Flare waves observed in Helium I 10 830 Å

Abstract

Abstract. Three traveling disturbances recorded in the absorption line of Helium I at 10 830 Å (He I), analogous to Hα Moreton waves, are analyzed. The morphology and kinematics of the wavefronts are described in detail. The He I wave appears as an expanding arc of increased absorption roughly corresponding to the Hα disturbance, although not as sharply defined. He I perturbations consist of a relatively uniform diffuse component and a patchy one that appears as enhanced absorption in He I mottles. It leads the Hα front by some 20 Mm and can be followed to considerably larger distances than in Hα observations. Behind the front stationary areas of reduced He I absorption develop, resembling EUV coronal dimming. The observed He I as well as the Hα disturbances show a deceleration of the order of 100-1000 m s −2 . Moreover, in the event where Hα, He I, and EUV wavefronts are observed, all of them follow closely related kinematical curves, indicating that they are a consequence of a common disturbance. The analysis of spatial perturbation profiles indicates that He I disturbances consist of a forerunner and a main dip, the latter being cospatial with the Hα disturbance. The properties and behavior of the wavefronts can be comprehended as a consequence of a fast-mode MHD coronal shock whose front is weakly inclined to the solar surface. The Hα disturbance and the main He I dip are a consequence of the pressure jump in the corona behind the shock front. The He I forerunner might be caused by thermal conduction from the oblique shock segments ahead of the shock-chromosphere intersection, or by electron beams accelerated in the quasi-perpendicular section of the shock.

Large-scale Globally Propagating Coronal Waves

“…The multiplicity of wavefronts could reflect several wave generation mechanisms acting in one event, for instance CME-and flare-related mechanisms. Some regions behind the He i front show a brightening that coincides with the locations of coronal dimming (see Section 3.4.2) in cotemporal EIT images observed behind EIT waves (Vršnak et al, 2002b). This weakening of absorption in He i is probably due to a reduction of EUV irradiation or heat flux from the corona.…”

“…Before the availability of high-cadence EUV imaging, the He i waves have thus acted as a link between Moreton and EIT waves. This has proven that the He i waves are cospatial with the coronal wavefronts (Vršnak et al, 2002b;, as well as with the Moreton wavefronts (Vršnak et al, 2002b;Warmuth, 2010).…”

“…Dimmings are most clearly seen in the EUV range (Thompson et al, , 2000a, but have also been reported in SXRs (Sterling and Hudson, 1997). In He i, brightenings that are associated with coronal dimmings can be detected (Vršnak et al, 2002b;de Toma et al, 2005). It is widely accepted that dimmings are primarily due to plasma evacuation (e.g., Hudson et al, 1996;Zarro et al, 1999;Harrison et al, 2003;Harra et al, 2007).…”

“…While this complicates the interpretation of the data, it provides an interesting link between chromospheric and coronal observations. Wave signatures in He i were first detected in images recorded by the CHIP instrument (MacQueen et al, 1998) at Mauna Loa Solar Observatory by Vršnak et al (2002b). They appear as dark fronts, i.e., they are seen in increased absorption, as shown in Figure 11.…”

“…The He i wavefronts were found to consist of a weaker frontal segment and a more pronounced main perturbation element. The former could be generated by the higher parts of an inclined coronal wave (see Section 6.1.1), while the latter is generated by the actual impact of the coronal wave on the chromosphere (Vršnak et al, 2002b).…”