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: http://star.arm.ac.uk/annrep/annrep2004/node45.html
Дата изменения: Tue Apr 19 13:35:58 2005 Дата индексирования: Tue Oct 2 04:48:25 2012 Кодировка: Поисковые слова: star trail |
A detailed study of two consecutive bright points observed simultaneously with CDS, EIT and MDI is presented. The analysis of the evolution of the photospheric magnetic features and their coronal counterpart shows that there is a linear dependence between the EIT FeXII 195Å flux and the total magnetic flux of the photospheric bipolarity. The appearance of the coronal emission is associated with the emergence of new magnetic flux and the disappearance of coronal emission is associated with the cancellation of one of the polarities. In one of the cases the disappearance takes place 3-4 hours before the full cancellation of the weakest polarity.
The spectral data obtained with CDS show that one of the bright points experienced short time variations in flux on a time scale of 420-650s, correlated with the transition region lines (OV 629.73Å and OIII 599.60Å) and also the HeI 584.34Å line. The coronal line (MgIX 368.07Å) undergoes changes as well, but on a longer scale. A wavelet analysis of the temporal series reveals that many of these events appear in a random fashion and sometimes after periods of quietness. However, we have found two cases of an oscillatory behaviour. A sub-section of the OV temporal series of the second bright point shows a damped oscillation of five cycles peaking in the wavelet spectrum at 546s, but showing in the latter few cycles a lengthening of that period. The period compares well with that detected in the SVI 933.40Å oscillations seen in another bright point observed with the SUMER spectrometer, which has a period of 491s. The derived electron density in the transition region was cm with some small variability, while the coronal electron density was cm (Ugarte-Urra et al. 2004a).
In a follow-up study, we used wide slit (90arcsec240arcsec) movies of four EIT coronal bright points (BPs) obtained with CDS. The wavelet analysis of the HeI 584.34Å, OV 629.73Å and MgIX 368Å time-series confirms the oscillating nature of the BPs, with periods ranging between 600 and 1100s. In one case we detect periods as short as 236s. We suggest that these oscillations are the same as those seen in the chromospheric network and that a fraction of the network bright points are most likely the cool footpoints of the loops comprising coronal bright points. These oscillations are interpreted in terms of global acoustic modes of the closed magnetic structures associated with BPs (Ugarte-Urra et al. 2004b).