Flare waves observed in Helium I 10 830 Å

A link between Hα Moreton and EIT waves

¹ Hvar Observatory, University of Zagreb, Kačićeva 26, 10000 Zagreb, Croatia e-mail: bvrsnak@geodet.geof.hr
² Institute for Geophysics, Astrophysics and Meteorology, University of Graz, Universitätsplatz 5, 8010 Graz, Austria e-mail: ajw@igam.uni-graz.at
³ Kiepenheuer-Institut für Sonnenphysik (KIS), Schöneckstr. 6, 79104 Freiburg, Germany e-mail: rbrajsa@kis.uni-freiburg.de

Corresponding author: B. Vršnak, bvrsnak@geodet.geof.hr

Received: 12 April 2002
Accepted: 29 July 2002

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.