5.4 Wave propagation

Wave propagation is another type of dynamic phenomena observed after the onset of a flare. The so-called Moreton wave is one of these waves, which is observed in Hα (Moreton and Ramsey, 1960; Moreton, 1960). Moreton waves are often associated with type II radio bursts (e.g., Uchida, 1974). The mechanism for producing the Moreton wave is given by Uchida (1968), where this wave is explained as the chromospheric counterpart of a fast MHD shock wave propagating in the corona. Recent observations of the Moreton wave based on high temporal resolution have revealed various aspects of this highly transient phenomenon (Khan and Hudson, 2000; Warmuth et al., 2001; Eto et al., 2002; Narukage et al., 2002Jump To The Next Citation Point; Okamoto et al., 2004; Warmuth, 2007). Figure 45View Image shows an observational result on the structure and evolution of a Moreton wave, derived from a detailed analysis of Hα and soft X-ray data.
View Image

Figure 45: Structure and evolution of a Moreton wave in soft X-rays and Hα observed on 1997 November 3. (a – f) Hα + 0.8 Å running difference images of a Moreton wave (black arrows). (g – j) Soft X-ray (middle panels) and running difference (bottom panels) images of an X-ray wave (white arrows). (k) Wave fronts of the Moreton wave at every minute (black lines) and the X-ray wave at every 48 seconds (white lines) overlaid on the photospheric magnetic field. Gray lines show the great circles through the flare site (gray arrow). The rectangle, circle, and lines shown in (l) are the field of view of (a – k), the limb of the Sun, and the great circles, respectively (from Narukage et al., 2002).

The so-called EIT wave gives another type of waves associated with a flare, which was discovered by the EUV Imaging Telescope (EIT) aboard SOHO as a transient wave-like phenomenon with enhanced coronal emissions (Moses et al., 1997; Thompson et al., 1998). The mechanism for producing an EIT wave has been investigated by Delannée and Aulanier (1999), Wang (2000), and Wu et al. (2001). Chen et al. (2002) presents an integrated model of EIT and Moreton waves in which these two waves are subproducts of an expanding magnetic loop responsible for a CME.

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