The anisotropic turbulent cascade at high frequencies, the effects of instabilities and the correct way to measure magnetic fields when considering small scales.

Details The anisotropic turbulent cascade at high frequencies, the effects of instabilities and the correct way to measure magnetic fields when considering small scales. The anisotropic turbulent cascade at high frequencies, the effects of instabilities and the correct way to measure magnetic fields when considering small scales.

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsh51d..02w&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #SH51D-02

Physics

Plasma Physics

[7829] Space Plasma Physics / Kinetic Waves And Instabilities, [7836] Space Plasma Physics / Mhd Waves And Instabilities, [7863] Space Plasma Physics / Turbulence

Scientific paper

Advances in spacecraft and instrument engineering and calibration are generating very high time cadence and very accurate magnetic field and particle data which are ideal for measuring high frequency turbulence, plasma instabilities and dissipation. There is, however, a complicating factor: the magnetic field causes anisotropy in the turbulence and also the input of energy by instabilities and thus the cascade towards dissipation at high frequencies is not isotropic. We must be careful to take this into account when interpreting data, but how? The direction we measure for the magnetic field is scale dependent and rapidly changing in time. We know from previous work that ion temperature anisotropy instabilities act in the solar wind (e.g. Bale et al., PRL , 2009), but what effect do these instabilities have on the magnetic field and the ion kinetic cascade at smaller scales, and how frequently do they act? In this presentation we will discuss recent results showing anisotropy of the inertial range turbulence and the transfer of energy to kinetic scales as well as the evidence from magnetic helicity that ion cyclotron and fire hose instabilities are acting. We will present new results showing the anisotropic and scale dependent effect of these instabilities on magnetic field power spectra in the inertial and dissipation ranges. We also show that the scale dependent (local) mean magnetic field is the correct way to measure magnetic field in small scale turbulence by deriving the coordinate transforms between the spacecraft frame and the direction of axi-symmetry of the turbulence and thus show that magnetic helicity should be interpreted more carefully than perhaps it has been.

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