Slow halo CMEs with shock signatures

¹ Tuorla Observatory, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland e-mail: [silpoh;nijole]@utu.fi
² Department of Physics, University of Turku, Finland

Received: 30 August 2005
Accepted: 17 November 2005

Abstract

Context.In the solar corona, shocks are formed when the speed of a disturbance exceeds the local magnetosonic speed. In the active region corona the Alfvén speed can drop to a few hundred km s^-1, but globally it is much higher. There has been a long debate on whether the shocks responsible for type II bursts are created by bow shocks in front of coronal mass ejections (CMEs), shocks in the flanks of CMEs, or by flare (blast) waves.Aims.We study the alternative explanations for type II bursts in events where we have a slow CME, flare(s), and associated type II burst emission.Methods.We use multi-wavelength observations to analyse the height-time evolution of CMEs and compare it with the evolution of shock signatures in radio and EUV.Results.Three flare-associated halo-type CME events were observed on October 30, 2004. Velocity estimates (260, 325, and 920 km s^-1) from the first plane-of-the-sky CME leading front observations suggested that the first two were very slow compared to halo CMEs, on average. The CMEs were associated with flares (M 4.2, X1.2, and M 5.9) and each event was also associated with coronal (metric) type II emission that is known to be a signature of a propagating shock front. After the flare starts, loop displacements and large-scale dimmings were observed in EUV. The two slow halo CMEs started as filament eruptions, but the CME velocities and/or bulk motions were affected at the times of flares. We find support for the idea that the cause of metric type II bursts in these two events is flare-related. The later CME velocity changes (acceleration around 4–5 solar radii) could also be explained by eruptions associated with later flares. The repeating homologous flare-halo CME events indicate a restoration of the same large-scale structures within 5–6 h.