanced. In this part the author first treats the orbits of individual particles (Chapter 10). He then turns to collections of particles, starting with the rather difficult BBGKY formalism. This is in line with his goal of developing each subject from basic principles. However, only the Vlasov equation emerges from this chapter, and there is no really deep explanation of the more basic kinetic theory being presented. The author then presents the collisionless theory of plasma waves, including Landau damping. He does not go into Landau damping in any great physical detail, contenting himself with describing how one must modify the velocity contour to surround any poles, referring the student to other texts for the true explanation. Only a hazy idea of the significance of this process is given. Chapter 14 concerns magnetohydrodynamics, and again the treatment is sketchy, but some quite interesting examples of magnetohydrodynamics in astrophysics are provided. Chapter 15 takes up magnetic reconnection and Taylor relaxation, while Chapter 16 sketches classical dynamo theory giving the famous Cowling theorem to motivate this theory. Chapter 17 is an epilogue, which touches on a number of topics not covered earlier. In summary, the author is quite successful in bringing many interesting and important plasma astrophysical phenomena to the attention of students, but only in a semiquantitative way. Thus, the more able graduate student cannot really master the subject from this text without also supplementing it with more technical books such as those of Shu, Krall and Trivelpiece and Montgomery and Tidman. In this sense the plasma physics section is more appropriate to an undergraduate course than to a graduate course as the author recommends. Furthermore, there are a number of quite important topics not covered in the book that are essential to any really basic understanding of plasma astrophysics. Examples of such topics are: the fact that thermal conductivity is strongly suppressed across even a weak magnetic field, the interaction caused by plasma instabilities of energetic particles, such as cosmic rays, with the background plasma, the buildup of a small scale magnetic field in turbulence by field line stretching, the Biermann battery mechanism, the Braginski equations for a two fluid plasma with transport coefficients, runaway electrons and the shock acceleration of cosmic rays. It is true there is an attempt to cover some of these topics through the excellent sets of problems for the student at the end of each chapter. However, I am not sure that the student will be able to gain any real understanding in this way, or even be able to solve some of them. In summary, the author has done a really excellent job in qualitatively imparting knowledge of an impressive number of phenomena to the student in an effective and pleasant manner. However, because of the self-imposed constraint to cover two large topics, fluids and plasmas, he has not succeeded in rigorously training the student in the practice of plasma astrophysics.