分类： 天文学 >> 天体物理学 提交时间： 2023-07-29
摘要： The pulsar radio emission mechanism remains an enigma since its discovery in 1967. The critical issue of origin of coherent emission is usually investigated separately from the micro-structure of individual pulses and characteristic emission frequency of pulsars... In this letter, these issues are interpreted in an unified scenario. The pulsar spin piles up magnetic field at the apex of last closed field line triggering magnetic reconnection. The resultant Alfven wave interacts with open field lines giving rise to coherent maser curvature and cyclotron emission. Such a scenario of coherent emission not only imposes new limit to emission site, nanoburst, and characteristic frequency, but also affects polarization and pair production required in maintaining a marginal stable circuit in radio emission of pulsars.
分类： 物理学 >> 电磁学、光学、声学、传热、经典力学和流体动力学 提交时间： 2016-07-01
摘要： We demonstrate a convenient approach for simultaneously manipulating the amplitude and polarization of light beams by means of the modulation of the correlation structure. As an illustration, we constructed a periodic correlation structure that can generate an arbitrary radially polarized array (RPA) beam of a radial or rectangular symmetry array in the focal plane from a radially polarized (RP) beam. The physical realizability conditions for such source and the far-field beam condition are derived. It is illustrated that the beamlet shape and the state of polarization (SOP) can be effectively controlled by the initial correlation structure and the coherence width. Furthermore, by designing the source correlation structure, a tunable OK-shaped RPA beam and an optical cage are demonstrated, which can find widespread applications in non-destructive manipulation of particles and living biological cells. The arbitrariness in the design of correlation structure prompted us to find more convenient approaches for controlling the statistics of light beams in terms of amplitude and polarization.
分类： 天文学 >> 天体物理学 提交时间： 2024-01-16
摘要： The pulsar radio emission mechanism remains an enigma since over half a century. A radiation process requires not only to explain the coherency and high degree of polarization of the emission, but also nanobursts, characteristic frequency of emission, and problems like death line, which can#2;not be well understood in the context of long standing cap models and recent models of magnetic reconnection. This article proposes a dynamo process in pulsar magnetosphere. Through centrifu#2;gal force the last closed magnetic field lines can be amplified to a critical value of, BT 104T, which triggers magnetic reconnection responsible for the instability required in coherent curvature radiation, so that a number of problems can be interpreted. The time scale of magnetic field pile up and relaxation can be short or long for young or old pulsars respectively, which naturally account for the diverse intermittencies exhibited in pulsars and FRBs.
分类： 天文学 >> 天体物理学 提交时间： 2023-12-04
摘要： The pulsar radio emission mechanism remains an enigma over half a century.A successful radiation process requires not only to explain the coherency and high degree of polarization of the emission, but alsomicrostructures, characteristic frequency of emission, and the death line problem, etc. These issues challenge both the long standing cap models and recent models of magnetic reconnection assuming stationary magnetosphere of a pulsar.This article proposes a radio emission through global-local interaction of pulsar magnetosphere. The centrifugal force at the light cylinder leads to the concentration of both field lines and plasma in an equatorial layer much less than that of a pulsar magnetosphere.The resultant magnetic reconnection with chain of plasmoids interprets not only the above problems in a simple and unified way, but also Rotating Radio Transients (RRATs) and Fast Radio Bursts (FRBs).
分类： 地球科学 >> 地理学 提交时间： 2023-06-13 合作期刊: 《干旱区科学》
Janisson B de JESUS
Tatiana M KUPLICH
Íkaro D de C BARRETO
Fernando L HILLEBRAND
Cristiano N da ROSA
摘要：The Caatinga biome is an important ecosystem in the semi-arid region of Brazil. It has significantly degraded due to human activities and is currently a region undergoing desertification. Thus, monitoring the variation in the Caatinga biome has become essential for its sustainable development. However, traditional methods for estimating aboveground biomass (AGB) are time-consuming and destructive. Remote sensing, such as optical and radar imaging, can estimate and correlate with vegetation. Nevertheless, radar imaging is still a novelty to be applied in estimating the AGB of this biome, which is an area with little research. Therefore, this study aimed to use Sentinel-1 images to estimate the AGB of the Caatinga biome in Sergipe State (northeastern Brazil) and to verify its influencing factors. Nineteen sample plots (30 m×30 m) were selected, and the stems of individuals with a circumference at breast height (1.3 m above the ground) equal to or greater than 6.0 cm were measured, and the AGB through an allometric equation was estimated. The Sentinel-1 images from 3 different periods (green, intermediate, and dry periods) were used to consider the phenological conditions of the Caatinga biome. All the pre-processing and extraction of attributes (co-polarized VV (vertical transmit and vertical receive), cross-polarized VH (vertical transmit and horizontal receive), and band ratio VH/VV backscatter, radar vegetation index, dual polarization synthetic aperture radar (SAR) vegetation index (DPSVI), entropy (H), and alpha angle (α)) were performed with Sentinel's Application Platform. These attributes were used to estimate the AGB through simple and multiple linear regressions and evaluated by the coefficients of determination (R2), correlation (r), and root mean squared error (RMSE). The results showed that the attributes individually had little ability to estimate the AGB of the Caatinga biome in the three periods. Combined with multiple regression, we found that the intermediate period presented the equation with the best results among the observed and estimated variables (R2=0.73; r=0.85; RMSE=8.33 Mg/hm2), followed by the greenness period (R2=0.72; r=0.85; RMSE=8.40 Mg/hm2). The attributes contributing to these equations were VH/VV, DPSVI, H, α, and co-polarized VV for the green period and cross-polarized VH for the intermediate period. The study showed that the Sentinel-1 images could be used to estimate the AGB of the Caatinga biome in the green and intermediate phenological periods since the SAR attributes highly correlated with the estimated variable (i.e., AGB) through multiple linear equations.
分类： 核科学技术 >> 粒子加速器 提交时间： 2023-02-22
摘要： As an important measure of improving the efficiency and usability of X-ray free electron laser facilities, parallel operation of multiple undulator lines realized by a beam switchyard has become a standard configuration in the recent built XFEL facilities. SXFEL-UF, the first soft X-ray free electron laser user facility in China, has finished construction and commissioning recently. The electron beams from the linac are separated and delivered alternately to the two parallel undulator beam lines through a beam switchyard. A stable and fast kicker magnet is used to achieve bunch-by-bunch separation. Optics measures are applied to mitigate the impact of various collective effects, such as coherent synchrotron radiation and micro-bunching instability, on the beam quality after passing through the deflection line of the beam switchyard. In this study, the comprehensive physical design of the beam switchyard is described and the latest results of its commissioning process are presented.
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