Collisional relaxation of electrons in a warm plasma and accelerated nonthermal electron spectra in solar flares

Extending previous studies of nonthermal electron transport in solar flares which include the effects of collisional energy diffusion and thermalization of fast electrons, we present an analytic method to infer more accurate estimates of the accelerated electron spectrum in solar flares from observations of the hard X-ray spectrum. Unlike for the standard cold-target model, the spatial characteristics of the flaring region, especially the necessity to consider a finite volume of hot plasma in the source, need to be taken into account in order to correctly obtain the injected electron spectrum from the source-integrated electron flux spectrum (a quantity straightforwardly obtained from hard X-ray observations). We show that the effect of electron thermalization can be significant enough to nullify the need to introduce an {\it ad hoc} low-energy cutoff to the injected electron spectrum in order to keep the injected power in non-thermal electrons at a reasonable value. Rather the suppression of the inferred low-energy end of the injected spectrum compared to that deduced from a cold-target analysis allows the inference from hard X-ray observations of a more realistic energy in injected non-thermal electrons in solar flares.Comment: accepted for publication in Ap

On the variation of solar flare coronal x-ray source sizes with energy

Observations with {\em RHESSI} have enabled the detailed study of the structure of dense hard X-ray coronal sources in solar flares. The variation of source extent with electron energy has been discussed in the context of streaming of non-thermal particles in a one-dimensional cold-target model, and the results used to constrain both the physical extent of, and density within, the electron acceleration region. Here we extend this investigation to a more physically realistic model of electron transport that takes into account the finite temperature of the ambient plasma, the initial pitch-angle distribution of the accelerated electrons, and the effects of collisional pitch-angle scattering. The finite temperature results in the thermal diffusion of electrons, that leads to the observationally-inferred value of the acceleration region volume being an overestimate of its true value. The different directions of the electron trajectories, a consequence of both the non-zero injection pitch-angle and scattering within the target, cause the projected propagation distance parallel to the guiding magnetic field to be reduced, so that a one-dimensional interpretation can overestimate the actual density by a factor of up to $\sim 6$. The implications of these results for the determination of acceleration region properties (specific acceleration rate, filling factor, etc.) are discussed.Comment: 45 pages, 9 figures, accepted for publication in Ap

Flux measurements in the near surface layer over a non-uniform crop surface in China

International audienceEddy covariance measurements were conducted on fluxes of moisture, heat and CO2 in a near-surface layer over a non-uniform crop surface in an agricultural ecosystem in the central plain of China from 10 June to 20 July 2002. During this period, the mean canopy height was about 0.50 m. The study site consisted of grass (10% of area), bean (15%), corn (15%) and rice (60%). Based on footprint analysis, we expected >90% of the measured flux (at a height of 4 m above ground surface) to occur within the nearest 600 m of upwind area. We examined interdiurnal variations in the components of the surface energy balance and in CO2 flux. Results show that the pattern of energy partition had no obvious variation during the season. Daytime absorption of CO2 flux by the crop canopy suddenly increased after thunderstorm events. We examined the energy budget closure and found it to be around 0.85. We compared energy partitioning for all rain-free days, and found energy imbalance was more significant for the 1~3 days after rainy events and energy components almost achieve balance for the other rain-free days. It indicated that the cold or warm rainwater infiltrating into soil made problems

Regional Archives in the People\u27s Republic of China: A Case Study of the Chongqing Municipal Archives and the Yunnan Provincial Archives

The emergence of China as an active member of the international community and the growing number of exchange programs between archival institutions in China and the United States have fostered broader professional contacts between American archivists and their counterparts in China. Consequently, American archivists have become increasingly curious about the professional practices of their Chinese colleagues. The following description and analysis offers a case study of the structure, function, and use of Chinese regional archives

A Novel Scheme to Search for Fractional Charge Particles in Low Energy Accelerator Experiments

In the Standard Model of particle physics, the quarks and anti-quarks have fractional charge equal to $\pm1/3$ or $\pm2/3$ of the electron's charge. There has been a large number of experiments searching for fractional charge, isolatable, elementary particles using a variety of methods, including $e^+e^-$ collisions using dE/dx ionization energy loss measurements, but no evidence has been found to confirm existence of free fractional charge particles, which leads to the quark confinement theory. In this paper, a proposal to search for this kind particles is presented, which is based on the conservation law of four-momentum. Thanks to the CLEOc and BESIII detectors' large coverage, good particle identification, precision measurements of tracks' momenta and their large recorded data samples, these features make the scheme feasible in practice. The advantage of the scheme is independent of any theoretical models and sensitive for a small fraction of the quarks transitioning to the unconfinement phase from the confinement phase.Comment: 9 page

Enantiomerically pure β-phenylalanine analogues from α–β-phenylalanine mixtures in a single reactive extraction step

An efficient and selective method for the extraction of α-amino acids in preference over their β-isomers using PdCl2(PPh3)2 was discovered, which enables the separation of product mixtures obtained in the enantioselective enzymatic formation of β-amino acids.