They allow the measurement of the strength of magnetic fields in Sunspots. They are produced by nuclear reactions in the Sun's core and therefore give information about processes deep in the Sun. They are a deadly radiation of massive particles that gradually destroys the Earth's crust. They show that the Sun is burning helium in its core. They show that the Sun's core is iron and that it is spinning rapidly, thereby creating the solar cycle.
Hydrostatic equilibrium is the balance of pressure and nuclear forces.
None of the above. The temperature, in degrees Kelvin, in the center of the Sun is closest to one billion. Too few solar neutrinos have been detected. Far too many solar neutrinos have been detected. The wrong kind of solar neutrinos have been detected.