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| Titel |
Hybrid Vlasov-Maxwell simulations of phase mixing as a mechanism to produce kinetic Alfvén waves |
| VerfasserIn |
Christian Vasconez, Francesco Valentini, Francesco Malara, Sergio Servidio, Francesco Pucci, Pierluigi Veltri |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250101544
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| Publikation (Nr.) |
 EGU/EGU2015-703.pdf |
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| Zusammenfassung |
| In the range of proton kinetic scales and even down to typical electron kinetic scales, many
solar-wind observational analyses, theoretical works and numerical simulations suggest that
the so-called kinetic Alfvén waves (KAWs) can play an important role in the mechanisms of
turbulent energy dissipation and heating. Mathematically, KAWs are a wave solution of the
two-fluid dispersion relation, in propagation quasi-perpendicular to a background magnetic
field. They can be considered as the continuation of the magnetohydrodynamic (MHD)
Alfvén mode at scales comparable to the ion skin depth, and/or the ion Larmour radius.
Recently, kinetic numerical simulations of these kind of waves in solar-wind environment
conditions suggested that resonant protons can be trapped by a pseudo-potential (Vásconez et
al., 2014), resembling the process of particle trapping in an electrostatic potential well. This
nonlinear resonant interaction produces significant deformations of the proton velocity
distributions, through the generation of flat-top profiles, frequently recovered in spacecraft
measurements.
Considering their high characteristic angles of propagation, KAWs have been invoked to
explain the excess of perpendicular energy in observational data of the solar-wind magnetic
field. So far, the physical mechanisms responsible for the production of the KAWs are still on
debate. Phase-mixing is a mechanism that creates small scale Alfvén fluctuations
in a direction transverse to a background magnetic field. When an Alfvén wave
propagates in a region where an Alfvén speed shear is present, the wave experiences
phase-mixing and small scale Alfvén fluctuations are produced in the direction
perpendicular to the background magnetic field. This phenomenon was first studied by
Heyvaerts and Priests, in 1982, in the MHD framework. These authors pointed out that
phase-mixing is the most efficient mechanism to ensure the dissipation of shear Alfvén
waves.
Since phase-mixing can generate quasi-perpendicular Alfvén fluctuations, in this work
we investigate whether it can triggers KAWs excitation. To this purpose, we need to
study this physical process in a range of scale that goes at least from the smallest
scales of MHD to a fraction of the ion skin depth, in linear and nonlinear regimes
of wave propagation. To resolve the kinetic scales, we use the low noise hybrid
Vlasov-Maxwell (HVM) code (Valentini et al., 2007) and an Hall-MHD code to
control the smallest MHD scales. For the linear regime, we present some physical
properties of the perturbations produced after the Alfvén speed shear, which are in good
agreement with those discussed by, e.g., Vásconez et al., in 2014 (and references
therein).
Then, moving to the nonlinear regime, the comparison of the results of Hall-MHD and
HVM simulations allow to point out the kinetic effects at play during the phase mixing
process. |
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