How much of the Sun to not evaporate is due to gravity and how much due to magnetism?

Question

As protons and electrons on the sun 'surface' have high kinetic energy something obviously should stop them to go away.Its gravity is very high compared to planets but is this enough as magnetism also helps holding them in loops tight to the surface?What is the situation at the poles where magnetism does not help...are there emissions of particles?Just one subquestion...are there coupled protons and electrons in H atoms on the Sun surface that cannot make loops and in that case just leave the Sun due to their KE?

Answer 1

As protons and electrons on the sun 'surface' have high kinetic energy something obviously should stop them to go away.

The Sun has an escape speed of ~600 km/s whereas the typical solar wind speeds tend to be below ~450 km/s [e.g., Wilson et al., 2021]. That is, the typical speeds are below that of the escape speed so how does the solar wind exist? This is actually a rather complicated question and NASA devoted a flagship mission to it, called Parker Solar Probe.

So what do we know?

First, the typical electron thermal speed in the corona (>5000 km/s) is way above the escape speed, so electrons can "run away" along magnetic field lines (we'll worry about geometry later). If they were let to run away indefinitely, the Sun would charge up. Instead, what happens is that an ambipolar electric field is created that acts to "pull" the much heavier (and much slower) ions out of the corona. In such models, there is a sort of equilibrium reached between this ambipolar electric field and gravity. Unfortunately, it doesn't generate a supersonic solar wind [e.g., see more details at https://physics.stackexchange.com/a/253491/59023 and https://physics.stackexchange.com/a/257548/59023].

Regardless, this ambipolar electric field does exist and we can indirectly measure it by observing a deficit of electrons that should be coming from the sun [e.g., see Halekas et al., 2021].

Second, we are fairly confident (for a lot of reasons that could fill books) that there is an additional acceleration acting on both the ions and electrons due to electromagnetic fluctuations (e.g., Alfven waves).

Third, the radial gradient in the magnetic field magnitude combined with the thermal pressure gradients results in a sort of magnetic de Laval nozzle that can additionally accelerate particles.

Its gravity is very high compared to planets but is this enough as magnetism also helps holding them in loops tight to the surface? What is the situation at the poles where magnetism does not help...are there emissions of particles?

Take a look at McComas et al. [2008] which shows observations from the Ulysses spacecraft (if it's blocked or pay-walled, then look at https://sci.esa.int/web/ulysses/-/43460-mccomas-d-j-et-al-2008).

What we find is that in regions where the Sun's magnetic field is mostly radial, the solar wind speed is indeed higher and the plasma is hotter and less dense. These regions are referred to as coronal holes. The regions closer to the solar equator (i.e., roughly the ecliptic plane) tend to have magnetic fields that are more complicated. In coronagraph images they look like old German helmets, thus they were named helmet streamers.

Solar wind originating from coronal holes corresponds to what we call fast wind and that from the streamers slow wind. There are tons of differences between fast and slow wind, including different heavy ion abundances and charge states. I won't belabor the point but the origin of the slow solar wind is a huge open question.

Just one subquestion...are there coupled protons and electrons in H atoms on the Sun surface that cannot make loops and in that case just leave the Sun due to their KE?

The particles are observed to exhibit distributions over different velocities, called particle velocity distribution functions [e.g., see Halekas et al., 2021 for examples]. Those that have high energies and small angles between the velocity vector and magnetic field do indeed have a higher probability of escape than those at larger angles. Further, the radial magnetic field strength gradient results in a focusing of escaping particles into a narrow beam (due to mirror forces).

References

• Halekas, J.S., et al., "The Sunward Electron Deficit: A Telltale Sign of the Sun's Electric Potential," Astrophys. J. 916, doi:10.3847/1538-4357/ac096e, 2021.
• McComas, D.J., et al., "Weaker solar wind from the polar coronal holes and the whole Sun," Geophys. Res. Lett. 35(18), doi:10.1029/2008GL034896, 2008.
• Wilson, L.B., et al., "A Quarter Century of Wind Spacecraft Discoveries," Reviews of Geophysics 59(2), pp. e2020RG000714, doi:10.1029/2020RG000714, 2021.