tailieunhanh - Astrophysics and Space Science Proceedings Phần 9

et al. Tất cả các điểm nghiên cứu với sự hiện diện của sóng ép được cho là sóng chậm magnetoacoustic tìm thấy bằng cách phá rừng và Gurman (1998); O'Shea et al. (2006, 2007). Gần đây, Gupta et al. | 434 . Gupta et al. have measured oscillations in coronal holes in the polar off-limb regions of the Sun. All these studies point to the presence of compressional waves thought to be slow magnetoacoustic waves as found by Deforest and Gurman 1998 O Shea et al. 2006 2007 . Recently Gupta et al. 2009 have reported the detection of these waves in the disk part of the polar coronal hole hereafter PCH . They also find a difference in nature of the compressional waves between bright network and dark internetwork regions in the PCH. In this contribution we extend such analysis to another dataset. More detail is given in Gupta et al. 2009 . 2 Observations and Data Analysis The data used in this analysis were taken on 25 February 1997 during 00 00-13 59UT with the 1 X 300 slit of SUMER and an exposure time of 60s in the NIV 765 A and NeVIII 770 A lines in a southern PCH. Details of the data reduction are given in Gupta et al. 2009 . The chromosphere and transition region show enhanced-intensity network boundaries and darker internetwork cells. Presumably the magnetic field is predominantly concentrated at the network boundaries and within coronal holes the footpoints of coronal funnels emanate from these network boundaries. As the observing duration of this dataset is very long the locations of bright and dark pixels along the slit change with time. For this reason we have analyzed the whole dataset pixel by pixel and timeframe by timeframe. For example for one given moment we first determined the average intensity along the slit. All pixels having an intensity higher than times this average intensity were chosen as bright pixels. If such pixels are bright for at least 60 min or 60 timeframes then these are considered to be a bright network location over that time interval. The bright pixel identification is done only for the low-temperature N IV line the network pixels obtained from it are assumed to be the same in the higher-temperature Ne VIII line. 3 Results and

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