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The inherent statistical heterogeneities associated with chain length, composition, and architecture of synthetic block copolymers compromise the quantitative interpretation of their self-assembly process. This study scrutinizes the contribution of molecular architecture on phase behaviors using discrete ABA triblock copolymers with precise chemical structure and uniform chain length. A group of discrete triblock copolymers with varying composition and symmetry were modularly synthesized through a combination of iterative growth methods and efficient coupling reactions. The symmetric ABA triblock copolymers self-assemble into long-range ordered structures with expanded domain spacings and enhanced phase stability, compared with the diblock counterparts snipped at the middle point. By tuning the relative chain length of two end blocks, the molecular asymmetry reduces the packing frustration, and thus increases the order-to-disorder transition temperature and enlarges the domain sizes. This study would serve as a quantitative model system to correlate the experimental observations with the theoretical assessments and to provide quantitative understandings for the relationship between molecular architecture and self-assembly.

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We present the temporal evolution of magnetic field topology (FT) and its 'bifurcation', i.e. 'magnetic reconnection' taking place in the magnetotail with a southward interplanetary magnetic field. According to the FT theory, the magnetic FT is uniquely determined by the signs of real part of Jacobian matrix eigenvalues of the critical points (CPs) i.e. magnetic null points, if their Jacobian matrix of the CPs is not degenerated. This is because FT can change only at CP locations. The signs of the real part of eigenvalues characterize the magnetic field patterns around CPs. At the CP locations, the magnetic field becomes zero. The CPs can be classified into different types of 'saddles points'. In the saddles points, the eigenvectors span characteristic both one-and twodimensional manifolds, according to their signs. These manifolds and CPs form FT 'skeletons' and determine essentially the global FT. These FT 'skeletons' that include the CPs, characteristic curves and surfaces which are spanned by the eigenvectors of their CPs provide a clear view of the three-dimensional FT. The change in the skeleton, i.e. the change in the FT, is the so-called 'bifurcation' and allows us to show the occurrence of the magnetic reconnections.

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ELF oscillations (f < 500 Hz) were observed during the electron beam emissions of the space experiments with particle accelerators (SEPAC) flown on the Spacelab I shuttle mission. The beams had energies up to 5 keV and currents up to 300 mA, and the oscillations were present in the data from a Langmuir probe, a floating probe, an electron energy analyzer, and a photometer. The VLF (1 kHz < f < 10 kHz) wave stimulation monitored by a wave receiver during one particular beam sequence has already been reported by Neubert et aL (1986). The amplitudes of the ELF and VLF oscillations observed during this sequence have almost identical variations with beam pitch angle, the strongest emissions being observed for parallel beams; the ELF power spectra for the strongest emissions have peaks about 10 dB above the broadband ELF noise at frequencies around 50-60 Hz. In another beam sequence the power spectra had a harmonic structure with the fundamental frequency around 200 Hz. The power density and frequency of the fundamental increased with the shuttle charge-up potential. The emission level observed during the beam sequences increased with the charge-up potential of the orbiter, •vhich largely depended on the wake structure. We find it most likely that the ELF oscillations are expressions of fluctuations in the return current and the shuttle potential and that these fluctuations are caused by processes involving charge imbalances in the near environment of the shuttle, possibly in a comoving plasma ing the SEPAC experiment are consistent with this idea. In (DGP), and the monitor television camera (MTV). The the SEPAC experiment we had, in addition, the opportunity EBA included a focusing coil, creating a pencil-shaped beam to observe the ELF power level for a wide range of beam with an initial diameter of about 2 cm, which could be depitch angles and also to observe optical emissions during fiected up to 300 from the vertical as defined in the orbiter beam injections. We have found that the ELF power level reference frame. The characteristics of the accelerators and and the turbulence created in the plasma depend strongly on the diagnostics and a general description of the observathe beam pitch angle, the strongest ELF noise and plasma tions made during the SEPAC experiment have been given turbulence being observed for parallel beams. ELF oscilla-by Obayashi et al. [1984], Taylor et al. [1985], and Sasaki The three experimental sequences of electron beam emissions analyzed here are called FO-2, FO-7-1, and FO-7-2 (FO is an abbreviation for functional objective). The beam parameters and some shuttle attitude information are summarized in Table 1. In FO-2 altogether 45 electron beam pulses (also called "shots") were injected during a 15-min period. i:he pulse length was 5 s for the first 18 pulses and 1 s for the rest. FO-7-1 and-2 were 5-min sequences in which 15 pulses of 5-s duration were injected. During the sequences the beam current Ib and energy Eb were varied from pulse to pulse, with the low-current and low-energy be...

A layer of plasma (such as that comoving with the space shuttle) can travel across a magnetic field if it is flanked by ‘‘polarization sheaths’’ that create the E field necessary for E×B drift. Both the spontaneous evolution and the stability of these positive and negative sheaths can be studied by their analogy with the charge configuration in magnetron-type electron devices. That is, conservation of canonical momentum results in ‘‘Brillouin flow,’’ i.e., shear flows in a pure ion sheath on one side and a pure electron sheath on the other side of the neutral plasma beam. The equilibrium parameters of this flow were obtained and the stability of the equilibria was analyzed. Both the ion and electron sheaths were found to be one-dimensionally stable and two-dimensionally unstable in a mode variously known as ‘‘magnetron,’’ ‘‘slipping stream,’’ or ‘‘diocotron.’’ This mode connects up with the Kelvin–Helmholtz instability in neutral matter. The analytical results were confirmed both qualitatively and quantitatively by numerical simulations. The saturation of the unstable modes, observed in the simulation, allows one to estimate how the polarization sheaths eventually diffuse.

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