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Register a new userElectron Landau Damping of Kinetic Alfven Waves in Simulated Magnetosheath Turbulence
تضبيط الإلكترون اللاندو للأمواج الألففين الحركية في التربولنس المحاكاة لمغناطيس الجزء الخارجي
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تعتقد أن التضخم يلعب دورا في تسخين بلازما الرياح الشمسية، على الرغم من أن العديد من الأسئلة لا تزال متبقية لحلها بشأن طبيعة دقيقة للعوامل القادرة على تحقيق هذا العمل في الهيليوسفير. وخصوصا في الفيزياء الخاصة بالتفاعلات بين الجسيمات والحقول الكهرومغناطيسية التضخمية في مدى التضخم الذي ينتج عنه الضخامة، يبقى ناقصا مفهوما. وقد تم التحليل الأخير لفترة من تلك الملاحظات المتعددة المقياس (MMS) لاستخدام تقنية الارتباط بين الحقل والجسيم لإثبات أن التخمين اللانداو للإلكترون يشمل في ضخامة التضخم في حافظة المغناطيس الأرضية. وباستحثاط من هذا الاكتشاف، نحن نؤدي تحليلا رقميا بدقة عالية للتضخم في فترة MMS للتحقيق في دور التخمين اللانداو للإلكترون في ضخامة الطاقة التضخمية. ونحن نستخدم تقنية الارتباط بين الحقل والجسيم على بيانات المحاكاة الخاصة بنا، ونقارن نتائجنا بالإشارات المعروفة في الفضاء السرعة للتخمين اللانداو خارج مدى الضخامة، ونقيم الطاقة الإلكترونية النهائية. ونجد توافقا مشبها بين النتائج الرقمية والملاحظاتية لبعض النقاط الرئيسية من الطاقة ونتخيل طبيعة الاختلافات في الضوء من العوامل التجريبية، مثل الاختلافات في الدقة، ومن الإشارات المتطورة بشأن طبيعة التفاعلات بين الحقل والجسيم في وجود الموجات الألففين الكينيتية المبعثرة.
Turbulence is thought to play a role in the heating of the solar wind plasma, though many questions remain to be solved regarding the exact nature of the mechanisms driving this process in the heliosphere. In particular, the physics of the collisionless interactions between particles and turbulent electromagnetic fields in the kinetic dissipation range of the turbulent cascade remains incompletely understood. A recent analysis of an interval of Magnetosphere Multiscale (MMS) observations has used the field-particle correlation technique to demonstrate that electron Landau damping is involved in the dissipation of turbulence in the Earths magnetosheath. Motivated by this discovery, we perform a high-resolution gyrokinetic numerical simulation of the turbulence in the MMS interval to investigate the role of electron Landau damping in the dissipation of turbulent energy. We employ the field-particle correlation technique on our simulation data, compare our results to the known velocity-space signatures of Landau damping outside the dissipation range, and evaluate the net electron energization. We find qualitative agreement between the numerical and observational results for some key aspects of the energization and speculate on the nature of disagreements in light of experimental factors, such as differences in resolution, and of developing insights into the nature of field-particle interactions in the presence of dispersive kinetic Alfven waves.
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In the present paper, we investigate the power-law behaviour of the magnetic field spectra in the Earths magnetosheath region using Cluster spacecraft data under solar minimum condition. The power spectral density of the magnetic field data and spectral slopes at various frequencies are analysed. Propagation angle and compressibility are used to test the nature of turbulent fluctuations. The magnetic field spectra have the spectral slopes between -1.5 to 0 down to spatial scales of 20 ion gyroradius and show clear evidence of a transition to steeper spectra for small scales with a second power-law, having slopes between -2.6 to -1.8. At low frequencies, f_sc<0.3f_ci(where f_ci is ion gyro-frequency), propagation angle approximately 90 degrees to the mean magnetic field, B_0, and compressibility shows a broad distribution, 0.1 < R > 0.9. On the other hand at f_sc>10f_ci, the propagation angle exhibits a broad range between 30-90 degree while R has a small variation: 0.2 < R > 0.5. We conjecture that at high frequencies, the perpendicularly propagating Alfven waves could partly explain the statistical analysis of spectra. To support our prediction of kinetic Alfven wave-dominated spectral slope behaviour at high frequency, we also present a theoretical model and simulate the magnetic field turbulence spectra due to the nonlinear evolution of kinetic Alfven waves. The present study also shows the analogy between the observational and simulated spectra.
A Hamiltonian two-field gyrofluid model for kinetic Alfven waves (KAWs) in a magnetized electron-proton plasma, retaining ion finite-Larmor-radius corrections and parallel magnetic field fluctuations, is used to study the inverse cascades that develop when turbulence is randomly driven at sub-ion scales. In the directions perpendicular to the ambient field, the dynamics of the cascade turns out to be nonlocal and the ratio $chi_f$ of the wave period to the characteristic nonlinear time at the driving scale affect some of its properties. For example, at small values of $chi_f$, parametric decay instability of the modes driven by the forcing can develop, enhancing for a while inverse transfers. The balanced state, obtained at early time when the two counter-propagating waves are equally driven, also becomes unstable at small $chi_f$, leading to an inverse cascade. For $beta_e$ smaller than a few units, the cascade slows down when reaching the low-dispersion spectral range. For higher $beta_e$, the ratio of the KAW to the Alfven frequencies displays a local minimum. At the corresponding transverse wavenumber, a condensate is formed, and the cascade towards larger scales is then inhibited. Depending on the parameters, a parallel inverse cascade can develop, enhancing the elongation of the ion-scale magnetic vortices that generically form.
The nature of the plasma wave modes around the ion kinetic scales in highly Alfvenic slow solar wind turbulence is investigated using data from the NASAs Parker Solar Probe taken in the inner heliosphere, at 0.18 Astronomical Unit (AU) from the sun. The joint distribution of the normalized reduced magnetic helicity ${sigma}_m ({theta}_{RB}, {tau})$ is obtained, where ${theta}_{RB}$ is the angle between the local mean magnetic field and the radial direction and ${tau}$ is the temporal scale. Two populations around ion scales are identified: the first population has ${sigma}_m ({theta}_{RB}, {tau}) < 0$ for frequencies (in the spacecraft frame) ranging from 2.1 to 26 Hz for $60^{circ} < {theta}_{RB} < 130^{circ}$, corresponding to kinetic Alfven waves (KAWs), and the second population has ${sigma}_m ({theta}_{RB}, {tau}) > 0$ in the frequency range [1.4, 4.9] Hz for ${theta}_{RB} > 150^{circ}$, corresponding to Alfven ion Cyclotron Waves (ACWs). This demonstrates for the first time the co-existence of KAWs and ACWs in the slow solar wind in the inner heliosphere, which contrasts with previous observations in the slow solar wind at 1 AU. This discrepancy between 0.18 and 1 AU could be explained, either by i) a dissipation of ACWs via cyclotron resonance during their outward journey, or by ii) the high Alfvenicity of the slow solar wind at 0.18 AU that may be favorable for the excitation of ACWs.
We present the first study of the formation and dissipation of current sheets at electron scales in a wave-driven, weakly collisional, 3D kinetic turbulence simulation. We investigate the relative importance of dissipation associated with collisionless damping via resonant wave-particle interactions versus dissipation in small-scale current sheets in weakly collisional plasma turbulence. Current sheets form self-consistently from the wave-driven turbulence, and their filling fraction is well correlated to the electron heating rate. However, the weakly collisional nature of the simulation necessarily implies that the current sheets are not significantly dissipated via Ohmic dissipation. Rather, collisionless damping via the Landau resonance with the electrons is sufficient to account for the measured heating as a function of scale in the simulation, without the need for significant Ohmic dissipation. This finding suggests the possibility that the dissipation of the current sheets is governed by resonant wave-particle interactions and that the locations of current sheets correspond spatially to regions of enhanced heating.
How turbulent energy is dissipated in weakly collisional space and astrophysical plasmas is a major open question. Here, we present the application of a field-particle correlation technique to directly measure the transfer of energy between the turbulent electromagnetic field and electrons in the Earths magnetosheath, the region of solar wind downstream of the Earths bow shock. The measurement of the secular energy transfer from the parallel electric field as a function of electron velocity shows a signature consistent with Landau damping. This signature is coherent over time, close to the predicted resonant velocity, similar to that seen in kinetic Alfven turbulence simulations, and disappears under phase randomisation. This suggests that electron Landau damping could play a significant role in turbulent plasma heating, and that the technique is a valuable tool for determining the particle energisation processes operating in space and astrophysical plasmas.
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