1. Limb prominences seen in UV, EUV and SXR
P. Heinzel (1,2), F. Fárník (1), U. Anzer (2)
and I. Dammasch (3)
(1) - Ondřejov Observatory, Czech Republic
(2) - MPA Garching, Germany
(3) - MSSL, Great Britain

2. SOHO/SUMER images of the prominence under study
September 5, 1996
SOHO/SUMER images of the prominence under study
Fe XII
Mg X
N V
S II

3. An overlay of the NV image (contours) over the FeXII image (left) and another overlay, the SII image (contours) over the MgX image (right).

4. Mechanisms of lowering the coronal brightness
absorption due to photoionisation in resonance continua
of HI (912 Å), HeI (504 Å) and HeII (228 Å)
volume-blocking due to presence of cool plasmas or low-density volumes (voids ?)
Kucera, Andretta & Poland (1998)
Anzer & Heinzel (2005)

5. Recent work: Batchelor & Schmahl (1995) – Yohkoh SXT darkenings
Daw, DeLuca and Golub (1995) – SXR limb darkening (NIXT)
Drago et al. (2001) – CDS, SUMER filaments
Heinzel, Schmieder and Tziotziou (2001) – EUV extensions
Engvold et al. (2001) – EIT prominence/filament darkening
Schmieder et al. (2004) – CDS, SUMER, TRACE, Hα
Del Zanna et al. (2004) – SOHO prominences

6. Photoionisation cross-section in EUV and SXR

7. τ = σH NHI + σHeI NHeI + σHeII NHeIIλ
625
284
171
100 50 10 r
0.4
0.1
r = τ / τ912
i = 0.5

9. SOHO/SUMER line profiles

10. τ = - ln (2 r/rbl - 1) rbl = r (Fe XII) = 0.73 r = r (Mg X ) = 0.39
τ625 = 2.7
τ912 = 6.8
τ171 = r = (rbl = 0.73)
τ50 = r = 0.73

11. Observation of the prominence under study made by SOHO/EIT in 171 A (upper left), 195 A (upper right), 284 A (lower left) and 304 A (lower right).

13. Dark structures in SXR Noticed by Batchelor & Schmahl (1994)
Dark features visible in SXT/Yohkoh images
Interpreted due to absorption
Questions:
How do they correlate with structures seen in UV-EUV ?
Do they really result from absorption of background SXR ?

14. An SOHO/EIT image of the prominence in 304 A (left) and two Yohkoh/SXT images of the same region in different color presentation (B&W are negative)

15. SOHO/EIT image in 304 A (left) and 195 A (right) overlaid as contours over an Yohkoh/SXT image. The structure in the middle seen in 304 A fits well into the soft X-ray emission gap.

16. Absorption of SXR by cool plasmas ?
negligible absorption by HI, HeI and even HeII
possible absorption by metals like in the quiet chromosphere, BUT needs sufficient column mass
(usually too low in prominences)
SXR absorption is probably not efficient in prominences (also Hudson – private comm.)
we thus interpret SXR darkenings as due to volume blocking, same as for FeXII line detected at 1242 Å

17. To be checked by modeling:
P. Foukal suggests an absorption even above HI Lyman continuum limit, as due to resonance continua of neutral metals in UV (like CI). But this seems to work only for rather high column densities.
Very high column densities may also lead to SXR absorption (including metals).
The proper effects on prominences (e.g. seen edge-on on the limb) have to be tested.

18. Diagnostics potential
combination of UV (blocking) and EUV (absorption/blocking)
(SUMER, CDS, EIT and TRACE images)
OR: combination of SXR (blocking) and EUV (absorption/blocking)
(Solar-B: EUV-imager (EIS) + XRT at higher spatial resolution)
Disentangling the absorption and blocking mechanisms
Determination of column densities of HI, HeI and HeII
Better understanding of the behaviour of
cool plasmas embedded in the hot coronal environment