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| Titel |
The strange physics of low frequency mirror mode turbulence in the high temperature plasma of the magnetosheath |
| VerfasserIn |
R. A. Treumann, C. H. Jaroschek, O. D. Constantinescu, R. Nakamura, O. A. Pokhotelov, E. Georgescu |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1023-5809
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| Digitales Dokument |
URL |
| Erschienen |
In: Nonlinear Processes in Geophysics ; 11, no. 5/6 ; Nr. 11, no. 5/6 (2004-12-13), S.647-657 |
| Datensatznummer |
250008994
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| Publikation (Nr.) |
 copernicus.org/npg-11-647-2004.pdf |
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| Zusammenfassung |
| Mirror mode turbulence is the lowest frequency perpendicular
magnetic excitation in magnetized plasma proposed already about
half a century ago by Rudakov and Sagdeev (1958) and
Chandrasekhar et al. (1958) from fluid theory. Its experimental
verification required a relatively long time. It was early
recognized that mirror modes for being excited require a
transverse pressure (or temperature) anisotropy. In principle
mirror modes are some version of slow mode waves. Fluid theory,
however, does not give a correct physical picture of the mirror
mode. The linear infinitesimally small amplitude physics is
described correctly only by including the full kinetic theory and
is modified by existing spatial gradients of the plasma parameters
which attribute a small finite frequency to the mode. In addition,
the mode is propagating only very slowly in plasma such that
convective transport is the main cause of flow in it. As the
lowest frequency mode it can be expected that mirror modes serve
as one of the dominant energy inputs into plasma. This is however
true only when the mode grows to large amplitude leaving the
linear stage. At such low frequencies, on the other hand,
quasilinear theory does not apply as a valid saturation mechanism.
Probably the dominant processes are related to the generation of
gradients in the plasma which serve as the cause of drift modes
thus transferring energy to shorter wavelength propagating waves
of higher nonzero frequency. This kind of theory has not yet been
developed as it has not yet been understood why mirror modes in
spite of their slow growth rate usually are of very large
amplitudes indeed of the order of |B/B0|2~O(1). It is
thus highly reasonable to assume that mirror modes are
instrumental for the development of stationary turbulence in high
temperature plasma. Moreover, since the magnetic field in mirror
turbulence forms extended though slightly oblique magnetic
bottles, low parallel energy particles can be trapped in mirror
modes and redistribute energy (cf. for
instance, Chisham et al. 1998). Such trapped electrons excite banded
whistler wave emission known under the name of lion roars and
indicating that the mirror modes contain a trapped particle
component while leading to the splitting of particle distributions
(see Baumjohann et al., 1999) into trapped and passing particles. The
most amazing fact about mirror modes is, however, that they evolve
in the practically fully collisionless regime of high temperature
plasma where it is on thermodynamic reasons entirely impossible to
expel any magnetic field from the plasma. The fact that magnetic
fields are indeed locally extracted makes mirror modes similar to
"superconducting" structures in matter as known only at extremely
low temperatures. Of course, microscopic quantum effects do not
play a role in mirror modes. However, it seems that all mirror
structures have typical scales of the order of the ion inertial
length which implies that mirrors evolve in a regime where the
transverse ion and electron motions decouple. In this case the
Hall kinetics comes into play. We estimate that in the marginally
stationary nonlinear state of the evolution of mirror modes the
modes become stretched along the magnetic field with k||=0 and
that a small number the order of a few percent of the particle
density is responsible only for the screening of the field from
the interior of the mirror bubbles. |
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