|Nonlinear Processes in Geophysics|
|The strange physics of low frequency mirror mode turbulence in the high temperature plasma of the magnetosheath|
|E. Georgescu 5  O. A. Pokhotelov 6  R. Nakamura 2  O. D. Constantinescu 1  C. H. Jaroschek 4  R. A. Treumann 3 |
|Institut für Geophysik und extraterrestrische Physik, Technische Universität, Braunschweig, Germany|
|Space Research Institute, Austrian Academy of Sciences, Graz, Austria|
|Dept. Phys. Astron., Dartmouth College, Hanover, NH 03755, USA|
|Universitätssternwarte, Ludwig-Maximilians-Universität, Munich, Germany|
|Max-Planck-Institute for extraterrestrial Physics, Garching, Germany|
|Institute of Physics of the Earth, Russian Academy of Sciences, 123810 Moscow, Russia|
|Others : 1000988
DOI : doi:10.5194/npg-11-647-2004
【 摘 要 】Mirror mode turbulence is the lowest frequency perpendicularmagnetic excitation in magnetized plasma proposed already abouthalf a century ago by Rudakov and Sagdeev (1958)andChandrasekhar et al. (1958) from fluid theory. Its experimentalverification required a relatively long time. It was earlyrecognized that mirror modes for being excited require atransverse pressure (or temperature) anisotropy. In principlemirror modes are some version of slow mode waves. Fluid theory,however, does not give a correct physical picture of the mirrormode. The linear infinitesimally small amplitude physics isdescribed correctly only by including the full kinetic theory andis modified by existing spatial gradients of the plasma parameterswhich attribute a small finite frequency to the mode. In addition,the mode is propagating only very slowly in plasma such thatconvective transport is the main cause of flow in it. As thelowest frequency mode it can be expected that mirror modes serveas one of the dominant energy inputs into plasma. This is howevertrue only when the mode grows to large amplitude leaving thelinear 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 ofgradients in the plasma which serve as the cause of drift modesthus transferring energy to shorter wavelength propagating wavesof higher nonzero frequency. This kind of theory has not yet beendeveloped as it has not yet been understood why mirror modes inspite of their slow growth rate usually are of very largeamplitudes indeed of the order of |B/B0|2~O(1). It isthus highly reasonable to assume that mirror modes areinstrumental for the development of stationary turbulence in hightemperature plasma. Moreover, since the magnetic field in mirrorturbulence forms extended though slightly oblique magneticbottles, low parallel energy particles can be trapped in mirrormodes and redistribute energy (cf. forinstance, Chisham et al. 1998). Such trapped electrons excite bandedwhistler wave emission known under the name of lion roars andindicating that the mirror modes contain a trapped particlecomponent while leading to the splitting of particle distributions(see Baumjohann et al., 1999) into trapped and passing particles. Themost amazing fact about mirror modes is, however, that they evolvein the practically fully collisionless regime of high temperatureplasma where it is on thermodynamic reasons entirely impossible toexpel any magnetic field from the plasma. The fact that magneticfields are indeed locally extracted makes mirror modes similar to"superconducting" structures in matter as known only at extremelylow temperatures. Of course, microscopic quantum effects do notplay a role in mirror modes. However, it seems that all mirrorstructures have typical scales of the order of the ion inertiallength which implies that mirrors evolve in a regime where thetransverse ion and electron motions decouple. In this case theHall kinetics comes into play. We estimate that in the marginallystationary nonlinear state of the evolution of mirror modes themodes become stretched along the magnetic field with k||=0 andthat a small numberthe order of a few percent of the particledensity is responsible only for the screening of the field fromthe interior of the mirror bubbles.
© Author(s) 2004. This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License.