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Register a new userAstrophysical gyrokinetics: kinetic and fluid turbulent cascades in magnetized weakly collisional plasmas
الجيروكينتيك الفلكي: التدرجات الكينيتية والسائلة المتشوشة في البلازمات المغناطيسية ذات الاصطدامات الضعيفة
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Added by
Alexander Schekochihin
Publication date
2007
fields
Physics
and research's language is
English
Authors
A. A. Schekochihin
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نقدم منظومة نظرية للتضخم البلازما في البلازمات الفلكية (ريح الشمس، الفضاء الفلكي، أسواق المجرات، القروض الانخفاض). وتتضمن الأفتراضات الرئيسية أن التضخم هو عرضي بالنسبة للميل المتوسط للمغناطيس، وأن الترددات تكون بطيئة مقارنة بتردد كرون الأيون. ويجب تحويل الطاقة المضافة في المقياس الخارجي إلى حرارة، والتي لا يمكن القيام بها دون الاصطدامات. وتنشئ كاسكاد الحركي الذي يجلب الطاقة إلى مقاييس الاصطدام في المكان والسرعة. وتعتمد طبيعته على فيزياء الانحرافات البلازمية. في كل من المقاييس الفريدة الماديا يتم تقليل المشكلة الحركية إلى مجموعة من المعادلات الأكثر قابلية للتحليل. في المقياس الغير حركي فوق مقياس جيرو الأيون، يقسم كاسكاد الحركي إلى كاسكاد الافينية التي تحكم بمعادلات الإرشد الحراري المحدود الإلكتروني في مقاييس الاصطدام والغير اصطدامية، وكاسكاد متفاعل من الانحرافات الضغطية التي تطبق معادلة حركية خطية على الخطوط المتحركة المرتبطة بالمكون الافيني. في المقياس الضائع بين مقياس جيرو الأيون والإلكترون، هناك مرتين كاسكاد: كاسكاد الإفيني الحركي (KAW) الذي يحكم بمعادلتي الإرشد الحراري المحدود الإلكتروني المشبعين وكاسكاد متفاعل في الفضاء المرحلي للانحرافات سعادة الأيون. ويجلب هذا الآخر الطاقة من الانحرافات في المقياس الغير حركي التي تم تخميدها بواسطة التفاعل الموجة-جسيم الغير اصطدامي في مقياس جيرو الأيون إلى مقاييس الاصطدام في الفضاء المرحلي والذي يؤدي إلى حرارة الأيون. وتم تخميد طاقة KAW في مقياس جيرو الإلكترون وتحويلها إلى حرارة الإلكترون. وتم تحديد علاقات التشغيل النوعية لكولموغوروف لهذه الكاسكاد. وتم مناقشة التطبيقات الفلكية والفضائية بتفصيل.
We present a theoretical framework for plasma turbulence in astrophysical plasmas (solar wind, interstellar medium, galaxy clusters, accretion disks). The key assumptions are that the turbulence is anisotropic with respect to the mean magnetic field and frequencies are low compared to the ion cyclotron frequency. The energy injected at the outer scale scale has to be converted into heat, which ultimately cannot be done without collisions. A KINETIC CASCADE develops that brings the energy to collisional scales both in space and velocity. Its nature depends on the physics of plasma fluctuations. In each of the physically distinct scale ranges, the kinetic problem is systematically reduced to a more tractable set of equations. In the inertial range above the ion gyroscale, the kinetic cascade splits into a cascade of Alfvenic fluctuations, which are governed by the RMHD equations at both the collisional and collisionless scales, and a passive cascade of compressive fluctuations, which obey a linear kinetic equation along the moving field lines associated with the Alfvenic component. In the dissipation range between the ion and electron gyroscales, there are again two cascades: the kinetic-Alfven-wave (KAW) cascade governed by two fluid-like Electron RMHD equations and a passive phase-space cascade of ion entropy fluctuations. The latter cascade brings the energy of the inertial-range fluctuations that was damped by collisionless wave-particle interaction at the ion gyroscale to collisional scales in the phase space and leads to ion heating. The KAW energy is similarly damped at the electron gyroscale and converted into electron heat. Kolmogorov-style scaling relations are derived for these cascades. Astrophysical and space-physical applications are discussed in detail.
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The physical foundations of the dissipation of energy and the associated heating in weakly collisional plasmas are poorly understood. Here, we compare and contrast several measures that have been used to characterize energy dissipation and kinetic-scale conversion in plasmas by means of a suite of kinetic numerical simulations describing both magnetic reconnection and decaying plasma turbulence. We adopt three different numerical codes that can also include interparticle collisions: the fully kinetic particle-in-cell VPIC, the fully kinetic continuum Gkeyll, and the Eulerian Hybrid Vlasov-Maxwell (HVM) code. We differentiate between (i) four energy-based parameters, whose definition is related to energy transfer in a fluid description of a plasma, and (ii) four distribution function-based parameters, requiring knowledge of the particle velocity distribution function. There is an overall agreement between the dissipation measures obtained in the PIC and continuum reconnection simulations, with slight differences due to the presence/absence of secondary islands in the two simulations. There are also many qualitative similarities between the signatures in the reconnection simulations and the self-consistent current sheets that form in turbulence, although the latter exhibits significant variations compared to the reconnection results. All the parameters confirm that dissipation occurs close to regions of intense magnetic stresses, thus exhibiting local correlation. The distribution function-based measures show a broader width compared to energy-based proxies, suggesting that energy transfer is co-localized at coherent structures, but can affect the particle distribution function in wider regions. The effect of interparticle collisions on these parameters is finally discussed.
We present a theoretical framework for describing electromagnetic kinetic turbulence in a multi-species, magnetized, pressure-anisotropic plasma. Turbulent fluctuations are assumed to be small compared to the mean field, to be spatially anisotropic with respect to it, and to have frequencies small compared to the ion cyclotron frequency. At scales above the ion Larmor radius, the theory reduces to the pressure-anisotropic generalization of kinetic reduced magnetohydrodynamics (KRMHD) formulated by Kunz et al. (2015). At scales at and below the ion Larmor radius, three main objectives are achieved. First, we analyse the linear response of the pressure-anisotropic gyrokinetic system, and show it to be a generalisation of previously explored limits. The effects of pressure anisotropy on the stability and collisionless damping of Alfvenic and compressive fluctuations are highlighted, with attention paid to the spectral location and width of the frequency jump that occurs as Alfven waves transition into kinetic Alfven waves. Secondly, we derive and discuss a general free-energy conservation law, which captures both the KRMHD free-energy conservation at long wavelengths and dual cascades of kinetic Alfven waves and ion entropy at sub-ion-Larmor scales. We show that non-Maxwellian features in the distribution function change the amount of phase mixing and the efficiency of magnetic stresses, and thus influence the partitioning of free energy amongst the cascade channels. Thirdly, a simple model is used to show that pressure anisotropy can cause large variations in the ion-to-electron heating ratio due to the dissipation of Alfvenic turbulence. Our theory provides a foundation for determining how pressure anisotropy affects the turbulent fluctuation spectra, the differential heating of particle species, and the ratio of parallel and perpendicular phase mixing in space and astrophysical plasmas.
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