Pulsed Alfvn waves in the solar wind

Abstract

Using 3s plasma and magnetic field data from the Wind spacecraft located in the solar wind well upstream from Earth, we report observations of isolated, pulse-like Alfvénic disturbances in the solar wind. These isolated events are characterized by roughly plane-polarized rotations in the solar wind magnetic field and velocity vectors away from the directions of the underlying field and velocity and then back again. They pass over Wind on timescales ranging from seconds to several minutes. These isolated, pulsed Alfvén waves are pervasive; we have identified 175 such events over the full range of solar wind speeds (320-550kms-1) observed in a randomly chosen 10day interval. The large majority of these events are propagating away from the Sun in the solar wind rest frame. Maximum field rotations in the interval studied ranged from 6° to 109°. Similar to most Alfvénic fluctuations in the solar wind at 1AU, the observed changes in velocity are typically less than that predicted for pure Alfvén waves (Alfvénicity ranged from 0.28 to 0.93). Most of the events are associated with small enhancements or depressions in magnetic field strength and small changes in proton number density and/or temperature. The pulse-like and roughly symmetric nature of the magnetic field and velocity rotations in these events suggests that these Alfvénic disturbances are not evolving when observed. They thus appear to be, and probably are, solitary waves. It is presently uncertain how these waves originate, although they may evolve out of Alfvénic turbulence. © 2011. The American Astronomical Society. All rights reserved.