ANALYSIS OF A UV EVENT IN A POLAR CORONAL HOLE


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Abstract

We present observations of a UV event which occurred in a polar coronal hole. They were obtained by SUMER on SOHO in several chromospheric and transition region spectral lines. Its birth site was about 50 arc sec inside the limb and in a network lane showing a net outflow before its initiation. The event had an extension of about 5 arc sec along the slit, a duration of about 3 min and was characterized by a large increase of intensity together with a significant line broadening with, however, downflows of about 50 km s−1 being dominant. Proper motions with a velocity of about 10 km s−1 were also observed. The event appeared at middle transition (O vi) temperatures and it simultaneously showed up in chromospheric (O i, Ly β) and low transition region (C ii) temperatures. We discuss this event in view of different scenarios to account for it. Our event could be a part of the large family of quiet-Sun explosive events observed by Ryutova and Tarbell (2000) taking place in polar coronal holes that are triggered by magnetic reconnection in the low solar atmosphere.

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