Turbulent cascade in the solar wind at kinetic scales and quasi-parallel whistler waves

Friday, 19 December 2014
Olga Alexandrova1, Catherine Lacombe1, Andre Mangeney1, Roland Grappin2, Milan Maksimovic3, Lorenzo Matteini4, Ondrej Santolik5, Nicole Cornilleau-Wehrlin1,6 and Yvonne de Conchy1, (1)Paris Observatory, Paris, France, (2)LUTH, Observatoire de Paris and LPP, Ecole Polytechnique, Paris, France, (3)CNRS, Paris Cedex 16, France, (4)Imperial College London, London, United Kingdom, (5)Academy of Sciences of the Czech Republic, Prague, Czech Republic, (6)Laboratoire de Physique des Plasmas, Saint-Maur Des Fossés Cedex, France
The nature of the magnetic field fluctuations in the solar wind between the ion and electron scales is still under debate. Using the Cluster/STAFF instrument, we make a survey of the power spectral density and of the polarization of these fluctuations at frequencies 1-400 Hz, during five years (2001-2005) when Cluster was in the free solar wind, i.e. not magnetically connected to the Earth's bow-shock.
In most of the analyzed time intervals, the fluctuations are non-polarized and they have a general spectral shape between the ion scales and a fraction of electron scales. The intensity of these spectra is well correlated to the ion thermal pressure. These non-polarized fluctuations seem to have a negligible frequency in the solar wind frame, and a wavevector anisotropy kperp>>k||. In the rest ~10% of the selected data, we observe narrow-band, right-handed, circularly polarized fluctuations, with wave vectors quasi-parallel to the mean magnetic field, superimposed on the spectrum of the permanent background turbulence. We interpret these coherent fluctuations as whistler mode waves. The life time of such waves varies between a few seconds and several hours. We analyze in details the long-lived whistler waves, i.e. with a life time longer than five minutes. We find several conditions for the appearance of such waves: (1) a low level of the background turbulence; (2) a low ion thermal pressure; (3) a slow solar wind speed; (4) an electron heat flux Qe>4μW/m2; (5) an electron mean free path larger than 0.5 AU, i.e., a low collisional frequency; (6) a change in the magnetic field direction. When the level of the background turbulence is high, we cannot affirm that whistler waves do not exist: they can be just masked by the turbulence. The six above conditions for the presence of parallel whistlers in the free solar wind are necessary but are not sufficient. When the electron parallel beta factor βe is larger than 3, the whistler waves are seen along the heat flux threshold of the whistler heat flux instability.