A Type II Radio Burst without a Coronal Mass Ejection

Abstract

Type II radio bursts are thought to be a signature of coronal shocks. In this paper, we analyze a short-lived type II burst that started at 07:40 UT on 2011 February 28. By carefully checking white-light images, we find that the type II radio burst is not accompanied by a coronal mass ejection, only with a C2.4 class flare and narrow jet. However, in the extreme-ultraviolet (EUV) images provided by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO), we find a wave-like structure that propagated at a speed of $\sim$ 600 km s${}^{-1}$ during the burst. The relationship between the type II radio burst and the wave-like structure is in particular explored. For this purpose, we first derive the density distribution under the wave by the differential emission measure (DEM) method, which is used to restrict the empirical density model. We then use the restricted density model to invert the speed of the shock that produces the observed frequency drift rate in the dynamic spectrum. The inverted shock speed is similar to the speed of the wave-like structure. This implies that the wave-like structure is most likely a coronal shock that produces the type II radio burst. We also examine the evolution of the magnetic field in the flare-associated active region and find continuous flux emergence and cancellation taking place near the flare site. Based on these facts, we propose a new mechanism for the formation of the type II radio burst, i.e., the expansion of the strongly-inclined magnetic loops after reconnected with nearby emerging flux acts as a piston to generate the shock wave.

18 pages, 10 figures, accepted for publication in ApJ

References in corpus (5)

• Differential Emission Measure Analysis of Multiple Structural Components of Coronal Mass Ejections in the Inner Corona
• Observations and Interpretation of a Low Coronal Shock Wave Observed in the EUV by the SDO/AIA
• The formation heights of coronal shocks from 2D density and Alfvén speed maps
• Radio signatures of CME-streamer interaction and source diagnostics of type II radio burst
• On the relationship of shock waves to flares and coronal mass ejections

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