Plasma astrophysics at Santa Barbara

Nature Vol. 299 14 October 1982 579 than the full-length dynorphin,_,7 — thus the potency of the octapeptide was markedly increased when a cocktail of peptidase inhibitors was added to the assay system. This is in keeping with a putative transmitter role for dynorphin,_8, whereas the longer peptide, dynorphinm, may be more important in a hormonal function, perhaps after secretion from the pituitary. The companion peptide, a-neoendorphin, is pharmacologically similar to dynorphin, 4,. Whether these two peptides represent a distinct new family of endogenous ligands for the 2: sites in brain and spinal cord remains to be established, but this is cer- tainly an intriguing possibility. The function of these sites remains obscure, but x-selective drugs, for example ethylke- tocyclazocine and bremazocine, are known to be effective analgesics and do not sub- stitute for morphine in preventing with- drawal symptoms in morphine-dependent animals” (they seem to have only low addiction liability). Drugs with such a profile clearly have therapeutic potential, and a better understanding of the x-receptors and their endogenous ligands is urgently needed . 1:1 . Hughes, J. et al. Nature 258, 577 (1975). l 2. Rossier, 1. Nature 298, 221 (1982). 3. Weber. F... Evans, C..l. & Barchas, .l.D. Nature 299, 770982). 4. Goldstein. A. el al. Proc. natn. Acad. Sci. U.S.A. 76, 66660979). S. Goldstein, A. et al. Proc. naln. Acad. Sci. U.S.A. 78, 72l9(l98l). 6. Kangawa, K. etal. Biochem. biophys. Res. Commun. 99. 871 (1981). 7. Cavkin, C., James, l.F. & Goldstein, A. Science 215. 4l3(l982). 8. Corbett, A.D. el al. Nature 299, 79 (1982). 9. Minamino, N. el al. Bioehern. biophys. Res. Commun. 95. 14750980). 10. Seizinger, 8., H0111, V. & Herz, A. Biochem. biophys. Res. Commun. 102. 197 (1981). Plasma astrophysics at Santa Barbara from R. Rosnerq&#39;, E. Zweibell and V. Trimble* AT the workshop on ‘Space and Astro- physical Plasmas’ held this summer at the Institute for Theoretical Physics at the University of California, Santa Barbara, three topics held sway: hydromagnetic shocks and particle acceleration, the interaction of hot and cold plasmas, and hydromagnetic flows. A diverse group of theorists from the laboratory plasma, space plasma and astrophysics com- munities took part. The possibility that particles can be accelerated to high energies in the vicinity of collisionless shocks — yielding solar and galactic cosmic rays and relativistic particles in extragalactic radio sources - has received wide attention in recent years. The outstanding problems include the structure of collisionless shocks (the mechanisms which produce a shock transition in collisionless plasmas), the ‘injection problem’ (how thermal particles can be extracted from the background plasma and accelerated to high energy), the shock efficiency and — most difficult — the interaction between the accelerating particle population and the processes determining shock structure and acceleration efficiency. These difficult issues can be at least partially addressed by observations of the terrestrial bowshock, produced by the interaction between the solar wind and the terrestrial magnetosphere. E. Greenstadt (TRW) showed that while the shock layer is quite thin when the interplanetary magnetic field is nearly normal to the direction of motion of the shock (quasi- perpendicular case), it is highly irregular and quite extended in space in the quasi- parallel case (magnetic field nearly parallel to the direction of motion of the shock). A fraction of the incoming solar wind particles is reflected back upstream, and these particles excite low-frequency electromagnetic waves which exert a retarding force on the solar wind. A diffuse energetic ion component is also observed. The terrestrial bowshock (and inter- planetary shocks) will evidently provide important ‘laboratory’ constraints on theoretical studies of shock structure and particle acceleration. The problems of internal shock structure, including the details of ion dynamics, were addressed by a number of theorists by powerful numerical simu- lations of collisionless shocks. The simulations show reflected ions and short- wavelength electromagnetic turbulence (D. Forslund, Los Alamos), in reasonable agreement with observations, with ions bent back upstream by the enhanced magnetic field in a postshock magnetic overshoot layer (C. C. Goodrich, University of Maryland). However, the need of an electrostatic potential layer to slow down electrons (in quasiparallel shocks) remains unresolved by the present simulations and observations. The basic model of shock acceleration, the starting point for discussions at the workshop, is a steady-state model in which ‘Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138. *University of Colorado, Boulder, Colorado 80309. *University of California, Irvine, California 92717. cosmic rays diffuse in the vicinity of a high Mach number quasi-parallel shock which is approximated to by a discontinuity in flow speed. Observations of cosmic-ray secondaries (E ~ 1 to 100 GeV) constrain the extent to which cosmic rays can be accelerated to high energies by successive passage through interstellar shocks because the volume in which acceleration occurs does not exceed 10 per cent of the confinement volume (W. I. Axford, Max- Planck Institut). The cosmic rays are scattered by magnetic irregularities, which are assumed to be nearly stationary in the frame of the flow; thus, because of the jump in flow speed across the shock, particles are accelerated by a first-order Fermi process as they scatter back and forth across the shock front, with the well known result that the energetic particle spectrum is quite similar to that of galactic cosmic rays. Some major issues remain unresolved, however. Thus the efficiency of the shock (the fraction of energy which goes into relativistic particles) is not determined, and the properties of the hypothetical scattering centres, which determine the cosmic ray diffusion coefficient, must be specified ad hoc. If the scattering mean- free path is not large compared with the width of the shock transition, the detailed ﬂuid velocity profile must determine the acceleration. But the flow itself is modified by the coupling between the cosmic ray momentum flux and pressure and the thermal gas, the modification being highly sensitive to the details of the coupling (D. Eichler, University of Maryland; I-l. Volk, Max-Planck Institut/ University of Chicago). There is a very different problem in the transfer of energy between hot and cold plasmas. Thus the structure of the transition layer between the solar chromosphere and corona is crucial to an understanding of the energetics of these plasmas, but the construction of valid model atmospheres is complicated by the high degree of spatial inhomogeneity (including the probable existence of unresolved features), flows in the boundary layer region and the general absence of steady-state conditions (R. Rosner, Harvard University). Even if these difficulties were not present, calculations show that the electron distribution function in the transition region (B. Shoub, Stanford University) is significantly non- Maxwellian. The distribution at a given height is ‘contaminated’ by superthermal particles representative of the temperature of overlying layers, possibly significantly altering the rates of collisional ionization and excitation (and the heat flux). Thus, the diagnostic problem remains a major difficulty in modelling solar thermal boundary layers. Different problems arise in heat transport in laser-irradiated plasmas (C. Max, Lawrence Livermore). Experiments show the heat flux often to be much less than predicted by classical transport 0028-0836/82/4l0579-02501.00 ©1982 Macmillan Joumalx l Id