The kinetic turbulence of solar wind originates in the solar corona. The solar wind carries the fluctuations and escapes from the corona along the open magnetic field lines, and enters into the interplanetary space. The energy exchange among waves and particles reaches a balance, and the decay rate is very small and able to travel to 1 AU and beyond. The magnitude of the magnetic turbulence is about 20% of the solar wind background magnetic field, consistent with the observations. The observation-motivated model not only provides a viable solution to both long-standing puzzles of electron halo formation and the origin of solar wind turbulence on kinetic scale, but also establishes a possible link between the solar wind turbulence and the acceleration of electrons by nanoflares in solar corona.
The near-future space missions Solar Probe Plus and Solar Orbiter will reach the location of 10 solar radii to the Sun. The near-future (recently launched) mission MMS (magnetospheric multiscale) can measure 3D velocity distribution functions and kinetic turbulence on electron scales. The observations of halo and kinetic turbulence will provide more information to test this model.
A 2.5D PIC simulation was carried out with box size of 32×32 di in solar wind rest frame. The magnetic field is constant and along x. No current and density are uniform. The beam density is estimated by the observation of the superhalo. The two-stream instability drives an electromagnetic Weibel-like instability, and wave-wave interactions transfer the injected energy up to ion scale (inverse energy cascade) and down to electron scale (forward energy cascade). At the same time, turbulent wave scattering produces an isotropic electron halo. The total simulation is about 10 ion gyroperiods.