Final Report Summary - SPIA (Magnetic connectivity through the Solar Partially Ionized Atmosphere)
The peculiarity of solar plasma consists in its very weak degree of ionization (1 charged particle over 10000 neutral ones in the photosphere), but rather strong collisional coupling. The main objective of our project was to explore a novel approach for the treatment of such plasmas by relaxing approximations applied in classical magnetohydrodynamics (MHD). The presence of neutrals leads to a partially uncoupled behavior of plasma components. In its turn, it produces a series of non-ideal effects – ambipolar diffusion, Hall effect and battery effect that cannot be treated by standard MHD. Curiously, unlike other fields of astrophysics, solar physics lacks direct observational confirmations of these non-ideal effects. The motivation for their study came from the theoretical point of view the time of the start of the project. The SPIA project performed a systematic investigation of the influence of non-ideal plasma effects due to neutrals combining analytical, numerical and observational approaches. From a technical point of view our major achievements have been the following: (i) we have formulated a common mathematical and numerical approaches for the treatment of partially ionized plasmas across different fields: interstellar medium, magnetosphere and ionosphere of the Earth and solar physics; (ii) we have developed a single-fluid non-ideal MHD code for modeling of solar partially ionized plasma, Mancha3D. From a scientific point of view, our major conclusions are: (1) we have evaluated the magnitude of the different non-ideal effects for representative conditions of the solar atmosphere; we have concluded that in quiet regions (covering about 90% of solar surface at any time), the Hall and ambipolar effects dominate the electric current, being largest at the borders of intergranular lanes, where the strongest gradients of all parameters exist; in sunspots, the ambipolar effect dominates by 3 orders of magnitude over the next-in-magnitude Hall term; these results can be taken as a guide for the future application of a non-ideal MHD modeling in different solar situations; (2) we have proposed and investigated an extremely efficient mechanism for chromospheric heating based on the action of ambipolar diffusion in the presence of wave motions; this mechanism is considered these days one as the most promising one to explain the chromospheric temperature rise, helping to solve a long-standing puzzle; (3) we have proposed a theoretically interesting possibility of generation of Alfvén waves already in the photosphere based on the action of the non-ideal Hall effect; understanding mechanisms of generation of Alfvén waves is important because they are among the best candidates to transport energy to the chromosphere and corona; (4) we were the first who performed simulations the non-linear phase of the contact instabilities in partially ionized solar prominences, one of the most enigmatic magnetic structures; we have shown that the presence of neutrals in the prominence plasma increases the growth rate of the instability and changes the velocities and the thermodynamic structure of the turbulent drops, since the neutral atoms de not “feel” the stabilizing effect of the magnetic field; (5) we have constructed first 3D simulations of solar magneto-convection including non-ideal ambipolar and battery effects and action of solar local dynamo; our results provide a coherent way to estimate the magnetic energy content of the Sun's quiet areas; (5) we have attempted a first-ever direct detection of ion-neutral effects in the solar atmosphere; we have designed non-conventional observing campaigns at the telescopes of Canary islands observatories, positively identifying the differential behavior of the ionized and neutral components in sunspots and prominences, at the limits of possibilities of current instrumentation. Overall, our project has opened a new and original research direction in solar physics, with perspectives for further exploration.