Strong magnetic fields alter the energy levels in atoms and ions, changing the spectral lines they produce. Take spectra of sunspots, measure wavelengths of split lines, gives you strength of magnetic field. Magnetic field not only slows convection but it also keeps plasma from moving into sunspot. Vice versa, charged particles will drag magnetic fields along with them. Magnetic field lines will get twisted by turbulence of Sun's atmosphere.
Sunspots are where magnetic field is strongest and field lines are closest together.
Connected by loops of magnetic field lines. Usually last for days and weeks, gas stays up until it cools or the magnetic field goes away. Possibly tied to rapid changes in magnetic field. Largest one we know about reached a million kilometers from the Sun. Solar flares also occur near sunspots and are the most energetic event on the Sun.