Band-splitting of coronal and interplanetary type II bursts. II. Coronal magnetic field and Alfvén velocity

Type II radio bursts recorded in the metric wavelength range are excited by MHD shocks traveling through the solar corona. They often expose the fundamental and harmonic emission band, both frequently being split in two parallel lanes that show a similar frequency drift and intensity behaviour. Our previous paper showed that band-splitting of such characteristics is a consequence of the plasma emission from the upstream and downstream shock regions. Consequently, the split can be used to evaluate the density jump at the shock front and to estimate the shock Mach number, which in combination with the shock speed inferred from the frequency drift provides an estimate of the Alfvén velocity and the magnetic field in the ambient plasma. In this paper such a procedure is applied to 18 metric type II bursts with the fundamental band starting frequencies up to 270 MHz. The obtained values show a minimum of the Alfvén velocity at the heliocentric distance R~ 2 amounting to v_A~ 400-500 km s^-1. It then increases achieving a local maximum of v_A~ 450-700 km s^-1 at R~ 2.5. The implications regarding the process of formation and decay of MHD shocks in the corona are discussed. The coronal magnetic field in the range 1.3<R<3 decreases as R^-3 to R^-4, or H^-1.5 to H ^-2 if expressed as a function of the height. The results are compared with other estimates of the coronal magnetic field in the range 1<R<10. Combined data show that below H<0.3 the magnetic field is dominated by active region fields, whereas above H=1 it becomes radial, behaving roughly as B=2x R^-2 with a plausible value of B~ 5 nT at 1 a.u.