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Pavol Schwartz (Astronomical Institute of the Slovak Academy of Sciences)

Non-LTE modelling of the prominence fine structure using hydrogen Lyman line profiles
Wann Am 05.05.2014 von 11:00 bis 12:00
Was
  • Kolloquium
  • Special Seminar
Wo SH, Hörsaal
Termin übernehmen vCal / iCal

A prominence on NW limb was observed on 18 May 2005 in several EUV spectral lines and in whole hydrogen Lyman  series (including Lyman alpha) by the CDS and SUMER spectrographs, respectively, both on board of the SOHO satellite. From Earth the prominence was observed by the National Solar Telescope in the H_alpha center. The prominence looks in H_alpha as two radially directed columns while in EUV transition-region spectral lines OV 629 A and NeVI 562 A we can see in emission both these two structures and material between them. Emission in these two lines, which temperatures of formation are 250000 and 400000 K, respectively, indicates presence of such rather hot plasma in prominence except of cool plasma (temperatures from approximately 10000 up to several ten thousands K) emitting in H_alpha and hydrogen Lyman lines. In the HeI 591 A the both H_alpha structures are seen in absorption in bright background. That indicates that the H_alpha structures contain very dense and cool plasma that emits less radiation in the HeI line than hotter and less dense plasma in between the two structures. The active part of the SUMER slit is crossing Northern one of the two H_alpha structures and partly area between them. Observations of SUMER were made with static slit and 18 observations of Ly_alpha and 36 observations of the higher Lyman lines were made. This large set of observed profiles of Ly_alpha -- Lyman_delta was statistically compared with synthetic profiles obtained using the multi-thread model with identical threads approximated by 2D horizontal slabs embedded in horizontal dipped magnetic field and irradiated from the chromosphere at their sides. The following four characteristics of profiles were used for statistical comparison: integral intensities, Lyman decrements -- ratios of integral intensities of individual Lyman lines and Lyman_beta, two-peaked profile asymmetries and ratio of intensities at profile central reversals and peaks. The multi-thread model with hotter outer threads was also tested and it was found: Temperatures in interior of the outer thread can be only slightly higher (several thousand K) that in internal threads. But still a prominence can have a hotter shell which radiates in transition-region lines such as OV 629 A and NeVI 562 A. Each thread has its own prominence-to-coronal transition region with an increase of temperature which is much steeper across than along magnetic field.