Band-splitting of coronal and interplanetary type II bursts ^*

III. Physical conditions in the upper corona and interplanetary space

¹ Hvar Observatory, University of Zagreb, Kačićeva 26, HR - 10000 Zagreb, Croatia
² INAF – Trieste Astronomical Observatory, Via G. B. Tiepolo 11, 34131 Trieste, Italy

Corresponding author: B. Vršnak, bvrsnak@geodet.geof.hr

Received: 9 July 2003
Accepted: 30 September 2003

Abstract

We analyse properties of 58 type II radio bursts recorded in the meter-to-kilometer wavelength range, focusing on episodes of band-split emission. The basic two parameters utilized are the frequency drift and the relative band-split of type II burst emission lanes. On average, in the meter-to-kilometer wavelength range D_f increases with the emission frequency as , revealing that source velocities are smaller at larger heliocentric distances. The relative band-split shows a weak but statistically significant dependence on the emission frequency, , indicating an increase of BDW with the heliocentric distance. Combining the shock velocity estimated from the frequency drift, with the Mach number inferred from the band-split, the Alfvén speed and the magnetic field in the ambient plasma can be estimated as a function of the heliocentric distance r. However, the outcome directly depends on the coronal/interplanetary density model used, which is poorly known in the upper corona and the near-Sun interplanetary space. So, we invert the problem: utilizing the results of the previous paper where it was shown that beyond the heliocentric distance of two solar radii () the average magnetic field decreases approximately as , we infer the density  in the upper corona and near-Sun interplanetary space. The obtained empirical dependence  is presented in the analytical form as a four-degree polynomial of , and is compared with some theoretical  models, considering also a deviation from the scaling used. The model matches the five-fold Saito density model (representing the active region corona) with the regime in the interplanetary space. Furthermore, it is shown that on average the magnetosonic speed attains a local minimum of  km s^-1 around and a broad local maximum of  km s^-1 in the range , beyond which it gradually decreases to several tens km s^-1 at 1 a.u. The local minimum becomes even deeper if the super-radial expansion of the magnetic field is taken into account. The implications regarding the formation and evolution of shocks in the corona and upper corona are discussed in the framework of CME-piston and flare-blast scenarios. The inferred general decrease of type II burst source velocities and broadening of band-splits with distance is interpreted in terms of the deceleration of mass ejections driving the shocks in the decreasing v_ms environment.