3.5 Coronal waves

The frequent detection of coronal waves observed in EUV was an exciting discovery from the SOHO EIT observations (e.g., Thompson et al., 1998). They were originally termed EIT waves, but are now often referred to as EUV waves or, more generally, as coronal waves. These waves were originally considered to be a candidate for a CME-associated Moreton wave. According to the theory by Uchida (1968), a flare may trigger an impulse that will propagate along the solar surface as a fast traveling front with an increase in emission. In the photosphere and chromosphere it can best observed in Hα as a Moreton wave. However, the EUV (EIT) waves propagate across the solar disk at typical speeds of 200 – 400 km s–1 (Thompson and Myers, 2009), slower than the 1000 km s–1 typical of Moreton waves. Observational evidence, such as the association of Type II radio bursts with coronal waves, suggests that at least some of them may be fast-mode MHD shocks. Although Biesecker et al. (2002) found a CME associated with nearly every EIT wave, it is accepted that not all CMEs are associated with waves. For example, Webb (2002) found that only about half of frontside halo CMEs have EIT waves, and Cliver et al. (2005) found that there are ∼ 5 times as many frontside CMEs as EIT waves. Thus, their nature is still under intense debate, Competing models include fast-mode MHD waves, slow-mode waves or solitons, and “pseudo waves” related to a current shell or successive restructuring of field lines at the CME front. Details of observations and models can be found in recent reviews (Warmuth, 2007; Vršnak and Cliver, 2008; Wills-Davey and Attrill, 2009; Gallagher and Long, 2011).

The relatively poor cadence (∼ 12 minutes) of the EIT observations of propagating EUV disturbances were partially alleviated by STEREO EUVI imagery. These have shown that the EUV wave kinematics are more consistent with coronal MHD waves (e.g., Long et al., 2008; Patsourakos and Vourlidas, 2009). Using the Atmospheric Imaging Assembly (AIA) on SDO, Liu et al. (2010) show that there can be multiple wave components with rippling effects. In one of the best observed wave events using the AIA EUV images, it was found that the shock and metric type II burst appeared simultaneously (Gopalswamy et al., 2012). Also the wave propagation can be inhibited and possibly reflected from coronal holes (e.g., Gopalswamy et al., 2009c). Veronig et al. (2010) presented evidence from STEREO/EUVI observations that the wave initially appears as a dome-shaped spherical structure surrounding the CME. Chen and Wu (2011) interpret an EUV event using SDO/AIA data as consisting of a fast mode wave followed by a slower disturbance.

  Go to previous page Go up Go to next page