arXiv subjects

Ming Xiong

Publications and source records attributed to Ming Xiong.

26 records · Page 2Linked to original sources

arXiv2013

Using Coordinated Observations in Polarised White Light and Faraday Rotation to Probe the Spatial Position and Magnetic Field of an Interplanetary Sheath

Coronal mass ejections (CMEs) can be continuously tracked through a large portion of the inner heliosphere by direct imaging in visible and radio wavebands. White-light (WL) signatures of solar wind transients, such as CMEs, result from Thomson scattering of sunlight by free electrons, and therefore depend on both the viewing geometry and the electron density. The Faraday rotation (FR) of radio waves from extragalactic pulsars and quasars, which arises due to the presence of such solar wind features, depends on the line-of-sight magnetic field component $B_\parallel$, and the electron density. To understand coordinated WL and FR observations of CMEs, we perform forward magnetohydrodynamic modelling of an Earth-directed shock and synthesise the signatures that would be remotely sensed at a number of widely distributed vantage points in the inner heliosphere. Removal of the background solar wind contribution reveals the shock-associated enhancements in WL and FR. While the efficiency of Thomson scattering depends on scattering angle, WL radiance $I$ decreases with heliocentric distance $r$ roughly according to the expression $I \propto r^{-3}$. The sheath region downstream of the Earth-directed shock is well viewed from the L4 and L5 Lagrangian points, demonstrating the benefits of these points in terms of space weather forecasting. The spatial position of the main scattering site $\mathbf{r}_{\rm sheath}$ and the mass of plasma at that position $M_{\rm sheath}$ can be inferred from the polarisation of the shock-associated enhancement in WL radiance. From the FR measurements, the local $B_{\parallel {\rm sheath}}$ at $\mathbf{r}_{\rm sheath}$ can then be estimated. Simultaneous observations in polarised WL and FR can not only be used to detect CMEs, but also to diagnose their plasma and magnetic field properties.

astro-ph.SR↗

arXiv2012

Effects of Thomson-Scattering Geometry on White-Light Imaging of an Interplanetary Shock: Synthetic Observations from Forward Magnetohydrodynamic Modelling

Stereoscopic white-light imaging of a large portion of the inner heliosphere has been used to track interplanetary coronal mass ejections. At large elongations from the Sun, the white-light brightness depends on both the local electron density and the efficiency of the Thomson-scattering process. To quantify the effects of the Thomson-scattering geometry, we study an interplanetary shock using forward magnetohydrodynamic simulation and synthetic white-light imaging. Identifiable as an inclined streak of enhanced brightness in a time-elongation map, the travelling shock can be readily imaged by an observer located within a wide range of longitudes in the ecliptic. Different parts of the shock front contribute to the imaged brightness pattern viewed by observers at different longitudes. Moreover, even for an observer located at a fixed longitude, a different part of the shock front will contribute to the imaged brightness at any given time. The observed brightness within each imaging pixel results from a weighted integral along its corresponding ray-path. It is possible to infer the longitudinal location of the shock from the brightness pattern in an optical sky map, based on the east-west asymmetry in its brightness and degree of polarization. Therefore, measurement of the interplanetary polarized brightness could significantly reduce the ambiguity in performing three-dimensional reconstruction of local electron density from white-light imaging.

astro-ph.SR↗

arXiv2012

Roles of Fast-Cyclotron and Alfven-Cyclotron Waves for the Multi-Ion Solar Wind

Using linear Vlasov theory of plasma waves and quasi-linear theory of resonant wave-particle interaction, the dispersion relations and the electromagnetic field fluctuations of fast and Alfven waves are studied for a low-beta multi-ion plasma in the inner corona. Their probable roles in heating and accelerating the solar wind via Landau and cyclotron resonances are quantified. We assume that (1) low-frequency Alfven and fast waves have the same spectral shape and the same amplitude of power spectral density; (2) these waves eventually reach ion cyclotron frequencies due to a turbulence cascade; (3) kinetic wave-particle interaction powers the solar wind. The existence of alpha particles in a dominant proton/electron plasma can trigger linear mode conversion between oblique fast-whistler and hybrid alpha-proton cyclotron waves. The fast-cyclotron waves undergo both alpha and proton cyclotron resonances. The alpha cyclotron resonance in fast-cyclotron waves is much stronger than that in Alfven-cyclotron waves. For alpha cyclotron resonance, an oblique fast-cyclotron wave has a larger left-handed electric field fluctuation, a smaller wave number, a larger local wave amplitude, and a greater energization capability than a corresponding Alfven-cyclotron wave at the same wave propagation angle θ, particularly at $80^\circ$ < θ< $90^\circ$. When Alfven-cyclotron or fast-cyclotron waves are present, alpha particles are the chief energy recipient. The transition of preferential energization from alpha particles to protons may be self-modulated by differential speed and temperature anisotropy of alpha particles via the self-consistently evolving wave-particle interaction. Therefore, fast-cyclotron waves as a result of linear mode coupling is a potentially important mechanism for preferential energization of minor ions in the main acceleration region of the solar wind.

astro-ph.SR↗

arXiv2010

Forward modelling to determine the observational signatures of white-light imaging and interplanetary scintillation for the propagation of an interplanetary shock in the ecliptic plane

Recent coordinated observations of interplanetary scintillation (IPS) and stereoscopic heliospheric imagers (HIs) are significant to continuously track the propagation and evolution of solar eruptions throughout interplanetary space. In order to obtain a better understanding of the observational signatures in these two remote-sensing techniques, the magnetohydrodynamics of the macro-scale interplanetary disturbance and the radio-wave scattering of the micro-scale electron-density fluctuation are coupled and investigated using a newly-constructed multi-scale numerical model. This model is then applied to a case of an interplanetary shock propagation within the ecliptic plane. The shock could be nearly invisible to an HI, once entering the Thomson-scattering sphere of the HI. The asymmetry in the optical images between the western and eastern HIs suggests the shock propagation off the Sun-Earth line. Meanwhile, an IPS signal, strongly dependent on the local electron density, is insensitive to the density cavity far downstream of the shock front. When this cavity (or the shock nose) is cut through by an IPS ray-path, a single speed component at the flank (or the nose) of the shock can be recorded; when an IPS ray-path penetrates the sheath between the shock nose and this cavity, two speed components at the sheath and flank can be detected. Moreover, once a shock front touches an IPS ray-path, the derived position and speed at the irregularity source of this IPS signal, together with an assumption of a radial and constant propagation of the shock, can be used to estimate the later appearance of the shock front in the elongation of the HI field of view. The results of synthetic measurements from forward modelling are helpful in inferring the in-situ properties of coronal mass ejection from real observational data via an inverse approach.

astro-ph.SR↗

arXiv2009

Magnetohydrodynamic simulation of the interaction between two interplanetary magnetic clouds and its consequent geoeffectiveness: 2. Oblique collision

The numerical studies of the interplanetary coupling between multiple magnetic clouds (MCs) are continued by a 2.5-dimensional ideal magnetohydrodynamic (MHD) model in the heliospheric meridional plane. The interplanetary direct collision (DC) / oblique collision (OC) between both MCs results from their same/different initial propagation orientations. Here the OC is explored in contrast to the results of the DC (Xiong et al., 2007). Both the slow MC1 and fast MC2 are consequently injected from the different heliospheric latitudes to form a compound stream during the interplanetary propagation. The MC1 and MC2 undergo contrary deflections during the process of oblique collision. Their deflection angles of $|δθ_1|$ and $|δθ_2|$ continuously increase until both MC-driven shock fronts are merged into a stronger compound one. The $|δθ_1|$, $|δθ_2|$, and total deflection angle $Δθ$ ($Δθ= |δθ_1| + |δθ_2|$) reach their corresponding maxima when the initial eruptions of both MCs are at an appropriate angular difference. Moreover, with the increase of MC2's initial speed, the OC becomes more intense, and the enhancement of $δθ_1$ is much more sensitive to $δθ_2$. The $|δθ_1|$ is generally far less than the $|δθ_2|$, and the unusual case of $|δθ_1|\simeq|δθ_2|$ only occurs for an extremely violent OC. But because of the elasticity of the MC body to buffer the collision, this deflection would gradually approach an asymptotic degree. Therefore, the deflection due to the OC should be considered for the evolution and ensuing geoeffectiveness of interplanetary interaction among successive coronal mass ejections (CMEs).

astro-ph.SR↗

arXiv2009

Magnetohydrodynamic Simulation of the Interaction between Interplanetary Strong Shock and Magnetic Cloud and its Consequent Geoeffectiveness

Numerical studies have been performed to interpret the observed "shock overtaking magnetic cloud (MC)" event by a 2.5 dimensional magnetohydrodynamic (MHD) model in heliospheric meridional plane. Results of an individual MC simulation show that the MC travels with a constant bulk flow speed. The MC is injected with very strong inherent magnetic field over that in the ambient flow and expands rapidly in size initially. Consequently, the diameter of MC increases in an asymptotic speed while its angular width contracts gradually. Meanwhile, simulations of MC-shock interaction are also presented, in which both a typical MC and a strong fast shock emerge from the inner boundary and propagate along heliospheric equator, separated by an appropriate interval. The results show that the shock firstly catches up with the preceding MC, then penetrates through the MC, and finally merges with the MC-driven shock into a stronger compound shock. The morphologies of shock front in interplanetary space and MC body behave as a central concave and a smooth arc respectively. The compression and rotation of magnetic field serve as an efficient mechanism to cause a large geomagnetic storm. The MC is highly compressed by the the overtaking shock. Contrarily, the transport time of incidental shock influenced by the MC depends on the interval between their commencements. Maximum geoeffectiveness results from that when the shock enters the core of preceding MC, which is also substantiated to some extent by a corresponding simplified analytic model. Quantified by $Dst$ index, the specific result gives that the geoeffectiveness of an individual MC is largely enhanced with 80% increment in maximum by an incidental shock.

astro-ph.SR↗

arXiv2009

Magnetohydrodynamic Simulation of the Interaction between Interplanetary Strong Shock and Magnetic Cloud and its Consequent Geoeffectiveness 2: Oblique Collision

Numerical studies of the interplanetary "shock overtaking magnetic cloud (MC)" event are continued by a 2.5 dimensional magnetohydrodynamic (MHD) model in heliospheric meridional plane. Interplanetary direct collision (DC)/oblique collision (OC) between an MC and a shock results from their same/different initial propagation orientations. For radially erupted MC and shock in solar corona, the orientations are only determined respectively by their heliographic locations. OC is investigated in contrast with the results in DC \citep{Xiong2006}. The shock front behaves as a smooth arc. The cannibalized part of MC is highly compressed by the shock front along its normal. As the shock propagates gradually into the preceding MC body, the most violent interaction is transferred sideways with an accompanying significant narrowing of the MC's angular width. The opposite deflections of MC body and shock aphelion in OC occur simultaneously through the process of the shock penetrating the MC. After the shock's passage, the MC is restored to its oblate morphology. With the decrease of MC-shock commencement interval, the shock front at 1 AU traverses MC body and is responsible for the same change trend of the latitude of the greatest geoeffectiveness of MC-shock compound. Regardless of shock orientation, shock penetration location regarding the maximum geoeffectiveness is right at MC core on the condition of very strong shock intensity. An appropriate angular difference between the initial eruption of an MC and an overtaking shock leads to the maximum deflection of the MC body. The larger the shock intensity is, the greater is the deflection angle. The interaction of MCs with other disturbances could be a cause of deflected propagation of interplanetary coronal mass ejection (ICME).

astro-ph.SR↗

arXiv2009

Magnetohydrodynamic simulation of the interaction between two interplanetary magnetic clouds and its consequent geoeffectiveness

Numerical studies of the interplanetary "multiple magnetic clouds (Multi-MC)" are performed by a 2.5-dimensional ideal magnetohydrodynamic (MHD) model in the heliospheric meridional plane. Both slow MC1 and fast MC2 are initially emerged along the heliospheric equator, one after another with different time interval. The coupling of two MCs could be considered as the comprehensive interaction between two systems, each comprising of an MC body and its driven shock. The MC2-driven shock and MC2 body are successively involved into interaction with MC1 body. The momentum is transferred from MC2 to MC1. After the passage of MC2-driven shock front, magnetic field lines in MC1 medium previously compressed by MC2-driven shock are prevented from being restored by the MC2 body pushing. MC1 body undergoes the most violent compression from the ambient solar wind ahead, continuous penetration of MC2-driven shock through MC1 body, and persistent pushing of MC2 body at MC1 tail boundary. As the evolution proceeds, the MC1 body suffers from larger and larger compression, and its original vulnerable magnetic elasticity becomes stiffer and stiffer. So there exists a maximum compressibility of Multi-MC when the accumulated elasticity can balance the external compression. With respect to Multi-MC geoeffectiveness, the evolution stage is a dominant factor, whereas the collision intensity is a subordinate one. The magnetic elasticity, magnetic helicity of each MC, and compression between each other are the key physical factors for the formation, propagation, evolution, and resulting geoeffectiveness of interplanetary Multi-MC.

physics.space-ph↗