太阳风中离子束流与电磁离子回旋波的相互作用

正电磁离子回旋波是指频率低于或者接近离子回旋频率的电磁波,其存在左旋和右旋两种偏振状态.通过回旋共振相互作用,电磁离子回旋波能直接与粒子发生能量交换,对太阳风等离子体加热和加速等能化现象起着重要作用.然而,太阳风中电磁离子回旋波的激发机制及其波粒相互作用尚未完全清楚.本学位论文深入、系统地研究了太阳风等离子体环境下离子束流对电磁离子回旋波激发机制的影响及其波粒相互作用,为进一步理解与解释太阳风中微观等离子体物理过程、     

电子回旋脉泽不稳定性驱动的太阳射电辐射机制

在解释天体爆发过程中的短时标相干射电辐射现象方面,非热电子驱动的回旋脉泽辐射得到了广泛应用.在以往的研究中,一个关键条件是非热电子具有各向异性速度分布,从而有效激发回旋脉泽不稳定性.然而,观测显示太阳和其他天体的非热电子经常呈现负幂律谱形式的能量分布.非热电子的这种负幂律能谱特征会严重抑制回旋脉泽不稳定性放大率,因此,进一步研究非热电子的负幂律能谱分布行为,能拓宽电子回旋脉泽辐射机制的适用性,很好地发展天体射电辐射机制理论.最近研究结果显示,负幂律谱电子的低能截止行为可以有效地激发电子回旋脉泽辐射,从而拓宽了其在天体物理研究中的应用范围.     

Reflection Driven MHD Turbulence in the Solar Atmosphere and Wind

Alfvénic turbulence is usually invoked and used in many solar wind models (Isenberg and Hollweg, 1982, J. Geophys. Res. 87:5023; Tu et al. 1984, J. Geophys. Res. 89:9695; Hu et  al. 2000, J. Geophys. Res. 105:5093; Li 2003, Astron. Astrphys. 406:345; Isenberg 2004, J. Geophys. Res. 109:3101) as a process responsible for the transfer of energy, released at large scale in the photosphere, towards small scale in the corona, where it is dissipated. Usually an initial spectrum is prescribed since the closest constraint to the spectrum is given by Helios measurements at 0.3 AU. With this work we intend to study the efficiency of the reflection as a driver for the nonlinear interactions of Alfvén waves, the development of a turbulent spectrum and its evolution in the highly stratified solar atmosphere inside coronal holes. Our main finding is that the perpendicular spectral slope changes substantially at the transition region because of the steep density gradient. As a result a strong turbulent heating occurs, just above the transition region, as requested by current solar wind models.     

Properties of Ion-Cyclotron Waves in the Open Solar Corona

Remote observations of coronal holes have strongly suggested the resonant interactions of ion-cyclotron waves with ions as a principal mechanism for plasma heating and acceleration of the fast solar wind. In order to study these waves, a WKB (Wentzel?CKramers?CBrillouin) linear perturbation analysis is used in the frame work of a collisionless multi-fluid model where we consider in addition to protons a second ion component made of alpha particles. We consider a non-uniform background plasma describing a funnel region in the open coronal holes and we use the ray tracing Hamiltonian-type equations to compute the ray path of the waves and the spatial variation of their properties. At low frequency (smaller than the proton cyclotron frequency), the results showed a distinct behavior of the two ion-cyclotron modes found in our calculations, namely the first one propagates anisotropically guided along the magnetic field lines while the second one propagates isotropically with no preferred direction.     

Proton and He^2＋ Temperature Anisotropies in the Solar Wind Driven by Ion Cyclotron Waves

We carried out one-dimensional hybrid simulations of resonant scattering of protons and He^2 ions by ion cyclotron waves in an initially homogeneous, collisionless and magnetized plasma. The initial ion cyclotron waves have a power spectrum and propagate both outward and inward. Due to the resonant interaction with the protons and He^2 ions, the wave power will be depleted in the resonance region. Both the protons and He^2 ions can be resonantly heated in the direction perpendicular to the ambient magnetic field and leading to anisotropic velocity distributions, with the anisotropy higher for the He^2 ions than for the protons. At the same time, the anisotropies of the protons and He^2 ions are inversely correlated with the plasma β‖p=8πnpkBT‖p/B0^2, consistent with the prediction of the quasilinear theory (QLT).     

Viscous Energy Dissipation by Flux Pile-Up Merging in the Solar Corona

Magnetic field annihilation in resistive viscous incompressible plasmas is analyzed. Anisotropic viscous transport is modeled by the dominant terms in the Braginskii viscous stress tensor. An analytical solution for steady-state magnetic merging, driven by vortical plasma flows in two dimensions, is derived. Resistive and viscous energy dissipation rates are calculated. It is shown that, except in the limiting case of zero vorticity, viscous heating can significantly exceed Joule heating at the merging site. The results strongly suggest that viscous dissipation can provide a significant fraction of the total energy release in solar flares, which may have far-reaching implications for flare models.     

Visco-Resistive Dissipation in Strongly Driven Transient Reconnection

Solar flare energy release mechanisms often neglect the role played by viscous effects. Here we perform incompressible planar reconnection simulations, driven by the Orszag–Tang vortex, for both classical and Braginskii forms of the viscosity. We show that strongly driven “saturated” flux pile-up current layers, which lead to weak reconnection rates at small resistivities, are accompanied by invariant global viscous losses. These results support the notion that viscous dissipation in flaring plasmas can account for a significant fraction of the flare energy release.     

Electromagnetic Ion Beam Instability in the Solar Corona

Remote-sensing measurements indicate that heavy ions in the corona undergo an anisotropic and mass-charge dependent energization.A popular explanation to this phenomenon is the damping of the Alfven/ion cyclotron waves.In this paper,we propose that the ion beam instability can be an important source of the Alfven/ion cyclotron waves, and we study the excitation of the ion beam instability in the corona at the heliocentric distance～3R_⊙ and the corresponding energy transfer process ther...     

Solar Wind Plasma Particles Organized by the Flow Speed

An understanding of solar variability over a broad spectral range and broad range of timescales is needed by scientists studying Earth’s climate. The Total and Spectral Solar Irradiance Sensor (TSIS) Spectral Irradiance Monitor (SIM), is designed to measure solar spectral irradiance (SSI) with unprecedented accuracy from 200 nm to 2400 nm. SIM started daily observations in March 2018. To maintain its accuracy over the course of its anticipated 5-year mission and beyond, TSIS SIM needs to be corrected for optical degradation, common for solar viewing instruments. The differing long-term trends of various independent solar-irradiance records attest to the challenge at hand.

The correction of TSIS SIM for optical degradation is based on piecewise linear fits that bring the three instrument channels into agreement. It is fundamentally different to the correction applied to the TSIS SIM predecessor on SORCE. The correction facilitates reproducibility, uncertainty estimation and is measurement-based. Corrected, integrated TSIS SIM SSI agrees with independent observations of total solar irradiance to within 45 ppm as well as various solar-irradiance models. TSIS SIM SSI is available at: http://lasp.colorado.edu/lisird/.

     

太阳风电子动力学不稳定性

电子是太阳风粒子中最为重要的组分之一,它可以通过多种机制对太阳风产生影响.太阳风中的电子通常具有温度各向异性和束流两种非热平衡分布特征,这些偏离热平衡分布的特征可以通过波粒相互作用激发电子不稳定性和等离子体波动,激发的等离子体波动又可以通过波粒相互作用调制太阳风粒子的分布,从而加热太阳风中的背景粒子.因此电子动力学不稳定性在太阳风的演化过程中扮演了极为重要的角色.详细介绍了太阳风中常见的电子动力学不稳定性,并基于等离子体动力论,详细介绍太阳风传播过程中所出现的各种不稳定性,尤其是在近日球层和太阳大气区域所出现的电子声热流不稳定性以及低混杂热流不稳定性,并分析其波粒相互作用机制,以便更加深入地研究太阳风传播过程中的电子分布函数演化.     

Are Homologous Radio Bursts Driven by Solar Post-Flare Loops？

Three particularly complex radio bursts (2001 October 19, 2001 April 10 and 2003 October 26) obtained with the spectrometers (0.65-7.6GHz) at the National Astronomical Observatories, Chinese Academy of Sciences (NAOC, Beijing and Yunnan) and other in- struments (NoRH, TRACE and SXT) are presented. They each have two groups of peaks occurring in different frequency ranges (broad-band microwave and narrow-band decimeter wavelengths). We stress that the second group of burst peaks that occurred in the late phase of the flares and associated with post-flare loops may be homologous radio bursts. We think that they are driven by the post-flare loops. In contrast to the time profiles of the radio bursts and the images of coronal magnetic polarities, we are able to find that the three events are caused by the active regions including main single-bipole magnetic structures, which are associated with multipole magnetic structures during the flare evolutions. In particular, we point out that the later decimetric radio bursts are possibly the radio counterparts of the homologous flares (called "homologous radio bursts" by us), which are also driven by the single-bipole mag- netic structures. By examining the evolutions of the magnetic polarities of sources (17GHz), we could presume that the drivers of the homologous radio bursts are new and/or recurring appearances/disappearances of the magnetic polarities of radio sources, and that the triggers are the magnetic reconnections of single-bipole configurations.     

Hydrodynamics of the Solar Wind Expansion

A pedagogical introduction to the classical hydrodynamic steady state solutions for flows in a spherically symmetric atmosphere is presented along with a detailed analysis of the stability of such flows and a discussion of the Parker/Bondi phase diagram of solutions in the Mach number – stellar radius plane. This leads naturally to the scenario presented by Velli (1994) for wind-accretion transitions, which helps to explain results from numerical simulations in many contexts and gives a more comprehensive answer to the question of why the solar wind and other steady state flows in spherical symetry must quite generally become supersonic.     

Velocity-Space Proton Diffusion in the Solar Wind Turbulence

We study the velocity-space quasi-linear diffusion of the solar wind protons driven by oblique Alfvén turbulence at proton kinetic scales. Turbulent fluctuations at these scales possess the properties of kinetic Alfvén waves (KAWs) that are efficient in Cherenkov-resonant interactions. The proton diffusion proceeds via Cherenkov kicks and forms a quasi-linear plateau – the nonthermal proton tail in the velocity distribution function (VDF). The tails extend in velocity space along the mean magnetic field from 1 to (1.5?–?3) V _A, depending on the spectral break position, on the turbulence amplitude at the spectral break, and on the spectral slope after the break. The most favorable conditions for the tail generation occur in the regions where the proton thermal and Alfvén velocities are about equal, V _Tp/V _A≈1. The estimated formation times are within 1?–?2 h for typical tails at 1 AU, which is much shorter than the solar wind expansion time. Our results suggest that the nonthermal proton tails, observed in situ at all heliocentric distances >?0.3 AU, are formed locally in the solar wind by the KAW turbulence. We also suggest that the bump-on-tail features – proton beams, often seen in the proton VDFs, can be formed at a later evolutional stage of the nonthermal tails by the time-of-flight effects.     

Constraints on the Solar Internal Magnetic Field from a Buoyancy Driven Solar Dynamo

We report here results from a dynamo model developed on the lines of the Babcock-Leighton idea that the poloidal field is generated at the surface of the Sun from the decay of active regions. In this model magnetic buoyancy is handled with a realistic recipe – wherein toroidal flux is made to erupt from the overshoot layer wherever it exceeds a specified critical field B _c (10⁵ G). The erupted toroidal field is then acted upon by the α-effect near the surface to give rise to the poloidal field. In this paper we study the effect of buoyancy on the dynamo generated magnetic fields. Specifically, we show that the mechanism of buoyant eruption and the subsequent depletion of the toroidal field inside the overshoot layer, is capable of constraining the magnitude and distribution of the magnetic field there. We also believe that a critical study of this mechanism may give us new information regarding the solar interior and end with an example, where we propose a method for estimating an upper limit of the difusivity within the overshoot layer. This revised version was published online in July 2006 with corrections to the Cover Date.     

Visibility-Graph Analysis of the Solar Wind Velocity

We analyze in situ measurements of the solar wind velocity obtained by the Advanced Composition Explorer (ACE) and the Helios spacecraft during the years 1998?–?2012 and 1975?–?1983, respectively. The data mainly belong to solar cycles 23 (1996?–?2008) and 21 (1976?–?1986). We used the directed horizontal-visibility-graph (DHVg) algorithm and estimated a graph functional, namely, the degree distance (D), which is defined using the Kullback–Leibler divergence (KLD) to understand the time irreversibility of solar wind time-series. We estimated this degree-distance irreversibility parameter for these time-series at different phases of the solar activity cycle. The irreversibility parameter was first established for known dynamical data and was then applied to solar wind velocity time-series. It is observed that irreversibility in solar wind velocity fluctuations show a similar behavior at 0.3 AU (Helios data) and 1 AU (ACE data). Moreover, the fluctuations change over the phases of the activity cycle.     

Nonstationary Phenomena in the Solar Wind and InterstellarMedium Interaction

We discuss the results of numerical modeling of the solar wind with the inhomogeneous interstellar medium. The density of the plasma component in the interstellar cloud is supposed to be space periodic. The interaction pattern is shown to be highly unsteady with hydrodynamic instabilities developing on the side portion of the heliopause. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Floor in the Solar Wind Magnetic Field Revisited

Svalgaard and Cliver (Astrophys. J. Lett. 661, L203, 2007) proposed that the solar-wind magnetic-field strength [B] at Earth has a ??floor?? value of ??4.6 nT in yearly averages, which is approached but not broached at solar minima. They attributed the floor to a constant baseline solar open flux. In both 2008 and 2009, the notion of such a floor was undercut by annual B averages of ??4 nT. Here we present a revised view of both the level and the concept of the floor. Two independent correlations indicate that B has a floor of ??2.8 nT in yearly averages. These are i) a relationship between solar polar-field strength and yearly averages of B for the last four 11-year minima (B _MIN), and ii) a precursor relationship between peak sunspot number for cycles 14??C?23 and B _MIN at their preceding minima. These correlations suggest that at 11-year minima, B consists of i) a floor of ??2.8 nT, and ii) a component primarily due to the solar polar fields that varies from ??0 nT to ??3 nT. The solar polar fields provide the ??seed?? for the subsequent sunspot maximum. Removing the ??2.8 nT floor from B _MIN brings the percentage decrease in B between the 1996 and 2009 minima into agreement with the corresponding decrease in solar polar-field strength. Based on a decomposition of the solar wind (from 1972??C?2009) into high-speed streams, coronal mass ejections, and slow solar wind, we suggest that the source of the floor in B is the slow solar wind. During 2009, Earth was in slow solar-wind flows ??70% of the time. We propose that the floor corresponds to a baseline (non-cyclic or ground state) open solar flux of ??8×10¹³ Wb, which originates in persistent small-scale (supergranular and granular) field.     

Alpha/proton Instability in the Presence of Proton and Alpha Temperature Anisotropy and its Application to the Deceleration of Alpha Particles in the Solar Wind

Observations of solar wind plasma find that the drift velocity between alpha beams and protons,vα,is reduced with the heliocentric distance,but the ratio of vα ...