The solar wind comes from the Sun, and it is laced with lines of magnetic force caused by currents flowing near the surface of the sun, and in the interplanetary medium. It is 'frozen into' the wind, because when you look at the dynamics of this interplanetary plasma, there is about as much energy density ( pressure) in the gas component as there is in the magnetic field component, with the scales tipped in favor of the gas. This means that the gas drags the magnetic field around, and the magnetic field mostly follows after the gas as it flows out from the sun. They are in 'equipartition' if their energy densities (pressures) are exactly equal, but this condition only happens rarely and in small localities where magnetic fields get amplified, as for instance, in shocks. As for 'why' this happens, it probably has to do with the way that the wind is ejected by magnetized plasma clouds that become buoyant in the solar chromosphere and corona. This naturally leads to conditions where approximate pressure balance would happen between magnetic fields and particles, and so 'magnetic flux freezing' tends to be a favored dynamical state.
All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the
NASA IMAGE/POETRY project.