Abstract
Abstract. This paper reviews recent aspects of solar wind physics and elucidates the role Alfvén waves play in solar wind acceleration and turbulence, which prevail in the low corona and inner heliosphere. Our understanding of the solar wind has made considerable progress based on remote sensing, in situ measurements, kinetic simulation and fluid modeling. Further insights are expected from such missions as the Parker Solar Probe and Solar Orbiter. The sources of the solar wind have been identified in the chromospheric network, transition region and corona of the Sun. Alfvén waves excited by reconnection in the network contribute to the driving of turbulence and plasma flows in funnels and coronal holes. The dynamic solar magnetic field causes solar wind variations over the solar cycle. Fast and slow solar wind streams, as well as transient coronal mass ejections, are generated by the Sun's magnetic activity. Magnetohydrodynamic turbulence originates at the Sun and evolves into interplanetary space. The major Alfvén waves and minor magnetosonic waves, with an admixture of pressure-balanced structures at various scales, constitute heliophysical turbulence. Its spectra evolve radially and develop anisotropies. Numerical simulations of turbulence spectra have reproduced key observational features. Collisionless dissipation of fluctuations remains a subject of intense research. Detailed measurements of particle velocity distributions have revealed non-Maxwellian electrons, strongly anisotropic protons and heavy ion beams. Besides macroscopic forces in the heliosphere, local wave–particle interactions shape the distribution functions. They can be described by the Boltzmann–Vlasov equation including collisions and waves. Kinetic simulations permit us to better understand the combined evolution of particles and waves in the heliosphere.
Highlights
1.1 Hannes Alfvén and his waveThe European Geosciences Union (EGU) has awarded the Hannes Alfvén Medal to me for the year 2018
To give at least one example of Alfvén waves, I show in Fig. 1 a nice case stemming from measurements of the WIND spacecraft made at 1 AU in 1995 (Wang et al, 2012)
The experimental and theoretical literature on the solar wind abounds and is unmanageable, given the results obtained by so many spacecraft that have been sent to space for investigation of the near-Earth and planetary plasma environments, and the Sun and its extended heliosphere that reaches out to more than 100 AU
Summary
The European Geosciences Union (EGU) has awarded the Hannes Alfvén Medal to me for the year 2018. Receiving this important award gives me great enjoyment, and I feel deeply honored. To give at least one example of Alfvén waves, I show in Fig. 1 a nice case stemming from measurements of the WIND spacecraft made at 1 AU in 1995 (Wang et al, 2012) These large-amplitude Alfvénic fluctuations (shown here component-wise for a 40 min period with a time resolution of 3 s) reveal a very high correlation between the variations of the magnetic field vector and flow velocity vector, which was evaluated in the de Hoffmann–Teller frame in which the convective electric field of the solar wind is transformed away.
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