2.2 Giant-arcade formation associated with filament eruption (or coronal mass ejection)

It has been found that a cusp-shaped loop or arcade is formed in the events that have much larger spatial scale than LDE flares (Tsuneta et al., 1992bJump To The Next Citation Point; McAllister et al., 1992; Watanabe et al., 1992; Hanaoka et al., 1994; Shiota et al., 2005Jump To The Next Citation Point). These are called giant arcades, which are usually associated with the disappearance of a dark filament. Tsuneta et al. (1992b) analyzed a giant arcade associated with the disappearance of a polar-crown filament observed on Nov. 12, 1991. The arcade has gradually evolved for more than 20 hours and formed a structure with the size of more than solar radius. A similar event was also observed on Apr. 14, 1994 (McAllister et al., 1996Jump To The Next Citation Point; Figure 6View Image).
View Image

Figure 6: A giant arcade observed in soft X-ray on April 14, 1994 (from McAllister et al., 1996). Shown in reversed contrast.

A large helmet streamer is usually observed after the eruption of a filament or CME, and this probably shows the side view of a giant arcade. An example of this is presented in Figure 7View Image (Hiei et al., 1993Jump To The Next Citation Point). It is interesting to note that the temperature is higher near the edge of the cusp-shaped loop of giant arcades, which is similar to LDE flares. Note also that the soft X-ray intensity in a giant arcade is usually very low, and this is why giant arcades had belonged to a different category than flares. However, Yohkoh/SXT has revealed that giant arcades have a lot of similarities to LDE flares except for their spatial scale and magnetic field strength. These two factors actually affect the time scale, amount of released energy, and emission measure of individual events.

View Image

Figure 7: Helmet streamer observed in soft X-ray on January 24, 1992 (modified from Hiei et al., 1993).

Outside the cusp-shaped structure the soft X-ray intensity often decreases with time, which is called dimming (Tsuneta, 1996Jump To The Next Citation Point; Sterling and Hudson, 1997Jump To The Next Citation Point; Harra and Sterling, 2001Jump To The Next Citation Point). Tsuneta (1996Jump To The Next Citation Point) attributed dimming to the inflow driven by reconnection that carries a large amount of plasma surrounding a current sheet into the sheet, thereby decreasing the gas density outside the current sheet (see also Shiota et al., 2005Jump To The Next Citation Point). Although usual dimmings associated with CMEs are attributed to stretching /opening of field lines by eruption (e.g., Sterling and Hudson, 1997Jump To The Next Citation Point; Harra and Sterling, 2001), there is a possibility that some of them are caused by the evacuation via the inflow into reconnection region like above (e.g., Shiota et al., 2005).


  Go to previous page Go up Go to next page