4 Open Questions and Challenges

Although significant progress was made in observing and modeling of the solar wind over the past decades, there are still several important questions that are unanswered. This situation stems from the lack of unambiguous observations, which point to a specific physical mechanism for coronal heating, and solar wind accelerations, as well as due to the limitations of present models and theories. In particular, the following questions remain open:

  1. What is the exact physical mechanism that produces the fast and slow solar wind? This question relates directly to the question of coronal heating mechanism.
  2. What is the role of waves (in a broad frequency range from kinetic to MHD) in the acceleration and heating of the solar wind?
  3. What is the role of density inhomogeneity and small scale turbulence (cascade) in the heating and the acceleration of the fast and slow solar wind?
  4. How are the non-Maxwellian velocity distributions of protons and ions in the solar wind formed?
  5. What determines the heavy ion composition (i.e., elemental abundance and charge states, see Zurbuchen, 2007) of the fast and slow solar wind?
  6. What is the role of electrons in solar wind acceleration and heating?
  7. How does Earth’s global space environment respond to solar wind variations?

The works reviewed here bear on the first five questions, showing that MHD waves with a given spectrum provide plausible acceleration mechanism of the fast solar wind in coronal holes, and heating of coronal plasma may occur through resonant and non-resonant dissipation of the waves energy. However, the fluid models do not provide the kinetic details of the dissipation processes, and the hybrid models show only limited aspects of the resonant dissipation processes. The formation and the evolution of the ion velocity distribution of the solar wind plasma is not modeled in detail from the Sun to 1 AU. Multi-fluid models address limited aspects (i.e., within the ion-fluids approximation) of the compositional variation of the solar wind in open and closed structures (Ofman, 2000, 2004a). In the reviewed models the electrons are only studied as a fluid and their role in solar wind heating and acceleration only includes basic aspects (i.e., heat conduction, collisions coupling to ions). The last question can be addressed with global models that include the Earth’s magnetosphere and ionosphere. However, the study of the kinetic processes that are at the roots of the solar-wind–magnetosphere–ionosphere interactions is far from complete.

The above questions are on the forefront of current research, and the answer can be obtained by combination of improved observations and modeling. A possible way to answer these questions is by obtaining in-situ measurements of the solar wind plasma in the region close to the Sun, where the acceleration and heating processes are still significant (McComas et al., 2007). This is the goal of the future European Solar Orbiter, and NASA’s Solar Probe Plus missions.

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