Here is a brief summary of the various aspects of coronal and wind dynamics that we have discussed, reflecting shortcomings of the current theoretical modelling efforts as well as some of the future avenues that we have identified.
Findings
- The observed densities of field-reversal switchbacks are much higher than can be reproduced in 3D MHD simulations of switchback formation through nonlinear wave dynamics. Does this reflect a shortcoming of the wave-origin model of switchbacks and suggests an additional source of large-amplitude Alfvén waves? E.g. in interchange reconnection events? On the other hand, the observed lack of field-reversals in sub-Alfvénic periods apparently seems to contradict the coronal-base origin of switchback through interchange reconnection.
- The near-HCS (Heliospheric Current Sheet) plasma appears to be unusually quiet (low levels of fluctuations compared to background values) in PSP observations. How can this be explained through the current models?
- The observed power spectra of the Elsasser variables close to the Sun are not consistently reproduced in models, with the minority Elsasser variable (usually z^-) being especially problematic, reflecting that we still fail to capture some essential solar wind dynamics.
- Quiet periods of the solar wind are more easily modelled using 3D compressible MHD simulations of open field regions that more “rough” periods. This is in connection to the first point. It is likely that we are missing violent transient events (e.g. interchange reconnection at the base of the corona) in the models, that can drive large deviations from background values.
- It is concluded that parametric decay should be important for solar wind acceleration, especially below 10 solar radii where the growth rate is the highest. Can we find a conclusive observational evidence of parametric decay happening at these distances remotely? Density perturbations can have various sources (e.g. turbulence) and are by themselves not conclusive evidence of parametric decay.
- The nature of the 1/f spectrum at large scales is still not clear and not reproduced in all models. Various models achieve a 1/f scaling, although these have various sources and explanations. Is the 1/f scaling an essentially perpendicular or parallel scaling with respect to the background magnetic field?
- There is no consensus as to the nature of the observed solar wind fluctuations. There are normally two interpretations, either as advected structures (Alfvén vortex, 2D structures, spaghetti-like flux tubes, etc.) or waves (Alfvén, Alfvénic, fast, slow, etc), or both. Additionally, solar wind fluctuations can also represent nonlinear waves which do not have a linear counterpart. It is unclear whether structures or waves are more prevalent, and whether these can be distinguished in single point in-situ measurements.
- The observed scaling of the fluctuation energy with radial distance is still not satisfactorily explained.
Ideas
- Distinct populations of fast and slow wind speeds can be observed at 1 AU and with Helios measurements up to 0.3 AU. It is unclear whether in PSP measurements this shows up close to the Sun. As the Sun is heading towards maximum activity, will the distinct populations show up in PSP data?
- There is a recurring depletion of number density observed close to the HCS. Would this be observable remotely in white light images? (E.g., WISPR, coronagraphs)
- What would the measurement of alpha (scaling between magnetic field and velocity perturbations, 1/sqrt(/mu /rho) for unidirectional Alfvén waves), originating in the Q-variable representation of the MHD equations tell us about solar wind fluctuations? How can various scenarios leading to the same alpha value be distinguished?
- It would probably constitute an interesting addition to current 3D MHD modelling efforts to find a MHD term which can approximate the effects of helicity barrier on large scales. In this way its effects on the large-scale dynamics could be tested without running very computationally expensive hybrid codes.