On the Dependence of Solar Wind Speed on Local Magnetic Field Orientation in Highly Alfvénic Streams

Wednesday, 17 December 2014: 9:30 AM
Lorenzo Matteini, Imperial College London, London, United Kingdom, Timothy Simon Horbury, Imperial College London, London, SW7, United Kingdom, Marcia Neugebauer, University of Arizona, Tucson, AZ, United States and Bruce E Goldstein, NASA Jet Propulsion Laboratory, Pasadena, CA, United States
Magnetic field fluctuations from the low frequency part of the solar wind turbulent spectrum have typical amplitudes of the order of the underlying mean field, and then result in large random variations in the instantaneous direction associated with the magnetic field vector on the scale of minutes/few hours. At the same time, since fluctuations are Alfvénic, these also introduce variations in solar wind speed, which reflecting the randomness of the process, are not expected to show a dependence on the direction of the magnetic field. On the contrary, and quite surprisingly, we have found that in highly Alfvénic solar wind streams there exists a remarkably strong correlation between the bulk speed of the plasma and the local direction of the magnetic field. This leads to a systematic increase in the solar wind speed when the magnetic field vector is locally transverse to the velocity, and to a decrease in the speed when the field is more aligned with the radial, regardless of the polarity of the underlying magnetic field. We demonstrate that such an effect is a consequence of Alfvénicity for a unidirectional flux of waves propagating away from the Sun and with approximatively constant magnetic field magnitude. This correlation is systematically present in highly Alfvénic streams observed by Ulysses and Helios from 0.3 AU to 5 AU. The variations in the speed being proportional to the local Alfvén speed results in a modulation of the solar wind profile that is particularly relevant close to the Sun.