Coronal magnetic field strength from Type II radio emission: complementarity with Faraday rotation measurements

¹ INAF – Astrophysical Observatory of Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy
e-mail: mancuso@oato.inaf.it
² Laboratory for Astroparticle Physics, University of Nova Gorica, 5000 Nova Gorica, Slovenia

Received: 10 September 2013
Accepted: 23 October 2013

Abstract

We analyzed the band splitting of a Type II radio burst observed on 1997 May 12 by ground- and space-based radio spectrometers. Type II radio emission is the most evident signature of coronal shock waves and the observed band splitting is generally interpreted as due to plasma emission from both upstream and downstream shock regions. From the inferred compression ratio we estimated, using the magnetohydrodynamic (MHD) Rankine-Hugoniot relations, the ambient Alfvén Mach number. By means of the electron density obtained by inverting white-light polarized brightness (pB) coronagraph data and the shock speed inferred from the Type II frequency drift, we finally derived a radial profile for the magnetic field strength in the middle corona. The result was compared with the field profile obtained in May 1997 (but above ~5 R_⊙) with Faraday rotation measurements of extragalactic radio sources occulted by the corona. The power law of the form B(r) = 3.76  r^-2.29  G inferred in that work nicely describes the combined set of data in a wide range of heliocentric distances (r ≈ 1.8−14  R_⊙).