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| Titel |
Fine velocity structures collisional dissipation in plasmas |
| VerfasserIn |
Oreste Pezzi, Francesco Valentini, Pierluigi Veltri |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250121456
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| Publikation (Nr.) |
 EGU/EGU2016-187.pdf |
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| Zusammenfassung |
| In a weakly collisional plasma, such as the solar wind, collisions are usually considered far
too weak to produce any significant effect on the plasma dynamics [1]. However, the
estimation of collisionality is often based on the restrictive assumption that the
particle velocity distribution function (VDF) shape is close to Maxwellian [2]. On the
other hand, in situ spacecraft measurements in the solar wind [3], as well as kinetic
numerical experiments [4], indicate that marked non-Maxwellian features develop in the
three-dimensional VDFs, (temperature anisotropies, generation of particle beams,
ring-like modulations etc.) as a result of the kinetic turbulent cascade of energy
towards short spatial scales. Therefore, since collisional effects are proportional to the
velocity gradients of the VDF, the collisionless hypothesis may fail locally in velocity
space.
Here, the existence of several characteristic times during the collisional relaxation of fine
velocity structures is investigated by means of Eulerian numerical simulations of a spatially
homogeneous force-free weakly collisional plasma. The effect of smoothing out velocity
gradients on the evolution of global quantities, such as temperature and entropy, is discussed,
suggesting that plasma collisionality can increase locally due to the velocity space
deformation of the particle velocity distribution.
In particular, by means of Eulerian simulations of collisional relaxation of a spatially
homogeneous force-free plasma, in which collisions among particles of the same species are
modeled through the complete Landau operator, we show that the system entropy growth
occurs over several time scales, inversely proportional to the steepness of the velocity
gradients in the VDF. We report clear evidences that fine velocity structures are dissipated by
collisions in a time much shorter than global non-Maxwellian features, like, for
example, temperature anisotropies. Moreover we indicate that, if small-scale structures
in the VDF are artificially smoothed out by fitting the VDF with some analytical
model as, for example, the bi-Maxwellian one, the physics related to small scale
structures (entropy growth, existence of many characteristic times) is definitively
lost.
These results support the idea that high-resolution measurements of the particle velocity
distributions are crucial for an accurate description of weakly collisional systems, such as the
solar wind, in order to answer relevant scientific questions, related, for example, to particle
heating and energization. Future space missions, planned to increase both energy and angular
resolution for the measurements of the particle VDFs, will provide insights for understanding
these processes.
[1] R. Bruno & V. Carbone, Living Rev. Sol. Phys. 2, 4 (2005).
[2] L. Spitzer Jr, Physics of Fully Ionized Gases, (Interscience Publishers, New York, NY,
1956).
[3] E. Marsch, Living Rev. Sol. Phys. 3(1), 1–100 (2006).
[4] S. Servidio, F. Valentini, D. Perrone, A. Greco, F. Califano, W.H. Matthaeus & P. Veltri, J.
Plasma Phys. 81(1), 325810107 (2015). |
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