Numerical Simulation For The EUV Brightenings

Traditionally, there are coronal mass ejections, prominence eruptions, two-ribbon flares and other type large-scale explosive events occurring in the solar atmosphere. Besides the large-scale explosive events, there is a host of small-scale explosive events in the coronal and chromosphere. These explosive events include compact flares,â…¹-ray bright points, transition region explosive events, surges, sprays, blinkers, EUV brightenings, etc. Because these small-scale explosive events are difficult to resolve spatially, many of them are less well understood than large-scale explosive events.We are greatly interested in the EUV brightenings observed by TRACE and SOHO in recent years. The 16 August 1999 EUV brightenings in the upper atmosphere above NOAA active region 8668 observed by TRACE at EUV Feâ…¨/â…©171 (?) is numerically studied by a third-order upwind compact scheme. The numerical results can give a possible explanation for its formation and evolution.The numerical results show that the initial reconnection jets nearâ…©-point are possible consistent with the EUV brightenings scattered in space and time in the observations. The evolution of magnetic field lines undergoes two coalescence instability steps, the strong and superposed ejections caused by the first and second coalescence of magnetic islands are possibly related to the lifted material which initially appeared as absorption features at 171(?) and later EUV-emitting structures respectively. The plasmoid is remarkably accelerated in the opposite direction forming the bi-directional reconnection jets. The bi-directional reconnection jets may correspond to the lifted material that either continued to move upward along the apparently open field lines or fell down to the surface. The jet-like flows are accelerated to the order of the Alfv en speed and ejected, leading to a Petschek-like fast reconnection configuration. The conversion of magnetic energy into kinetic energy is strongest exceeding the Ohmic heating, the magnetic energy is continuously decreasing, which reveals that the reorganization of the magnetic field lines is associated with a series of magnetic reconnection events, including multi-site reconnections, coalescence of magnetic islands, and acceleration of the ejections.In a word, the numerical simulation results above reproduce qualitatively the evolution process of the 16 August 1999 EUV brightenings, deriving to a possible explanation for its formation and evolution. Through our study on such a typical EUV brightenings, we can get a more profound understanding of the EUV brightenings, and also promoting for the research of the small-scale explosive events in the solar atmosphere.