Randomness and fuzziness are the two most important uncertainties inherent in human cognition, which have attracted great attention in artificial intelligence research. In this paper, regarding linguistic terms or concepts as the basic units of human cognition, we propose a new cognitive model-cloud model, which can synthetically describe the randomness and fuzziness of concepts and implement the uncertain transformation between a qualitative concept and its quantitative instantiations. Furthermore, by analyzing in detail the statistical properties of normal cloud model, that is, an important kind of cloud models based on normal distribution and Gauss membership function, we show that normal cloud model can not only be viewed as a generalized normal distribution with weak constraints but also avoid the flaw of fuzzy sets to quantify the membership degree of an element as an accurate value between 0 and 1 and, therefore, may be more adaptive for the uncertainty description of linguistic concepts. Finally, two demonstration examples about the fractal evolution of plants and network topologies based on cloud models are given to illustrate the promising applications of cloud models in some more complex knowledge representation tasks. C 2009 Wiley Periodicals, Inc.

Aims. We investigate the electron acceleration in convective electric fields of cascading magnetic reconnection in a flaring solar corona and show the resulting hard X-ray (HXR) radiation spectra caused by Bremsstrahlung for the coronal source.Methods. We perform test particle calculation of electron motions in the framework of a guiding center approximation. The electromagnetic fields and their derivatives along electron trajectories are obtained by linearly interpolating the results of high-resolution adaptive mesh refinement (AMR) MHD simulations of cascading magnetic reconnection. Hard X-ray (HXR) spectra are calculated using an optically thin Bremsstrahlung model. Results. Magnetic gradients and curvatures in cascading reconnection current sheet accelerate electrons: trapped in magnetic islands, precipitating to the chromosphere and ejected into the interplanetary space. The final location of an electron is determined by its initial position, pitch angle and velocity. These initial conditions also influence electron acceleration efficiency. Most of electrons have enhanced perpendicular energy. Magnetic curvature and gradient driven acceleration efficiency along the magnetic field increases with the increase of the magnetic field resolution. It was shown that for a sufficiently high resolution of the electromagnetic fields, adaptive mesh refinement (AMR) MHD simulations are needed. Trapped electrons can be accelerated to energies up to half of MeV, precipitating ones to more than 60 keV in the refined smaller-scale magnetic structures. Trapped electrons are considered to cause the observed bright spots along coronal mass ejection CME-trailing current sheets as well as the flare loop-top HXR emissions. Another observable effect is the locations of the precipitating energetic electrons with respect to the polarity inversion line (PIL). A stronger asymmetry of the energetic electron precipita-Article number, page 1 of 27 arXiv:1504.06486v1 [astro-ph.SR] 24 Apr 2015 X. Zhou et al.: Electron acceleration by cascading reconnection in the solar corona tions around the PIL is obtained with sufficiently well resolved smaller-scale magnetic structures than that in the coarsely resolved ones due to the asymmetry in the parallel magnetic curvature acceleration.

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We present observations of distinct UV spectral properties at different locations during an atypical X-shaped flare (SOL2014-11-09T15:32) observed by the Interface Region Imaging Spectrograph (IRIS). In this flare, four chromospheric ribbons appear and converge at an X-point where a separator is anchored. Above the X-point, two sets of non-coplanar coronal loops approach laterally and reconnect at the separator. The IRISslit was located close to the X-point, cutting across some of the flare ribbons and loops. Near the location of the separator, the Si IV 1402.77Åline exhibits significantly broadened line wings extending to 200 km s −1 with an unshifted line core. These spectral features suggest the presence of bidirectional flows possibly related to the separator reconnection. While at the flare ribbons, the hot Fe XXI 1354.08Åline shows blueshifts and the cool Si IV 1402.77 Å, C II 1335.71 Å, and Mg II 2803.52Ålines show evident redshifts up to a velocity of 80 km s −1 , which are consistent with the scenario of chromospheric evaporation/condensation.