A model of the Alfvén speed in the solar corona

• Published in: Astronomy and astrophysics (Berlin) Vol. 435; no. 3; pp. 1123 - 1135
• Main Authors: Warmuth, A., Mann, G.
• Format: Journal Article
• Language: English
• Published: Les Ulis EDP Sciences 01.06.2005
• Subjects: shock waves; Sun: corona; Sun: magnetic fields; Sun: radio radiation; Solar corona; Dipoles; Sun: corona; Model study; Quiet sun; Magnetoacoustic waves; Shock waves; Radio radiation; Sun: magnetic fields; Type II burst; Models; Magnetic fields; Electron density; Solar magnetic field; Sun: radio radiation; Mach number
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361:20042169

We present an analytic model of the Alfvén speed vA in the solar corona. The coronal magnetic field is modeled by a radial component representing the global field and by a dipole representing an active region. The free parameters of the model are constrained by actual observations of solar magnetic fields and coronal electron densities. The coronal magnetic field strength in the quiet Sun is determined by coronal seismology, using EIT waves as proxies for the fast magnetosonic speed $v_{{\rm ms}}$, and thus for the magnetic field strength. Depending on the orientation of the dipole, we find local minima of vA (and $v_{{\rm ms}}$) at the coronal base at distances of 0.2–0.3 solar radii from the center of the modelled active region (AR), as well as above the AR at comparable heights. For all dipole orientations, a global maximum is found at 3.5 solar radii. We apply our model to the study of the formation and propagation of coronal shock waves which are observed as flare waves and as type II radio bursts, using a sample of eight solar events. We find that flare waves are initially highly supermagnetosonic (with magnetosonic Mach numbers of $M_{{\rm ms}} \approx 2$–3). During their propagation, they decelerate until $M_{{\rm ms}}=1$ is reached. This behavior can be explained by a strong shock or large-amplitude simple wave that decays to an ordinary fast magnetosonic wave. The observed starting frequencies and Mach numbers of the associated type II bursts are consistent with the predictions of the model.