scholarly journals Inertial-range kinetic turbulence in pressure-anisotropic astrophysical plasmas

2015 ◽  
Vol 81 (5) ◽  
Author(s):  
M. W. Kunz ◽  
A. A. Schekochihin ◽  
C. H. K. Chen ◽  
I. G. Abel ◽  
S. C. Cowley
Keyword(s):  
Solar Wind ◽  
Free Energy ◽  
Distribution Function ◽  
Low Frequency ◽  
Energetic Cost ◽  
Astrophysical Plasmas ◽  
Order Unity ◽  
Quantitative Measurements ◽  
Maxwellian Plasma ◽  
Field Lines

A theoretical framework for low-frequency electromagnetic (drift-)kinetic turbulence in a collisionless, multi-species plasma is presented. The result generalises reduced magnetohydrodynamics (RMHD) and kinetic RMHD (Schekochihinet al.,Astrophys. J. Suppl. Ser., vol. 182, 2009, pp. 310–377) to the case where the mean distribution function of the plasma is pressure-anisotropic and different ion species are allowed to drift with respect to each other – a situation routinely encountered in the solar wind and presumably ubiquitous in hot dilute astrophysical plasmas such as the intracluster medium. Two main objectives are achieved. First, in a non-Maxwellian plasma, the relationships between fluctuating fields (e.g. the Alfvén ratio) are order-unity modified compared to the more commonly considered Maxwellian case, and so a quantitative theory is developed to support quantitative measurements now possible in the solar wind. Beyond these order-unity corrections, the main physical feature of low-frequency plasma turbulence survives the generalisation to non-Maxwellian distributions: Alfvénic and compressive fluctuations are energetically decoupled, with the latter passively advected by the former; the Alfvénic cascade is fluid, satisfying RMHD equations (with the Alfvén speed modified by pressure anisotropy and species drifts), whereas the compressive cascade is kinetic and subject to collisionless damping (and for a bi-Maxwellian plasma splits into three independent collisionless cascades). Secondly, the organising principle of this turbulence is elucidated in the form of a conservation law for the appropriately generalised kinetic free energy. It is shown that non-Maxwellian features in the distribution function reduce the rate of phase mixing and the efficacy of magnetic stresses, and that these changes influence the partitioning of free energy amongst the various cascade channels. As the firehose or mirror instability thresholds are approached, the dynamics of the plasma are modified so as to reduce the energetic cost of bending magnetic-field lines or of compressing/rarefying them. Finally, it is shown that this theory can be derived as a long-wavelength limit of non-Maxwellian slab gyrokinetics.

scholarly journals Characterization of ultra low frequency (ULF) pulsations and the investigation of their possible source

2009 ◽  
Vol 27 (8) ◽  
pp. 3287-3296 ◽  
Author(s):  
S. H. Mthembu ◽  
S. B. Malinga ◽  
A. D. M. Walker ◽  
L. Magnus
Keyword(s):  
Solar Wind ◽  
Dynamic Pressure ◽  
Temporal Structure ◽  
Sudden Change ◽  
Low Frequency ◽  
Hf Radar ◽  
Doppler Velocity ◽  
Full Field ◽  
Spectral Techniques ◽  
Field Lines

Abstract. In this paper we present the results from the observation of ultra low frequency (ULF) pulsations in the Doppler velocity data from SuperDARN HF radar located at Goose Bay (61.94° N, 23.02° E, geomagnetic). Fourier spectral techniques were used to determine the spectral content of the data and the results show Pc 5 ULF pulsations (with a frequency range of 1 to 4 mHz) where the magnetic field lines were oscillating at discrete frequencies of about 1.3 and 1.9 mHz. These pulsations are classified as field lines resonance (FLR) since the 1.9 mHz component exhibited an enhancement in amplitude with an associated phase change of approximately 180° across a resonance latitude of 71.3°. The spatial and temporal structure of the ULF pulsations was examined by investigating their instantaneous amplitude which was calculated as the amplitude of the analytic signal. The results presented a full field of view which exhibit pulsations activity simultaneously from all beams. This representation shows that the peak amplitude of the 1.9 mHz component was observed over the longitudinal range of 13°. The temporal structure of the pulsations was investigated from the evolution of the 1.9 mHz component and the results showed that the ULF pulsations had a duration of about 1 h. Wavelet analysis was used to investigate solar wind as a probable source of the observed ULF pulsations. The time delay compared well with the solar wind travel time estimates and the results suggest a possible link between the solar wind and the observed pulsations. The sudden change in dynamic pressure also proved to be a possible source of the observed ULF pulsations.

scholarly journals Low-frequency magnetic variations at the high-<i>β</i> Earth bow shock

2019 ◽  
Vol 37 (5) ◽  
pp. 877-889
Author(s):  
Anatoli A. Petrukovich ◽  
Olga M. Chugunova ◽  
Pavel I. Shustov
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Spatial Scales ◽  
Low Frequency ◽  
Background Field ◽  
Bow Shock ◽  
Stable Phase ◽  
Main Magnetic Field ◽  
Astrophysical Plasmas ◽  
Earth’S Bow Shock

Abstract. Observations of Earth's bow shock during high-β (ratio of thermal to magnetic pressure) solar wind streams are rare. However, such shocks are ubiquitous in astrophysical plasmas. Typical solar wind parameters related to high β (here β>10) are as follows: low speed, high density, and a very low interplanetary magnetic field of 1–2 nT. These conditions are usually quite transient and need to be verified immediately upstream of the observed shock crossings. In this report, three characteristic crossings by the Cluster project (from the 22 found) are studied using multipoint analysis, allowing us to determine spatial scales. The main magnetic field and density spatial scale of about a couple of hundred of kilometers generally corresponds to the increased proton convective gyroradius. Observed magnetic variations are different from those for supercritical shocks, with β∼1. Dominant magnetic variations in the shock transition have amplitudes much larger than the background field and have a frequency of ∼ 0.3–0.5 Hz (in some events – 1–2 Hz). The wave polarization has no stable phase and is closer to linear, which complicates the determination of the wave propagation direction. Spatial scales (wavelengths) of variations are within several tens to a couple of hundred of kilometers.

Recent observations of magnetic cavities: from MHD to kinetic scale

2020 ◽  
Author(s):  
Quanqi Shi ◽  
Et al
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Distribution Function ◽  
Electron Distribution Function ◽  
Circular Cross Section ◽  
High Energy ◽  
Space Physics ◽  
Small Scale ◽  
Astrophysical Plasmas ◽  
Magnetic Bottle

&lt;p&gt;Magnetic cavities, also termed magnetic holes, dips or depression structures, have an observable magnetic field decrease in a short time span and have been widely observed in the solar wind plasmas, comet magnetospheres, terrestrial/planetary magnetosheaths, magnetospheric cusps and magnetotail plasmas since 1970s. In early observations, the structures were found in MHD scale, from tens to thousands of &amp;#961;i (proton gyroradius) with corresponding temporal scales from seconds to tens of minutes. Later, kinetic scale magnetic cavities were detected in the earth&amp;#8217;s magnetotail and magnetosheath, with size less than &amp;#961;i and sometimes close to several &amp;#961;e (electron gyroradius) and often associated with a significant electron vortex around the structure. Surprisingly, it has been found that such a small structure contains an abundance of phenomena, including different kinds of ion and electron distributions, electron or ion vortices, various types of waves, and even particle acceleration and declarations. In this presentation, we will show our recent observations of magnetic cavities from MHD scale to kinetic scale in the solar wind, magnetosheath, cusp and magnetotail. In the magnetosheath, downstream of the bow shock, the mirror mode instability can generate magnetic dip and peak trains. Using data from the new NASA satellite constellation MMS, we have found that electrons exhibit a new &amp;#8216;donut&amp;#8217; shaped distribution function related to particle deceleration processes. Using boundary normal and velocity determination techniques, we found that MHD scale magnetic cavity structures can expand or shrink, and they can enter the cusp regions along with the entry plasmas. In the turbulent magnetosheath and quiet magnetotail, we have observed kinetic scale magnetic cavity structures with scales comparable or less than one &amp;#961;i. An EMHD model and other theories will also be introduced and compared. We found that in the sheath the electron scale magnetic cavity has a circular cross section and it is a magnetic bottle in 3-D. We have also found that these structures shrink due to increases in the surrounding magnetic field, and this shrinkage of the small scale magnetic cavity can induce an electric field that accelerates the electrons to a significantly higher energy. Qualitatively distinct from other acceleration mechanisms, this process indicates a new type of non-adiabetic acceleration, and has been confirmed by the observed electron distribution function and test particle simulations. This discovery in space physics also has implications for understanding energy conversion in astrophysical plasmas, the origin of cosmic high-energy particles and plasma turbulence.&lt;/p&gt;

scholarly journals Low frequency magnetic variations at high-<i>β</i> Earth bow shocks

2019 ◽  
Author(s):  
Anatoli A. Petrukovich ◽  
Olga M. Chugunova ◽  
Pavel I. Shustov
Keyword(s):  
Solar Wind ◽  
Spatial Scales ◽  
Low Frequency ◽  
Stable Phase ◽  
Main Magnetic Field ◽  
Astrophysical Plasmas ◽  
Survey Statistics ◽  
Earth’S Bow Shock ◽  
Bow Shocks ◽  
High Mach

Abstract. Earth's bow shock in high β (ratio of thermal to magnetic pressure) solar wind environment is a rare phenomenon. However such an object is ubiquitous in astrophysical plasmas. Typical solar wind parameters related with high β (here β > 10) are: low speed, high density and very low IMF 1–2 nT. These conditions are usually quite transient and need to be verified immediately upstream of the observed shock crossings. We survey statistics of high-β shock observations by near-Earth spacecraft since 1995. About a hundred crossings were initially identified mostly with oblique or quasi-perpendicular geometry and high Mach number. In this report 22 crossings by Cluster project are studied with multipoint analysis, allowing to determine spatial scales. Observed shock structure is different from that for supercritical shocks with β ~ 1. The main magnetic field increase is smeared to couple tens of seconds and is dominated with magnetic variations ~ 0.1–0.5 Hz (in some events – 1–2 Hz). Their polarization has no stable phase and is closer to linear, while spatial scales are of the order of hundred km at 0.1–0.5 Hz.

SAMPL6 Octanol-Water Partition Coefficients from Alchemical Free Energy Calculations with MBIS Atomic Charges

2019 ◽  
Author(s):  
Maximiliano Riquelme ◽  
Esteban Vöhringer-Martinez
Keyword(s):  
Free Energy ◽  
Electron Density ◽  
Free Energy Calculations ◽  
Basis Set ◽  
Energetic Cost ◽  
Atomic Charges ◽  
Free Energies ◽  
Energy Calculations ◽  
Alchemical Free Energy ◽  
Alchemical Free Energy Calculations

In molecular modeling the description of the interactions between molecules forms the basis for a correct prediction of macroscopic observables. Here, we derive atomic charges from the implicitly polarized electron density of eleven molecules in the SAMPL6 challenge using the Hirshfeld-I and Minimal Basis Set Iterative Stockholder(MBIS) partitioning method. These atomic charges combined with other parameters in the GAFF force field and different water/octanol models were then used in alchemical free energy calculations to obtain hydration and solvation free energies, which after correction for the polarization cost, result in the blind prediction of the partition coefficient. From the tested partitioning methods and water models the S-MBIS atomic charges with the TIP3P water model presented the smallest deviation from the experiment. Conformational dependence of the free energies and the energetic cost associated with the polarization of the electron density are discussed.

scholarly journals Cancellation exponents and multifractal scaling laws in the solar wind magnetohydrodynamic turbulence

1996 ◽  
Vol 14 (8) ◽  
pp. 777-785 ◽  
Author(s):  
V. Carbone ◽  
R. Bruno
Keyword(s):  
Solar Wind ◽  
Velocity Field ◽  
Scaling Laws ◽  
Interplanetary Medium ◽  
Low Frequency ◽  
Scaling Exponent ◽  
Magnetohydrodynamic Turbulence ◽  
Low Speed ◽  
Multifractal Scaling

Abstract. Some signed measures in turbulence are found to be sign-singular, that is their sign reverses continuously on arbitrary finer scales with a reduction of the cancellation between positive and negative contributions. The strength of the singularity is characterized by a scaling exponent κ, the cancellation exponent. In the present study by using some turbulent samples of the velocity field obtained from spacecraft measurements in the interplanetary medium, we show that sign-singularity is present everywhere in low-frequency turbulent samples. The cancellation exponent can be related to the characteristic scaling laws of turbulence. Differences in the values of κ, calculated in both high- and low-speed streams, allow us to outline some physical differences in the samples with different velocities.

Low-frequency waves in the solar wind near Neptune

1991 ◽  
Vol 18 (6) ◽  
pp. 1071-1074 ◽  
Author(s):  
Ming Zhang ◽  
J. W. Belcher ◽  
J. D. Richardson ◽  
V. M. Vasyliunas ◽  
R. P. Lepping ◽  
...  
Keyword(s):  
Solar Wind ◽  
Low Frequency ◽  
Low Frequency Waves

Foreshock ULF fluctuations near the Moon: THEMIS observations

2021 ◽  
Author(s):  
Anna Salohub ◽  
Jana Šafránková ◽  
Zdeněk Němeček
Keyword(s):  
Solar Wind ◽  
Rest Frame ◽  
Low Frequency ◽  
Solar Wind Plasma ◽  
Turbulent Plasma ◽  
Bow Shock ◽  
Ulf Waves ◽  
The Moon ◽  
Shock Surface ◽  
The Earth

&lt;p&gt;The foreshock is a region filled with a turbulent plasma located upstream the Earth&amp;#8217;s bow shock where interplanetary magnetic field (IMF) lines are connected to the bow shock surface. In this region, ultra-low frequency (ULF) waves are generated due to the interaction of the solar wind plasma with particles reflected from the bow shock back into the solar wind. It is assumed that excited waves grow and they are convected through the solar wind/foreshock, thus the inner spacecraft (close to the bow shock) would observe larger wave amplitudes than the outer (far from the bow shock) spacecraft. The paper presents a statistical analysis of excited ULF fluctuations observed simultaneously by two closely separated THEMIS spacecraft orbiting the Moon under a nearly radial IMF. We found that ULF fluctuations (in the plasma rest frame) can be characterized as a mixture of transverse and compressional modes with different properties at both locations. We discuss the growth and/or damping of ULF waves during their propagation.&lt;/p&gt;

Foreshock wave transmission into the magnetosheath and magnetosphere: results from global hybrid-Vlasov simulations

2021 ◽  
Author(s):  
Lucile Turc ◽  
Markus Battarbee ◽  
Urs Ganse ◽  
Andreas Johlander ◽  
Yann Pfau-Kempf ◽  
...  
Keyword(s):  
Solar Wind ◽  
Mach Number ◽  
Cone Angle ◽  
Low Frequency ◽  
Compressional Wave ◽  
Wave Transmission ◽  
Wave Power ◽  
Solar Wind Flow ◽  
Magnetic Pulsations ◽  
Wave Properties

&lt;p&gt;The foreshock, extending upstream of the quasi-parallel shock and populated with shock-reflected particles, is home to intense wave activity in the ultra-low frequency range.&lt;em&gt; &lt;/em&gt;The most commonly observed of these waves are the &amp;#8220;30 s&amp;#8221; waves, fast magnetosonic waves propagating sunward in the plasma rest frame, but carried earthward by the faster solar wind flow. These waves are thought to be the main source of Pc3 magnetic pulsations (10 &amp;#8211; 45 s) in the dayside magnetosphere. A handful of case studies with suitable spacecraft conjunctions have allowed simultaneous investigations of the wave properties in different geophysical regions, but the global picture of the wave transmission from the foreshock through the magnetosheath into the magnetosphere is still not known. In this work, we use global simulations performed with the hybrid-Vlasov model Vlasiator to study the Pc3 wave properties in the foreshock, magnetosheath and magnetosphere for different solar wind conditions. We find that in all three regions the wave power peaks at higher frequencies when the interplanetary magnetic field strength is larger, consistent with previous studies. While the transverse wave power decreases with decreasing Alfv&amp;#233;n Mach number in the foreshock, the compressional wave power shows little variation. In contrast, in the magnetosheath and the magnetosphere, the compressional wave power decreases with decreasing Mach number. Inside the magnetosphere, the distribution of wave power varies with the IMF cone angle. We discuss the implications of these results for the propagation of foreshock waves across the different geophysical regions, and in particular their transmission through the bow shock.&lt;/p&gt;

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