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Scientific paper
Aug 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999nucfu..39.1071m&link_type=abstract
Nuclear Fusion, Volume 39, Issue 8, pp. 1071-1073 (1999).
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1
Scientific paper
Duncan Bryant is a retired space plasma physicist who spent most of his career at the Rutherford-Appleton Laboratory in Oxfordshire, England. For many years he has been challenging a widely accepted theory, that auroral electrons are accelerated by double layers, on the grounds that it contains a fundamental error (allegedly, an implicit assumption that charged particles can gain energy from conservative fields). It is, of course, right that models of particle acceleration in natural plasmas should be scrutinized carefully in terms of their consistency with basic physical principles, and I believe that Dr Bryant has performed a valuable service by highlighting this issue. He maintains that auroral electron acceleration by double layers is fundamentally untenable, and that acceleration takes place instead via resonant interactions with lower hybrid waves. In successive chapters, he asserts that essentially the same process can account for electron acceleration observed at the Earth's bow shock, in the neighbourhood of an `artificial comet' produced as part of the Active Magnetospheric Particle Explorers (AMPTE) space mission in 1984/85, in the solar wind, at the Earth's magnetopause, and in the Earth's magneto- sphere. The evidence for this is not always convincing: waves with frequencies of the order of the lower hybrid resonance are often observed in these plasma environments, but in general it is difficult to identify clearly which wave mode is being observed (whistlers, for example, have frequencies in approximately the same range as lower hybrid waves). Moreover, it is not at all clear that the waves which are observed, even if they were of the appropriate type, would have sufficient intensity to accelerate electrons to the extent observed. The author makes a persuasive case, however, that acceleration in the aurora, and in other plasma environments accessible to in situ measurements, involves some form of wave turbulence.
In Chapter 2 it is pointed out that the Debye number (the number of particles in a sphere of radius equal to the Debye length) is actually rather higher in the solar wind and the Earth's magnetosphere than it is in any laboratory plasma: in this sense space plasmas are more `ideal' than laboratory ones. Changes in magnetic field topology occur in both the magnetosphere and tokamaks, but in the former case the term `magnetic reconnection' tends to be used only in a steady state context: temporary or sporadic changes in field topology at the magneto- sphere/magnetosheath boundary, for example, are described instead as `flux transfer events'. Reconnection in tokamaks, on the other hand, is generally regarded as an intrinsically time dependent process. Such subtle distinctions in terminology should be borne in mind by any fusion researchers reading this book.
Dr Bryant's writing style is informal and often entertaining. A good example of this, from Chapter 3, is the following: ``Auroral arcs can be bright enough to use as a reading lamp, although it would be something of a waste to use it as such, since the aurora is vastly more interesting than any document (even this one).'' Chapter 3, indeed, is the best part of the book, covering as it does the author's principal area of expertise, namely the aurora. The author gives a very clear account of auroral phenomenology, in particular observations of auroral electrons, before considering the merits of rival acceleration mechanisms.
The approach is largely non-mathematical, with few equations: those that do appear are not numbered (it would have been better if they had been). It has to be said that the author is not always rigorous or consistent. For example, acceleration a is first defined `in its most general sense' to be rate of change of speed, rather than velocity: thus, according to this definition, a = 0 in a static magnetic field. A few pages later, the same symbol is used to denote the modulus of the rate of change of velocity: this, of course, is finite in a static magnetic field. Such elementary distinctions matter, because in order to address the issue of whether or not electrons are `accelerated' in static or quasi-static fields, one must first define unambiguously what `acceleration' means. It is stated in Chapter 2 that the Larmor radius of a particle is proportional to its magnetic rigidity divided by the magnetic field component normal to the particle trajectory. This, of course, is incorrect: it is the particle's momentum component normal to the field which defines the Larmor radius. The book contains a number of statements which are either misleading or demonstrably incorrect. For example, at the end of Chapter 3, and again at the end of Chapter 4, neutral beam injection (NBI) in tokamaks is invoked as a precedent for lower hybrid wave excitation by cross-field drifts. Although it is true that lower hybrid waves can couple to energetic ions in a tokamak, and could in principle be amplified by fusion alpha particles [see N.J. Fisch, J.-M. Rax, Phys. Rev. Lett. 69 (1992) 612], NBI has not, to the best of my knowledge, been used as a source of such waves. In Chapter 9, referring to solar flares, the author states that ``characteristic products of the accelerated electrons are X rays generated by synchrotron radiation in the remaining magnetic fields''. In fact, flare accelerated electrons produce X rays via bremsstrahlung, the magnetic field and particle energies being such that synchrotron radiation occurs at microwave frequencies instead (the more general term `gyrosynchrotron radiation' tends to be used by solar flare researchers, in recognition of the fact that the electrons producing the bulk of the emission are only mildly relativistic). Indeed, bremsstrahlung X rays and gyrosynchrotron microwaves provide important sources of information on the distribution function of flare accelerated electrons, but the author makes only a brief mention of such observations, preferring to concentrate on direct measurements of flare accelerated electrons at the Earth's orbit, despite acknowledging that uncertainties in propagation effects make it very difficult to reconstruct conditions at the Sun from such measurements.
Similar remarks apply to the final chapter, on acceleration of cosmic ray electrons. Again, attention is focused almost exclusively on measurements of particles rather than the radiation signature of those particles, in this case synchrotron radiation by ultrarelativistic electrons. No mention is made of radio and X ray data, indicating that electrons with energies of up to around 1014eV are being accelerated at shocks associated with shell type supernova remnants. Interestingly, resonant acceleration of electrons by lower hybrid waves has been invoked by A.A. Galeev [Sov. Phys.-JETP 59 (1984) 965] as a mechanism for the production of cosmic ray electrons: although Galeev's paper is not cited in this book, the process he describes is very similar to that proposed by Dr Bryant for electron acceleration in the aurora and other near Earth plasma environments.
The book contains a number of physics errors. For example, on page 17 the time derivative of a magnetic field is equated to an induced electric field, rather than the curl of one. On page 21, the author invokes Larmor's formula for the power radiated by a non-relativistic charged particle, and then combines it with the relativistic relation between acceleration and energy to estimate the maximum acceleration rate. The book has also been badly proofread. For example, Figure 1.15 appears twice: where it is first used, on page 8, it is clear that the accompanying caption and text refer to a different figure. I found several errors in the reference list (one of my own publications is cited as two separate papers, with both citations containing inaccuracies). Having said that, the reference list is impressively comprehensive and eclectic. It includes, for example, Swift's `Gulliver's Travels': a spacecraft in the magnetosphere is compared to Gulliver in Brobdingnag, the magnetosphere being, in some respects, a vastly scaled-up version of a laborator
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