The case for a common spectrum of particles accelerated in the heliosphere: Observations and theory


In the last decade a significant discovery has been made in the heliosphere: the spectrum of particles accelerated in both the inner heliosphere and in the heliosheath is the same: a power law in particle speed with a spectral index of -5, when the spectrum is expressed as a distribution function; or equivalently, a differential intensity spectrum that is a power law in energy with a spectral index of -1.5. In the inner heliosphere this common spectrum occurs at quite low energies and is most evident in instruments designed to measure suprathermal particles. In the heliosheath, the common spectrum is observed over the full energy range of the Voyager energetic particle instruments, up to energies of ~100 MeV. The remarkable discovery of a common spectrum is compounded by the realization that no traditional acceleration mechanism, i.e. diffusive shock acceleration or stochastic acceleration, can account for the common spectrum. There is thus an opportunity to once again demonstrate the relevance of heliospheric physics by developing a new acceleration mechanism that yields the common spectrum, with the expectation that such a new acceleration mechanism will find broader applications in astrophysics. In this paper, the observations of the common spectrum in the heliosphere are summarized, with emphasis on those that best reveal the conditions in which the acceleration must operate. Then, building on earlier work, a complete derivation is presented of an acceleration mechanism, a pump acceleration mechanism, that yields the common spectrum, and the various subtleties associated with this derivation are discussed.

Evidence for the Core Field Polarity of Magnetic Flux Ropes against the Reconnection Guide Field


We present the observational evidence by THEMIS-D spacecraft to demonstrate that magnetic flux rope can have a core field polarity opposite to the guide field during asymmetric reconnection with a high magnetic shear across the magnetopause. In the presented event, a bipolar reconnection outflow was observed at the magnetopause, across which the magnetic shear was ~149o corresponding to a predicted guide field BG = −15.6 nT. The ratios for magnetic field strength and ion density between the magnetospheric and magnetosheath sides of the magnetopause were ~1.6 and ~0.15, respectively. During the event, the component of magnetic field along the M axis (the intermediate variance direction) remained negative and stable for most of the time, and its mean value agreed with the BG. Two magnetic flux ropes with a strong core field were identified, but one flux rope had a core field polarity against the guide field BG. Our finding differs from current theoretical simulations that the core field polarity of the flux ropes inherits from the guide field.

Effect of different calcium contents on the microstructure and mechanical properties of Mg-5Al-1Bi-0.3Mn magnesium alloy.

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Effect of different calcium contents on the microstructure and mechanical properties of Mg-5Al-1Bi-0.3Mn magnesium alloy.

Microsc Res Tech. 2014 Oct 23;

Authors: Xiao-Ping L, Su-E D, Ya-Qing Z


The effect of different Ca contents on the microstructure and mechanical properties of Mg-5Al-1Bi-0.3Mn (AMB501) magnesium alloys was investigated by conventional melting and casting technique using different Ca contents (1.0, 2.0, and 3.0 wt %). Increasing the Ca content resulted in higher hardness and yield strength, but decreased elongation. The improved tensile properties of the AM50-1Bi-xCa alloys were due to the changes in AMB501 alloy microstructure when the Ca content increased, as demonstrated by scanning electron microscope, energy dispersive spectrum, and X-ray diffractometer. The alloy microstructure indicated that the amount of β-Mg17 Al12 phase on grain boundaries decreased and the morphology of β-Mg17 Al12 phase on grain boundaries changed from quasicontinuous-net shape to dispersed particles. The Mg17 Al12 phase disappeared and a new secondary phase Al2 Ca appeared after a 3.0 wt % Ca addition. Microsc. Res. Tech., 2014. © 2014 Wiley Periodicals, Inc.

PMID: 25339286 [PubMed - as supplied by publisher]

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