Kinetic Theory and Fast Wind Observations of the Electron Strahl

We develop a model for the strahl population in the solar wind -- a narrow, low-density and high-energy electron beam centered on the magnetic field direction. Our model is based on the solution of the electron drift-kinetic equation at heliospheric distances where the plasma density, temperature, and the magnetic field strength decline as power-laws of the distance along a magnetic flux tube. Our solution for the strahl depends on a number of parameters that, in the absence of the analytic solution for the full electron velocity distribution function (eVDF), cannot be derived from the theory. We however demonstrate that these parameters can be efficiently found from matching our solution with observations of the eVDF made by the Wind satellite's SWE strahl detector. The model is successful at predicting the angular width (FWHM) of the strahl for the Wind data at 1 AU, in particular by predicting how this width scales with particle energy and background density. We find the strahl distribution is largely determined by the local temperature Knudsen number $\gamma \sim |T dT/dx|/n$, which parametrizes solar wind collisionality. We compute averaged strahl distributions for typical Knudsen numbers observed in the solar wind, and fit our model to these data. The model can be matched quite closely to the eVDFs at 1 AU, however, it then overestimates the strahl amplitude at larger heliocentric distances. This indicates that our model may be improved through the inclusion of additional physics, possibly through the introduction of "anomalous diffusion" of the strahl electrons

Wave-particle energy exchange directly observed in a kinetic Alfvén-branch wave

Alfvén waves are fundamental plasma wave modes that permeate the universe. At small kinetic scales, they provide a critical mechanism for the transfer of energy between electromagnetic fields and charged particles. These waves are important not only in planetary magnetospheres, heliospheres and astrophysical systems but also in laboratory plasma experiments and fusion reactors. Through measurement of charged particles and electromagnetic fields with NASA’s Magnetospheric Multiscale (MMS) mission, we utilize Earth’s magnetosphere as a plasma physics laboratory. Here we confirm the conservative energy exchange between the electromagnetic field fluctuations and the charged particles that comprise an undamped kinetic Alfvén wave. Electrons confined between adjacent wave peaks may have contributed to saturation of damping effects via nonlinear particle trapping. The investigation of these detailed wave dynamics has been unexplored territory in experimental plasma physics and is only recently enabled by high-resolution MMS observations

Excitation of ion-acoustic waves by non-linear finite-amplitude standing Alfv\'en waves

We investigate, using a multi-fluid approach, the main properties of standing ion-acoustic modes driven by nonlinear standing Alfv\'en waves. The standing character of the Alfv\'enic pump is because we study the superposition of two identical circularly polarised counter-propagating waves. We consider parallel propagation along the constant magnetic field and we find that left and right-handed modes generate via ponderomotive forces the second harmonic of standing ion-acoustic waves. We demonstrate that parametric instabilities are not relevant in the present problem and the secondary ion-acoustic waves attenuate by Landau damping in the absence of any other dissipative process. Kinetic effects are included in our model where ions are considered as particles and electrons as a massless fluid, and hybrid simulations are used to complement the theoretical results. Analytical expressions are obtained for the time evolution of the different physical variables in the absence of Landau damping. From the hybrid simulations we find that the attenuation of the generated ion-acoustic waves follows the theoretical predictions even under the presence of a driver Alfv\'enic pump. Due to the nonlinear induced ion-acoustic waves the system develops density cavities and an electric field parallel to the magnetic field. Theoretical expressions for this density and electric field fluctuations are derived. The implications of these results in the context of standing slow mode oscillations in coronal loops is discussed

Interplay between Anisotropy- and Skewness-driven Whistler Instabilities in the Solar Wind under the Core–Strahlo Model

Temperature anisotropy and field-aligned skewness are commonly observed nonthermal features in electron velocity distributions in the solar wind. These characteristics can act as a source of free energy to destabilize different electromagnetic wave modes, which may alter the plasma state through wave–particle interactions. Previous theoretical studies have mainly focused on analyzing these nonthermal features and self-generated instabilities individually. However, to obtain a more accurate and realistic understanding of the kinetic processes in the solar wind, it is necessary to examine the interplay between these two energy sources. By means of linear kinetic theory, in this paper we investigate the excitation of the parallel propagating whistler mode, when it is destabilized by electron populations exhibiting both temperature anisotropy and field-aligned strahl or skewness. To describe the solar wind electrons, we adopt the core–strahlo model as an alternative approach. This model offers the advantage of representing the suprathermal features of halo and strahl electrons, using a single skew–kappa distribution already known as the strahlo population. Our findings show that when the electron strahlo exhibits an intrinsic temperature anisotropy, this suprathermal population becomes a stronger and more efficient source of free energy for destabilizing the whistler mode. This suggests the greater involvement of the anisotropic strahlo in processes conditioned by wave–particle interactions. The present results also suggest that the contribution of core anisotropy can be safely disregarded when assessing the importance of instabilities driven by the suprathermal population. This allows for a focused study, particularly regarding the regulation of the electron heat flux in the solar wind

Magnetic Alfvén-cyclotron fluctuations of anisotropic nonthermal plasmas

©2015. American Geophysical Union. All Rights Reserved. Remote and in situ observations in the solar wind show that ion and electron velocity distributions persistently present deviations from thermal equilibrium. Ion anisotropies seem to be constrained by instability thresholds which are in agreement with linear kinetic theory. For plasma states below these instability thresholds, the quasi-stable solar wind plasma sustains a small but detectable level of magnetic fluctuation power. These fluctuations may be related to spontaneous electromagnetic fluctuations arising from the discreteness and thermal motion of charged particles. Here we study magnetic Alfvén-cyclotron fluctuations propagating along a background magnetic field in a plasma composed of thermal and suprathermal protons and electrons via the fluctuation-dissipation theorem. The total fluctuating magnetic power is estimated in a proton temperature anisotropy-beta diagram for three different families of proton distribution

Weak kinetic Alfven waves turbulence during the 14November2012 geomagnetic storm: Van Allen Probes observations

Artículo de publicación ISIIn the dawn sector, L approximate to 5.5 and MLT approximate to 4-7, from 01:30 to 06:00UT during the 14 November 2012 geomagnetic storm, both Van Allen Probes observed an alternating sequence of locally quiet and disturbed intervals with two strikingly different power fluctuation levels and magnetic field orientations: either small (approximate to 10(-2)nT(2)) total power with strong GSM B-x and weak B-y or large (approximate to 10nT(2)) total power with weak B-x and strong B-y and B-z components. During both kinds of intervals the fluctuations occur in the vicinity of the local ion gyrofrequencies (0.01-10Hz) in the spacecraft frame, propagate oblique to the magnetic field, ( approximate to 60 degrees), and have magnetic compressibility C=|B-vertical bar|/|B|approximate to 1, where B-vertical bar(B) are the average amplitudes of the fluctuations parallel(perpendicular) to the mean field. Electric field fluctuations are present whenever the magnetic field is disturbed, and large electric field fluctuations follow the same pattern for quiet and disturbed intervals. Magnetic frequency power spectra at both spacecraft correspond to steep power laws approximate to f(-) with 4 < < 5 for f less than or similar to 2Hz, and 1.1 < < 1.7 for f 2Hz, spectral profiles that are consistent with weak kinetic Alfven wave (KAW) turbulence. Electric power is larger than magnetic power for all frequencies above 0.1Hz, and the ratio increases with increasing frequency. Vlasov linear analysis is consistent with the presence of compressive KAW with ki less than or similar to 1, right-handed polarization and positive magnetic helicity, in the plasma frame, considering a multiion plasma. All these results suggest the presence of weak KAW turbulence which dissipates the energy associated with the intermittent sudden changes in the magnetic field during the main phase of the storm.JHU/APL contract under NASA Prime contract 921647 NAS5-0107

Alfvenic Fluctuations Associated with Jupiter's Auroral Emissions

The Alfvn wave mode transmits fieldaligned currents and largescale turbulence throughout Jupiter's magnetosphere. Magnetometer data from the Juno spacecraft have provided the first observations of Alfvnic fluctuations along the polar magnetic flux tubes connected to Jupiter's main auroral oval and the Jovian satellites. Transverse magnetic field perturbations associated with Io are observed up to ~90 away from main Io footprint, supporting the presence of extended Alfvnic wave activity throughout the Io footprint tail. Additional broadband fluctuations measured equatorward of the statistical auroral oval are composed of incompressible magnetic turbulence that maps to Jupiter's equatorial plasma sheet at radial distances within ~20 R(sub J). These fluctuations exhibit a k(sub ||) power spectrum consistent with strong magnetohydrodynamic turbulence. This turbulence can generate up to ~100 mW/m(exp 2) of Poynting flux to power the Jovian aurora in regions connected to the inner magnetosphere's central plasma sheet