Contribution of domain rotation to the magnetic susceptibility of spherical two-domain particles of magnetite

Summary. Starting with a simplified picture of the presumed magnetization configuration in a two-domain sphere of magnetite, the initial susceptibility due to rotation of the magnetization in the domains has been calculated. In particles between 100 and 300 nm in diameter the calculated susceptibility for randomly oriented particles drops rapidly from about 6 to a little over 3, which is the value to be expected for large multidomain particles. The results agree well with the available experimental results and imply that in magnetite particles in this size range the low field susceptibility is dominated by the domain rotation and that wall motion is effectively inhibited.     

Thermosolutal-convective instability in a stellar atmosphere

The thermosolutal-convective instability of a stellar atomsphere is studied in the presence of suspended particles. The criteria for monotonic instability are derived and are found to hold good also in the presence of uniform rotation and uniform magnetic field on the thermosolutal-convective instability. The thermosolutal-convective instability of a stellar atmosphere is also studied in the presence of suspended particles and radiative transfer effects and the criteria for monotonic instability are obtained in terms of source function.     

In-situ particle acceleration in extragalactic radio hot spots: observations meet expectations

We discuss, in terms of particle acceleration, the results from optical VLT observations of hot spots associated with radio galaxies. On the basis of observational and theoretical grounds, the following is shown. (i) Relatively low radio-radio power hot spots are the optimum candidates for being detected at optical waves. This is supported by an unprecedented optical detection rate of 70 per cent out of a sample of low radio power hot spots. (ii) The shape of the synchrotron spectrum of hot spots is mainly determined by the strength of the magnetic field in the region. In particular, the break frequency, related to the age of the oldest electrons in the hot spots, is found to increase with decreasing synchrotron power and magnetic field strength. Both observational results are in agreement with an in-situ particle acceleration scenario.     

一次降水性积层混合云系的微物理特征分析

利用山东省2007年10月27日1架次机载粒子测量系统（Particles Measuring System,PMS）积层混合云探测资料,分析了云中粒子浓度和尺度、液态含水量,以及小云滴和大云滴谱的垂直分布特征,比较了催化前后云微物理特征的变化。结果表明,催化前,云层中小云滴谱型为单峰,谱宽随高度增加先变窄后变宽,大云滴谱型在云低层为单峰,中高层为双峰谱,谱宽随高度增加先变宽后变窄,并且没有探测到降水粒子。催化后,小云滴尺度在低层减小、高层增加,整层液态水含量减小;大云滴浓度增加,尺度增大,出现降水粒子,固态粒子类型增多。在3 700~4 000 m高度层内小于10μm粒子明显增加,说明凝结过程比较明显,并且10~27.5μm粒子开始出现,启动了云滴的碰并机制。小云滴谱变化较小,基本为单峰谱,但在较大云滴处谱型略有起伏,在3 000m和3 300m高度的谱宽增宽。大云滴粒子谱有较大的变化,低层变成双峰谱,谱宽最宽可达650μm,中高层为双峰或多峰,峰值从小值向较大值移动。2D-P探头在催化云高层探测到降水粒子,谱型呈单调下降形态,谱宽最大为600μm。     

Experimental study of the effect of temperature on ion cluster formation using ion mobility spectrometry

Ion mobility spectrometry offers a robust and effective technique to study ion clusters in ambient conditions. Here, we have experimentally studied the influence of temperature on the positive ion cluster formation of 2-propanol vapor in air, along with parallel measurements for n-butyl acetate vapor in air. For both of these low proton affinity compounds in the ppm concentration range, temperatures below 0 °C tend to favor formation of dimers and trimers. The measurements indicate that approximate estimations for the fractions of these n-mers (n > 1) in the ion spectra, can be obtained by classical theory for ion induced nucleation. Presence of natural background vapors however slightly blurs the data, especially for the fraction of monomers, so that accurate prediction of the fractions of n-mers in the spectra would require more accurate information on the gas composition. The findings concerning thermal behavior of ions help to understand better ion phenomena also in field conditions.     

On the formation and growth of atmospheric nanoparticles

In this paper we summarize recent experimental, theoretical and observational results on the formation and growth of atmospheric nanoparticles. During the last years significant progress has occurred to explain atmospheric nucleation and initial steps of the growth. Due to climatic and health effects of fine and ultrafine particles the formation and growth of new aerosol particles is of growing interest. The question “How and under which conditions does the formation of new atmospheric aerosol particles take place?” has exercised the minds of scientists since the time of John Aitken, who in the late 1880s built the first apparatus to measure the number of dust and fog particles. However, only during the last 15–20 years has the measurement technology developed to such a level that size distributions of nanometer-size particles and concentrations of gases participating in particle formation can be measured in the atmosphere. Also from a theoretical point of view atmospheric nucleation mechanisms have not been resolved: several mechanisms such as ion-induced (or ion mediated) nucleation, ternary and kinetic (barrier-less) nucleation have been suggested. In the most recent theory, the activation of existing neutral and/or ion clusters has been suggested.     

High-energy particle acceleration and production of ultra-high-energy cosmic rays in the giant lobes of Centaurus A

The nearby radio galaxy Centaurus A is poorly studied at high frequencies with conventional radio telescopes because of its very large angular size, but is one of a very few extragalactic objects to be detected and resolved by the Wilkinson Microwave Anisotropy Probe ( WMAP ). We have used the five-year WMAP data for Cen A to constrain the high-frequency radio spectra of the 10° giant lobes and to search for spectral changes as a function of position along the lobes. We show that the high-frequency radio spectra of the northern and southern giant lobes are significantly different: the spectrum of the southern lobe steepens monotonically (and is steeper further from the active nucleus) whereas the spectrum of the northern lobe remains consistent with a power law. The inferred differences in the northern and southern giant lobes may be the result of real differences in their high-energy particle acceleration histories, perhaps due to the influence of the northern middle lobe, an intermediate-scale feature which has no detectable southern counterpart. In light of these results, we discuss the prospects for Fermi Gamma-ray Space Telescope detections of inverse-Compton emission from the giant lobes and the lobes' possible role in the production of the ultra-high-energy cosmic rays (UHECR) detected by the Pierre Auger Observatory. We show that the possibility of a Fermi detection depends sensitively on the physical conditions in the giant lobes, with the northern lobe more likely to be detected, and that any emission observed by Fermi is likely to be dominated by photons at the soft end of the Fermi energy band. On the other hand, we argue that the estimated conditions in the giant lobes imply that UHECRs can be accelerated there, with a potentially detectable γ-ray signature at TeV energies.     

Dark matter annihilation at cosmological redshifts: possible relic signal from annihilation of weakly interacting massive particles

We discuss the possibility of observing the products of the dark matter annihilation that was going on in the early Universe. Of all the particles that could be generated by this process, we consider only photons, as they are both uncharged and easily detectable. The younger the Universe was, the higher the dark matter concentration n and the annihilation rate (proportional to n ²) were. However, the emission from the very early Universe cannot reach us because of the opacity. The main part of the signal was generated at the moment the Universe had just become transparent for the photons produced by the annihilation. Thus, the dark matter annihilation in the early Universe should have created a sort of relic emission. We obtain its flux and the spectrum.
If weakly interacting massive particles (WIMPs) constitute dark matter, it is shown that we may expect an extragalactic gamma-ray signal in the energy range 0.5–20 MeV with a maximum near 8 MeV. We show that an experimentally observed excess in the gamma-ray background at 0.5–20 MeV could be created by the relic signal from the annihilation of WIMPs only if the dark matter structures in the Universe had appeared before the Universe became transparent for the annihilation products  ( z ≃ 300)  . We discuss in more detail physical conditions whereby this interpretation could be possible.     

On the origin of the 511-keV emission in the Galactic Centre

Diffuse 511-keV line emission, from the annihilation of cold positrons, has been observed in the direction of the Galactic Centre for more than 30 yr. The latest high-resolution maps of this emission produced by the SPI instrument on INTEGRAL suggest at least one component of the emission is spatially coincident with the distribution of ∼70 luminous, low-mass X-ray binaries detected in the soft gamma-ray band. The X-ray band, however, is generally a more sensitive probe of X-ray binary populations. Recent X-ray surveys of the Galactic Centre have discovered a much larger population (>4000) of faint, hard X-ray point sources. We investigate the possibility that the positrons observed in the direction of the Galactic Centre originate in pair-dominated jets generated by this population of fainter accretion-powered X-ray binaries. We also consider briefly whether such sources could account for unexplained diffuse emission associated with the Galactic Centre in the microwave (the Wilkinson Microwave Anisotropy Probe 'haze') and at other wavelengths. Finally, we point out several unresolved problems in associating Galactic Centre 511-keV emission with the brightest X-ray binaries.