分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Over the past decade, Boozer has argued that three-dimensional (3D) magnetic reconnection fundamentally differs from two-dimensional (2D) reconnection due to the fact that the separation between any pair of neighboring field lines almost always increases exponentially over distance in a 3D magnetic field. According to Boozer, this feature makes 3D field-line mapping chaotic and exponentially sensitive to small non-ideal effects; consequently, 3D reconnection can occur without intense current sheets. We test Boozer's theory via ideal and resistive reduced magnetohydrodynamic simulations of the Boozer-Elder coronal loop model driven by sub-Alfvenic footpoint motions [A. H. Boozer and T. Elder, Physics of Plasmas 28, 062303 (2021)]. Our simulation results significantly differ from their predictions. The ideal simulation shows that Boozer and Elder under-predict the intensity of current density due to missing terms in their reduced model equations. Furthermore, resistive simulations of varying Lundquist numbers show that the maximal current density scales linearly rather than logarithmically with the Lundquist number.
分类: 地球科学 >> 空间物理学 提交时间: 2017-03-10
摘要: Two single-layer X/Ku dual-band dual-polarization reflectarray antennas of different sizes with double parallel dipole elements are presented. Elements of the two bands are set to two orthogonal linear polarizations and placed in interlaced grid. The proposed reflectarrays operate in two frequency-bands within X-band centered at 10 GHz and Ku-band centered at 13.58 GHz. The smaller size reflectarray with elements arranged in a 13 �13 grid for X-band and in a 12 �12 grid for Ku-band is designed and simulated first. Based on the excellent dual-band performance of the small size reflectarray, then a larger size prototype has been designed, manufactured and measured. Measured results demonstrate the maximum gain of 28.54 dB with 50.93% aperture efficiency at 10 GHz and 31.06 dB with 51.34% aperture efficiency at 13.58 GHz, which show desirable dual-band dual-polarization radiation performance.
分类: 地球科学 >> 空间物理学 提交时间: 2017-03-10
摘要: A novel single-layer reflectarray element for broadband use is proposed. Ansoft HFSS is used to analyze the reflection phase for the element. By varying the element size, a slow slope of the reflection phase curve with the phase range over 600° is achieved. A center-fed 121-element reflectarray antenna is designed using the proposed elements and a linearly polarized pyramidal horn antenna is used as the feed of the reflectarray. The simulated results show that the gain of 24.42dB at the center frequency of 13.58GHz with 3-dB gain bandwidth of 36.8% (from 11.08∼16.08GHz), demonstrating a wideband performance compared to the conventional designs.
分类: 地球科学 >> 空间物理学 提交时间: 2017-03-10
摘要: A broadband center-fed reflectarray antenna is presented in this paper. Patch with slot ring on ground plane elements are used for bandwidth improvement. It is shown that quasi-linear phase responses with broad frequency band can be achieved by such a phasing element. An 11�1 elements reflectarray is designed and simulated. The results show that the simulated gain is 23.92 dB at the center frequency of 13.58 GHz with 3-dB bandwidth of 52%, which demonstrate significant broadband operation compared with that of conventional reflectarray.
分类: 地球科学 >> 空间物理学 提交时间: 2017-03-10
摘要: A novel single-layer microstrip reflectarray element with fractal structure is proposed. Ansoft HFSS is used to analyze the reflect phase for the fractal element in honeycomb lattice. A 469-element prime focus microstrip reflectarray antenna composed of the proposed fractal elements is designed, manufactured, and measured. The measured gain level of 29.8 dB is obtained at the center frequency of 13.58 GHz with 1-dB gain bandwidth of 15.3%.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Magnetohydrodynamic turbulence regulates the transfer of energy from large to small scales in many astrophysical systems, including the solar atmosphere. We perform three-dimensional magnetohydrodynamic simulations with unprecedentedly large magnetic Reynolds number to reveal how rapid reconnection of magnetic field lines changes the classical paradigm of the turbulent energy cascade. By breaking elongated current sheets into chains of small magnetic flux ropes (or plasmoids), magnetic reconnection leads to a new range of turbulent energy cascade, where the rate of energy transfer is controlled by the growth rate of the plasmoids. As a consequence, the turbulent energy spectra steepen and attain a spectral index of -2.2 that is accompanied by changes in the anisotropy of turbulence eddies. The omnipresence of plasmoids and their consequences on, e.g., solar coronal heating, can be further explored with current and future spacecraft and telescopes.