P' Heinzel 1,2, F' Frnk 1, U' Anzer 2

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Limb prominences seen in UV, EUV and SXR

• P. Heinzel (1,2), F. Fárník (1), U. Anzer (2)
• and I. Dammasch (3)
• (1) - Ondrejov Observatory, Czech Republic
• (2) - MPA Garching, Germany
• (3) - MSSL, Great Britain

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SOHO/SUMER images of the prominence under study
September 5, 1996
Fe XII
Mg X
N V
S II
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An overlay of the NV image (contours) over the
FeXII image (left) and another overlay, the SII
image (contours) over the MgX image (right).
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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)

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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, Ha
• Del Zanna et al. (2004) SOHO prominences

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Photoionisation cross-section in EUV and SXR
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t sH NHI sHeI NHeI sHeII NHeII
r t / t912 i 0.5
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SOHO/SUMER line profiles
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t - ln (2 r/rbl - 1)

• rbl r (Fe XII) 0.73
• r r (Mg X ) 0.39
• t625 2.7
• t912 6.8
• t171 0.27 r 0.64 (rbl 0.73)
• t50 0.006 r 0.73

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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).
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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 ?

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

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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.

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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