Physics – Geophysics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsh53a1294p&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #SH53A-1294
Physics
Geophysics
[2149] Interplanetary Physics / Mhd Waves And Turbulence, [2159] Interplanetary Physics / Plasma Waves And Turbulence, [4490] Nonlinear Geophysics / Turbulence
Scientific paper
A school of thought that appears to be gaining acceptance holds that solar wind turbulence consists of an energetically dominant Alfvén wave cascade at MHD scales and a transition to a quasi-perpendicular propagating kinetic Alfvén wave (KAW) cascade at kinetic scales where k⊥ρL>1 and ρL is the thermal proton gyro-radius. It was shown by Gary (1986) that the normalized magnetic-helicity of such KAWs is near unity and that the waves posses a right-hand sense of polarization, that is, a right-hand sense of rotation with respect to the mean magnetic field. For turbulence composed of a spectrum of KAWs propagating in predominantly one direction relative to the mean magnetic field, say, k.B0>0, this implies that the normalized magnetic-helicity spectrum should show a positive signature. In fact, this is what is usually seen in the solar wind. Therefore, the positive magnetic-helicity spectrum seen in the solar wind at kinetic scales may be explained by the existence of an energetically dominant KAW cascade and may not be caused by ion-cyclotron resonance absorption of parallel propagating ion-cyclotron waves as was previously thought. High frequency (>1 Hz) measurements of the reduced-magnetic helicity spectrum from the Stereo and Cluster spacecraft usually show a positive magnetic-helicity signature in the kinetic regime, consistent with the existence of a KAW cascade. The data also show that the helicity spectrum reaches a peak and then gradually decreases to zero. This behavior appears to be inconsistent with linear wave theory (Vlasov theory) which predicts a plateau rather than a peak in the spectrum. A possible explanation is that the energies of inward and outward propagating waves tends toward a balanced equipartition state deep in the kinetic regime as opposed to a state, like in the inertial range, where outward propagating waves dominate the energy. At the same time, the spectrum is expected to remain highly anisotropic in the kinetic regime with the energy confined to a small cone angle in the direction perpendicular to the local mean magnetic field. Possible discrepancies between spacecraft measurements and linear wave theory are studied and tentative resolutions are suggested.
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