Magnetic flux expulsion as an acceleration mechanism for stellar winds

Observations of the Sun show that magnetic flux is emerging through the surface in small scales in rather copious amounts. In order to maintain a steady state field strength, this flux must either be locally dissipated or explelled or both. We believe that magnetic reconnection and subsequent flux explusion is the most effective manner in which to achieve this. If new flux emerges into an already preexisting coronal magnetic field, the ambient field must be pushed aside to allow room for the new flux. If the ambient field strength decreases outward with radial distance as is expected for all stars, it may pinch off the emerging flux through magnetic reconnection and expell it outward. The net force on an isolated diamagnetic plasmoid produced by this process is shown to assume a particularly simple form, depending only on the plasmoid's mass, its temperature, and the radial gradient of the logarithm of the undisturbed magnetic pressure. If a sufficient number of these magnetic elements are produced per unit time, this process translates to a net outward magnetic force on the coronal plasma which can be greater that the gas pressure force. Thus, a stellar wind can be produced by magnetic forces alone without the need for a high coronal gas pressure — a mechanism which could be effective in explaining why stars, such as the late-type giants, which possess cool coronae nevertheless exhibit vigorous coronal expansions.     

Diamagnetic aspects of the coronal transient phenomenon

If a magnetic structure emerges into the solar atmosphere containing a preexisting magnetic field, the preexisting field must be moved aside to allow space for the emerging flux. Under these conditions, provided the ambient field strength decreases outward with radial distance, the ambient field may tend to pinch off the emerging field through magnetic reconnection and expell it outward. Here, a model is developed to demonstrate this effect and to evaluate its relevance to the coronal transient phenomenon. In particular, we argue that, if magnetic reconnection can be initiated in such a configuration, the upward force produced by the ambient field is enhanced and, depending upon the internal conditions of the configuration, a coronal transient could be initiated in this manner.     

气动谐振管加热机理数值研究

放置在高速喷流中的半开口气动谐振管在一定的条件下会出现加热现象.针对这一现象,利用数值计算方法计算了一维非定常非线性Euler方程组,采用MacCormark格式,对不同管口条件下谐振管内的振荡过程进行了数值研究.研究发现,当管口压力变化频率处在谐振管的线性谐振频率附近时,管内形成的激波以及激波的反射增强导致了谐振管端壁区域明显的加热效果.对不同的驱动气体和不同的谐振管型式(圆柱形和圆锥形)的数值研究也给出了有价值的结论.研究结果与实验事实或现象是相吻合的.      

Current collection through the transport of electrons across magnetic field lines

Collection of electrons by a long conducting cylinder in a flowing plasma is studied by means of numerical simulations. The plasma flow simulates the relative motion between a spacecraft and plasma. The sheath structures and the levels of electron current collections for the cases with and without an ambient magnetic field ( ) are studied. It is found that for the flow perpendicular to the magnetic field, the current is considerably enhanced depending on the relative drift velocity. In the case of a non-zero magnetic field perpendicular to the cylinder axis, the potential structure is a two-dimensional double layer with dimensions L L_|, where L and L_| are the dimensions perpendicular and parallel to , respectively. L is found to be the current limiting radius given by the Parker-Murphy model. For the flow along , the electron current is found to be smaller than that for the flow perpendicular to . This is explained in terms of the potential structures.     

Radiation from relativistic shocks in turbulent magnetic fields

Using our new 3-D relativistic particle-in-cell (PIC) code parallelized with MPI, we investigated long-term particle acceleration associated with a relativistic electron–positron jet propagating in an unmagnetized ambient electron–positron plasma. The simulations were performed using a much longer simulation system than our previous simulations in order to investigate the full nonlinear stage of the Weibel instability and its particle acceleration mechanism. Cold jet electrons are thermalized and ambient electrons are accelerated in the resulting shocks. Acceleration of ambient electrons leads to a maximum ambient electron density three times larger than the original value as predicted by hydrodynamic shock compression. In the jet (reverse) shock behind the bow (forward) shock the strongest electromagnetic fields are generated. These fields may lead to time dependent afterglow emission. In order to calculate radiation from first principles that goes beyond the standard synchrotron model used in astrophysical objects we have used PIC simulations. Initially we calculated radiation from electrons propagating in a uniform parallel magnetic field to verify the technique. We then used the technique to calculate emission from electrons in a small simulation system. From these simulations we obtained spectra which are consistent with those generated from electrons propagating in turbulent magnetic fields with red noise. This turbulent magnetic field is similar to the magnetic field generated at an early nonlinear stage of the Weibel instability. A fully developed shock within a larger simulation system may generate a jitter/synchrotron spectrum.     

Dynamic trapping of electrons in the porcupine ionospheric ion beam experiment

Electrons are needed to maintain quasineutrality in a case where positive ions are injected across the magnetic field into a limited volume in a magnetized plasma. In the absence of collisions, a positive potential builds up and traps the electrons which enter the region along the magnetic field. If the added density of ions exceeds the ambient density, large potential differences along the magnetic field can be maintained this way. The process explains several features of the Porcupine xenon ion beam injection experiment, where strong magnetic-field-aligned electric fields were measured in the vicinity of a xenon ion beam which was injected into the ambient ionosphere from a spinning subpayload.     

Morphological and convective instabilities during solidifiation

The stability of the flow between two vertical, infinite, rigid, coaxial cylinders held at different temperatures is analyzed by linear stability theory. For a Prandtl number of 22.8 and a radius ratio of 0.02, the flow is unstable to an axisymmetric perturbation at a critical Grashof number of 2150; the wave speed of the instability is comparable to the maximum velocity of the unperturbed flow. When the rigid outer cylinder-fluid interface is replaced by a crystal-melt interface which can change shape, two new modes of instability occur at lower Grashof numbers. There is an asymmetric instability with a critical Grashof number of 180 and an axisymmetric instability with a critical Grashof number of 460; for both of these modes the wave speed of the instability is several orders of magnitude smaller than the unperturbed flow velocity.These calculations were motivated by and are in general agreement with our recent experiments on succinonitrile. A long vertical cylindrical sample of succinonitrile was heated by an electrical current through a coaxial vertical wire so that a vertical melt annulus formed between the coaxial heater and the surrounding crystal-melt interface. Above a critical Grashof number of circa 200, a helical crystal-melt interface formed which rotated steadily about the cylinder axis; the wave speed was several orders of magnitude less than the base flow velocity.     

Installation and preliminary data analysis of Penang magnetic data Acquisition system (MAGDAS) in Malaysia

MAGDAS PEN was established on 19th September 2019 as one of the MAGDAS observatory arrays located at Universiti Sains Malaysia (USM) (5.15°, 100.50°). The main objective of the MAGDAS project is to monitor global electromagnetic and the ambient plasma density in the geospace environment. This installation has contributed to a better understanding of the Sun-Earth coupling system. This paper presents the installation process of one of the MAGDAS magnetometers named YU-8 T magnetic sensor and provides a preliminary analysis of geomagnetic HDZ components amplitude-time that was observed at PEN station. A one-month HDZ-geomagnetic field data was processed from 1st November to 30th November 2019. The daily variations with a consistent pattern in delta H geomagnetic field components are observed throughout the day with eastward electric field effects that are observed during solar peak hours. The delta H-component gradually increases around 0700LT and reaches the maximum reading at 1300LT with a range of value ~ 40-70nT. The value slowly decreases that started from 1400LT until the night time. The reading during the night time shows a constant variation with magnitude varies in between ?10nT to + 10nT. The average H-component value of the night time is used as the baseline for the observation system. Overall, the observed trends portray a good sign of solar quiet field and Sq with no solar-terrestrial disturbances.     

上游双扰动对圆柱气动特性影响的数值模拟

通过用商用计算流体力学软件FLUENT 6.0中的SIMPLE方法求解二维非定常层流模型,对雷诺数Re=200时上游双扰动体对下游圆柱气动特性影响进行了数值模拟研究.计算结果表明当双扰动体相隔较近(H/d=1.1,1.33)时,其后形成单一的Karman涡街,它们对下游圆柱的干扰作用类似于单扰动体时的情况,但是流动结构发生变化的临界间距Lcri较之有效宽度相同的单扰动体的要大,因此可以推断出在该种布置下双扰动体和圆柱绕流会出现2种流动模式.在空穴流动模式下面,下游圆柱的阻力和升力脉动最多减小94%和79%,系统阻力(包括圆柱和扰动体)也能减小33%.而当双扰动体尾流为双Karman涡街时(双扰动体相隔较远H/d=3.33,5.0),圆柱迎风面部分区域仍受到来流的冲击作用,气动力减小有限.     

Sq(H) field variations at Sonmiani geomagnetic observatory,Pakistan for solar cycle 24

In this study, Sq(H) field results for Sonmiani geomagnetic observatory (SON), Pakistan are presented first time for solar cycle i.e., Solar Cycle 24. Sonmiani observatory was established in 2008 and was included in the list of INTERMAGNET Magnetic Observatory (IMO) in 2012. The analysis of Sq(H) field is performed to examine diurnal, annual and seasonal behaviour of Sq(H) field at Sonmiani along with Honolulu which is used as reference observatory. Both the observatories lie at the Equatorial Ionisation Anomaly (EIA) crest. In general strong dependency of Sq(H) field on solar cycle has been observed. First peak of SC24 was visible in Sq(H) field at both observatories. However, at HON maximum Sq(H) was observed in 2015 instead of 2014, a year later than the year of solar maximum. Prominent longitudinal difference has been noted between both observatories. At HON, maximum Sq(H) was observed during equinox while at SON it was noted during equinox and summer as a consequence of shift in latitude of Sq focus. Phase shift of Sq(H) at SON followed a general trend, that is maximum Sq(H) shifted to later hours in solar maximum as compared to solar minimum. Whereas, an opposite trend was noted at HON which might be due to coastal effect. In case of seasonal phase shift, at both observatories maximum Sq(H) occurred at later hours in summer than in winter which is opposite to the results obtained by many workers. These points need further investigation and geomagnetic as well as wind, and electric field data of other observatories situated at the EIA crest region is required to interpret the Sq(H) phase shift extensively.     

低速流预处理Roe格式中的数值粘性

使用Roe格式计算低马赫数流动,需采用预处理技术.在预处理技术中,需要选择适合的预处理参数,但有时很难找到,因而得不到合理的计算结果.通过数值实验,发现使用预处理方法时,格式的数值粘性对低速流动的模拟有显著的影响.提出了在预处理Roe格式中加入可调参数,用以控制数值粘性对计算结果的影响.带有可调节数值粘性参数的预处理Roe格式的数值粘性更接近低速流动的真实耗散,反映了低速流动的真实物理现象.对方腔流动和圆柱绕流的低雷诺数定常流动所做的数值实验,得到了比较满意的结果,表明这一改进是有效的.对同一低雷诺数不同低马赫数的定常圆柱绕流所做的数值实验,均得到了满意的结果,表明了方法的适用性.     

横电磁传输室理论与技术的若干问题

横电磁传输室(TEM cell)由于其全封闭式结构,不会污染环境或影响操作人员的健康,因而比敞开式带状线测试室优越。从内部场分布看,扁结构(矩形截横面)优于方结构(方形横截面)。电磁场数值解法(先求导积分方程然后化为矩阵代数方程)不仅对传统结构有效,而且可用来计算一些可想象的新结构。文中给出了作者针对小型室和中型室进行大量测试得到的实验结果。并据此导出了最高可用频率(UUFL)的经验公式。最后,提出了在今后研究工作中应优先解决的问题。     

Fractal Structure of the Heliospheric Plasma Sheet at the Earth''''s Orbit

An analysis of the data from the Wind and IMP-8 spacecraft revealed that a slow solar wind, flowing in the heliospheric plasma sheet, represents a set of magnetic tubes with plasma of increased density (N > 10cm-3 at the Earth's orbit). They have a fine structure at several spatial scales (fractality), from 2°-3°(at the Earth's orbit, it is equivalent to 3.6-5.4h, or (5.4-8.0)×106km) to the minimum about 0.025°, i.e. the angular size of the nested tubes is changed nearly by two orders of magnitude. The magnetic tubes at each observed spatial scale are diamagnetic, i.e. their surface sustains a flow of diamagnetic (or drift) current that decreases the magnetic field within the tube itself and increases it outside the tube. Furthermore, the value of β= 8π[N(Te + Tp)]/B2 within the tube exceeds the value of βoutside the tube. In many cases total pressure P = N(Te + Tp) + B2/8πis almost constant within and outside the tubes at any one of the aforementioned scales.     

太阳风局部加速的磁流体力学过渡特征

本文讨论了轴对称磁力线管中太阳风从亚声速和亚阿尔文速度加速到超声速和超阿尔文速度的过程。考虑到流动和场的二维效应以后,除去临界声速线M2=1以外,还有临界阿尔文速线M_A2=1和M2+M_A2=1。这时,磁力线管中存在三次过渡,使磁流体力学方程组的类型从椭圆型变为双曲型,再变为椭圆型,最后变为双曲型。由于日冕中的典型声速和阿尔文速度具有相同的量级,而且人们对磁力线管的急剧膨胀很有兴趣,这就需要讨论二维磁流体力学方程组的自(冫合)解。太阳风的局部流管中有可能出现多次过渡的流动特征。      

Modified gradiometer technique applied to Double Star (TC-1)

On TC-1 (Tan Ce 1), the equatorial spacecraft of the Double Star mission, a strong spin-synchronized magnetic interference from the solar panels was observed. In-flight correction techniques for spinning spacecraft that are based on minimizing spin tones in the spin-aligned component and in the magnitude of the ambient magnetic field are therefore not possible in this case. However, due to the fortunate situation that the spacecraft carries two flux-gate magnetometers on the same boom (at 0.5 m distance from each other), the spacecraft field effects could be removed from the spin-averaged data to achieve 0.2 nT relative accuracy, by using a gradiometer technique. Methodology and results are presented. The obtained accuracy allows the use of the data in multi-spacecraft studies together with the Cluster satellites.     

A star identification algorithm based on radial and dynamic cyclic features of star pattern

A full-sky star identification algorithm based on radial and dynamic cyclic patterns is presented with the aim of solving the “lost-in-space” problem. The dynamic cyclic pattern match is applied with a maximum cumulate comparison method to identify sensor-catalog pairings in initial match, which substantially eliminates the effects of the star position noise, magnitude noise, and false stars. After initial match pairings of stars are obtained, a chain part extension technique is employed to quickly search for the longest match chain as the final result. Experimental results indicate that the proposed algorithm is highly robust to star position noise, magnitude noise and false stars. In a series of simulations, the identification rate of the algorithm is 97.50% with 2.0 pixels star position noise, 96.90% with 0.4 Mv star magnitude noise and 95.30% with four false stars respectively. Moreover, the algorithm achieves an identification rate of 58.08% when only six stars are in the field of view.     

折叠翼无人机冷气发射装置动态特性分析及优化

针对冷气投放装置工作过程中存在的机械系统和气压系统耦合非线性动力学问题,提出了一种满足折叠翼无人机（UAV）结构形式和空中发射技术要求的冷气发射动态特性分析方法及优化设计方法。以某型折叠翼无人机为研究对象,基于联合仿真建立了无人机气动发射系统动力学模型,搭建了冷气发射系统试验样机,并完成压缩气体空中发射试验,验证了仿真模型的准确性。在此基础上,分析了无人机与冷气发射装置主要系统参数对无人机发射动态性能的影响,针对该系统进行了参数优化设计。结果表明:储气瓶体积和充气压力是影响无人机冷气发射动态特性的关键参数,随着储气瓶体积和充气压力增大,最大发射速度和加速度明显增大,储气瓶体积从15 L增加至30 L,最大发射速度增加了52.7%,最大发射加速度增长了60.9%呈正相关影响;充气压力从0.4 MPa增加至0.7 MPa,最大发射速度增长了50.5%,最大发射加速度增长了69.9%;发射角度对无人机发射性能影响较小,可忽略不计。      

二维扰动对下游圆盘压力分布的影响

通过测量阻力和表面压力,研究了干扰圆柱尾迹产生的二维扰动对下游圆盘气动特性的影响,得到圆盘迎风面压力分布的变化规律,并结合以往流场结构的研究,对其机理进行了分析.结果表明在圆盘与干扰圆柱间距比L/D较小时,圆盘迎风面低压区宽度随间距增大迅速增加;当L/D>0.7时,对于同一直径的干扰圆柱,圆盘迎风面低压区宽度基本保持不变;对于不同直径的干扰圆柱,在圆盘前产生低压区的宽度随圆柱直径的增加而变大.此外,利用这种干扰方法,可以降低圆盘阻力,实验得到圆盘阻力可减少约9%,相应的圆柱/圆盘系统的阻力可减小4%.      

Energetic ion emission for active spacecraft control

Future space missions aiming at the accurate measurement of cold plasmas and DC to very low frequency electric fields will require that the potential of their conductive surfaces be actively controlled to be near the ambient plasma potential. In the near-Earth space these spacecraft are usually solar-cell powered; consequently, parts of their surface are most of the time exposed to solar photons. Outside the plasmasphere, a positive surface potential due the dominance of surface-emitted photoelectrons over ambient plasma electrons is to be expected. Photo- and ambient electrons largely determine the potential and positive values between a few Volts up to 100 V have been observed. Active ion emission is the obvious solution of this problem. A liquid metal ion emitter and a saddle field ion emitter are nearing the stage of flight unit fabrication. We will attempt to clamp the spacecraft potential to values close to the plasma potential. We present first results from vacuum chamber tests and describe the emission behaviour and characteristics of emitters producing, respectively, In⁺ and N₂⁺ beams with an energy of ≥ 5 keV.