Interaction between emerging flux and coronal hole - observations and simulations

VAN DRIEL-GESZTELYI, L.; BAKER, D.; MURRAY, M.J.; DÉMOULIN, P.; ATTRILL, G.; MATTHEWS, S.A.; MANDRINI, C.H.; TÖRÖK, T.

Montreal

Congreso; 37th COSPAR Scientific Assembly; 2008

COSPAR

Flux emergence in the vicinity of or inside a coronal hole (CH) is expected to induce magnetic reconnection between the closed emerging and open CH magnetic field lines, resulting in an evolution of the CH as formerly closed field lines become topologically open, while at the same time, open field lines close down. Through two case studies we show observational signatures of this (interchange) reconnection process and discuss its implications. First, using SOHO EIT and MDI data, we study a small active region (AR10869) emerging in the close vicinity of a low-latitude coronal hole in April 2006. The interfacing magnetic polarities between the AR and the CH were opposite, favourable for magnetic reconnection. We indeed observe the coupled formation of bright closed loops between the CH and the AR and coronal dimming on the far side of the AR, which we interpret as evidence of interchange reconnection. This process effectively modifies the CH boundary (making it retreat), while simultaneously displacing open field lines to the far side of the AR. In order to study this process in detail, we perform 2.5D MHD simulations, which qualitatively reproduce important aspects of the observations. We expect to find upflows of plasma at the location where previously closed field lines are opening up as well as on the reconnecting side, but since we had no spectroscopic data for this event, we can not verify this. Therefore we analyze Hinode/EIS line-of-sight velocity maps of another low-latitude CH with a small AR in its midst observed on 18 Oct. 2007. We find that while closed loops of the bipole are dominated by downflows in the Fe XII, Fe XIII and Fe XV lines, the strongest coronal plasma upflows are indeed located around and particularly at the "far side" of the bipolar AR, i.e. having the same polarity as the dominant polarity of the CH. The emerging biplole and the series of interchange reconnections it induces create a significant additional plasma upflow in the CH, thus we identify this outflow must contribute to the acceleration of the fast solar wind.