This book illustrates new developments in the fields of space and solar physics, stellar physics, extragalactic physics and cosmology. It also elaborates upon the progress of laboratory plasma physics. One of the topics discussed is the existence of collective processes, both linear and non-linear, that can explain key elements of accretion physics, magnetic reconnection, the formation of ‘strange’ particle distributions, particle scattering phenomena, etc. Astrophysical plasma are dominated by turbulent or quasi-turbulent processes which interactively associate instabilities, radiation processes and plasma-wave scattering. The resulting scenario, which is outside thermodynamics and conventional statistical physics, is too difficult to describe theoretically, but today there are large-scale experiments and powerful computational tools allowing for the exploration of an almost similarly complex variety of phenomena. Several contributions to this book present indications of the influence of nonlinear phenomena in astrophysical applications. This work marks the fast growth of plasma astrophysics thanks to new observations in the high energy band of the spectrum on the one hand and the possibility of validating and bringing to light relevant new theories by increasingly sophisticated machines on the other.
The 1999 Varenna Course on “Plasma Astrophysics”, took place 33 years after the first one with the same title. A part of the subjects discussed and the titles of the lectures were similar but the contents were much different. It is maybe worth noting that two persons participated in both the first and last courses held in Varenna: R. Kulsrud as a lecturer, and A. Ferrari, as a student in 1966. Meanwhile the subjects represented in the course and the series of meetings that followed it, both at Varenna and elsewhere around the world, have grown and expanded to make plasma astrophysics one of the new areas in which the most unexpected and attractive developments have occurred.
In the 1966 course lecturers were a collection of astronomers and plasma physicists with largely different backgrounds, expertise, and a somewhat limited interest in interacting. In 1999 great progress towards the integration of expertise in the two fields could be observed: a common language had been found. Of course, the two communities, astrophysicists and plasma physicists, still have divergences of opinions regarding the way to approach problems, but by now both have a general knowledge of the terminologies and of techniques used in the two fields. This is well illustrated in the program and lectures of the course.
In 1966 the astrophysicists built their models of celestial objects basically in terms of single particle motions or pure hydrodynamics; magnetic fields very rarely entered into the picture, and when they did it was mostly to interpret non-thermal (synchrotron) emission processes. Laboratory plasma physics had instead already reached a rather evolved stage in the understanding of electrodynamic processes, magnetohydrodynamics, microscopic theories, etc., displaying an incredible richness of phenomena (instabilities, formation of non-thermal distributions, etc.) that could be applicable in interpreting “strange” astrophysical observations. Of course, many of these phenomena require a detailed knowledge of the distribution functions of plasmas, which is not typically available in astrophysics. In fact, one of the points of discussion was: is it possible to adapt the interpretation of laboratory plasma phenomena to astrophysical situations where important ingredients for these interpretations are unobservables?
Soon, however, plasma phenomena were accepted by everybody as the obvious way to interpret such processes as particle acceleration, non-thermal radiation, stellar magnetospheres, stellar activity, etc. One should mention that the discovery of pulsars also marked, in this particular issue, a passage point: it became clear that, in an astrophysical universe dominated by gravitational forces, electrodynamic processes do play a fundamental role in defining the appearances of sources, particularly in the high-energy part of the distribution function of particles and photons, and in phenomena related to MHD and micro-instabilities.
However, it should be observed that, in the same period, space plasma probes allowed to test, in interplanetary space and in planetary magnetospheres, the existence of complex particle distribution functions associated with magnetic structures, driving instability phenomena and non-thermal processes. The extension of these results to other astrophysical settings could not be immediate, but was certainly a step forward for the adoption of plasma concepts and techniques in astrophysics.
Another spectacularly relevant surprise was the realization derived from X-ray astronomy that most of the “visible” mass in the largest identifiable objects in the universe, galaxy clusters, is in the plasma state with temperatures in the same range as that of advanced laboratory experiments on magnetically confined plasmas.
The results of these developments were amply illustrated in the lectures for the 1999 course, which included space and solar physics (Smith, Tsurutani, Noci, Einaudi), stellar physics (Rosner, Lamb, Trümper, Costa), extragalactic physics, and cosmology (Baum, Ferrari, Sunyaev, Mushotzky).
Still, progress in laboratory plasma physics has continued at a fast pace. The main novelty has been emphasized by B. Coppi in his introductory lecture, in particular the existence of collective processes, both linear and nonlinear, that can explain key elements of accretion physics, magnetic reconnection, the formation of “strange” particle distributions, particle scattering phenomena, etc.
Astrophysical plasmas are dominated by turbulent or quasi-turbulent processes which interactively associate instabilities, radiation processes, plasma-wave scattering, etc. The resulting scenario, which is outside thermodynamics and conventional statistical physics, is too difficult to describe theoretically, but today we have large-scale experiments and powerful computational tools allowing for the exploration of an almost similarly complex variety of phenomena. Several lectures have already presented indications of the influence of nonlinear phenomena in astrophysical applications (Benford, Melrose, Einaudi, Ferrari), and this is one of the main lines in the future of plasma astrophysics.
We regret that as a result of the pressing commitments to ongoing research activities, several lecturers could not deliver the text of their presentations in time for these proceedings. In any case, no written account could adequately recreate the inquisitive and exciting atmosphere which characterized every day of the meeting. We think that Varenna will continue to mark, through the upcoming new courses, the fast growth of plasma astrophysics thanks to wondrous new observations in the high-energy band of the spectrum on the one hand and the possibility of validating and bringing to light relevant new theories by increasingly sophisticated machines on the other.
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