Ethylene, a-olefin and norbornene copolymerization with the stereorigid catalyst systems iPr[FluCp]ZrC12/MAO and Me2Si[Ind]2ZrCl2/MAO b, SUMMARY:The stereorigid ansa-metallocene catalysts [2,4-cyclopentadien-1-ylidene(isopropylidene)fluoren-9-ylidene]zirconium dichloride/methylaluminoxane (iPr[FIuCp]ZrCl,/MAO) and [(dimethylsilylene)bis(~5-inden-1-ylidene)]zirconium dichloride/methylaluminoxane (Me,Si[Ind],-ZrCl,/MAO) are different in respect to the symmetry and, therefore, to the behaviour of the stereospecifity, in the size of the angle between the planes of the n-ligand systems and in the homopolymerization behaviour of ethylene and propene. The present paper concerns itself with comparing the investigations of the ethylene/a-olefin and the ethylenekycloolefin copolymerization with both catalyst systems. The main kinetic result for both catalysts is that the a-olefins show an accelerating effect on the ethylene polymerization rate during the copolymerization whereas norbornene shows exclusively a rate-decreasing effect. The acceleration effect can be explained by an increase in the concentration of active centres and/or an increase of the rate constant of the ethylene insertion. The analysis of the microstructures of the formed copolymers by the use of statistical models demonstrates that in copolymers with a high content of a-olefins, the experimental trial distributions can only be described satisfactorily with second-order Markovian statistics. That means that in this case the last two monomers have influence on the insertion of the subsequent monomer. A further main result of all investigated copolymerizations is that the r-parameters for the a-olefin insertion are more favourable for the system iPr[FluCp]ZrCl,/ MA0 than for the system Me,Si[Ind]2ZrCI,/MAO; the reason for this is the different size in the coordination gap aperture of the n-ligands. Furthermore, the r-parameters show only a small dependence on the chain length of the a-olefin. Finally, it is remarkable that the stereospecifity (syndiotactic or isotactic a-olefin blocks) remains intact in the copolymers. 0 1993, Hiithig & Wepf Verlag, Base1CCC 0025-1 16X/93/$05.00

Previous work has shown that Fermi acceleration can be an effective heating mechanism during magnetic island coalescence, where electrons may undergo repeated reflections as the magnetic field lines contract. This energization has the potential to account for the power-law distributions of particle energy inferred from observations of solar flares. Here, we develop a generalized framework for the analysis of Fermi acceleration that can incorporate the effects of compressibility and non-uniformity along field lines, which have commonly been neglected in previous treatments of the problem. Applying this framework to the simplified case of the uniform flux tube allows us to find both the power-law scaling of the distribution function and the rate at which the power-law behavior develops. We find that a guide magnetic field of order unity effectively suppresses the development of power-law distributions.

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Soon after the discovery of the geomagnetic tail at the start of the space age (Ness, 1965), it was found that the tail can sometimes move rapidly in a north-south direction (Speiser & Ness, 1967). This flapping motion was deduced from a reversal in the polarity of the Earth-Sun component of geomagnetic tail field, B x , concomitant with a decrease in the total magnetic field strength. The typical duration of this up-down motion is a couple of minutes, with an amplitude of a few R E (Runov et al., 2009; Sergeev et al., 2003; Toichi & Miyazaki, 1976). Tail flapping is now a well-known phenomenon that has been repeatedly reported by spacecraft making observations close to the tail current sheet (CS, or neutral sheet (NS)) in the near-tail region (R ∼−15 to −30 R E). Not well established is what gives rise to it. Over the years both internal (see e.g.,

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