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\noindent {\bf JOINT CDS/EIT PROGRAMME - JOP121}

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\noindent {\bf Strengthening of H$^{0}$ Lyman Continuum absorption in disk 
  filaments prior to eruption}

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\noindent Oddbj\o rn Engvold, Univ. Oslo \\ Einar Tandberg-Hanssen, Univ. 
Alabama \\ Jack B. Zirker, NSO/NOAO \\ Terry Kucera, NASA/GSFC \\ 
Paal Brekke, ESA/GSFC \\ Joe Gurman, NASA/GSFC

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\noindent{\small Received: 11 February 2000}

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\noindent {\it Scientific Justification}

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This programme is based on results obtained from a study of ``Filament
Absorption and Emission in Highly Ionized Iron and Neutral Hydrogen''
using images obtained with SOHO/EIT (Engvold et al. 2000; currently
being submitted for Solar Phys.) and the paper on ``Neutral Hydrogen
Column Depths in Prominences Using EUV Absorption Features'' by Kucera
et al. (1998, Solar Phys.  {\bf 183}, 107). The study by Engvold et
al. suggests that the absorption in neutral hydrogen (Ly C) increases
significantly 1-3 days prior eruption of the filaments. Also,
immediately before, and during eruption one observes significant
emission from the prominences/filaments in high temperature lines.

The absorption strength is obtained from the measured contrast of the
absorbing filaments relative to the brighter background. The two
earlier studies seem to show a wavelength dependence of the absorption
as expected from H$^{0}$ Ly C, as long as the filaments are in the
stable pre-eruptive phase. Closely before eruption the noted hot line
emission mixes in with an increasing absorption and change the
contrast in a way which somehow seems to depend on the temperature of
the lines.

It is the objective of this programme to monitor and study the
development of the H$^{0}$ Ly C absorption and hot filament line
emission as some of the filament goes into eruption. Furthermore, the
noted strengthening of the H$^{0}$ Ly C absorption is also an
interesting and potentially important signature of an approaching
eruption. Also, lines at wavelengths shorter than the He$^{0}$
threshold at 504\AA \ are included in the programme to see if an
additional contribution from neutral He may be detected in the assumed
strengthening effect.

The programme will involve monitoring of a number of larger filaments,
possibly 3 - 5 each day, with a cadence of 3 sequences per day over a
period of two weeks. We also wish to supplement each of the CDS raster
scans with one set of EIT images (He I 304\AA , Fe IX/X 171\AA , Fe
XII 195\AA , and Fe XV 284\AA ).  The exact locations of each sequence
will be determined from ground-based full disk H$\alpha$ images.

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\noindent {\it CDS}

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NIS Study: O\_HLYC

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Spectrometer: Normal Incidence

Slit: 4$\times$240 arc sec

Step (DX, DY): 4 arc sec, 0 arc sec

Raster Location: Centered on specified filaments

Exposure Time: 40 s

Duration of Rasters: 1404

Number of Rasters: 5-6 rasters, each at selected filament positions, 
3 times per day 

Total Duration: $\leq$8424

Line Selection: He I 537.04\AA , Fe XIV 334.17\AA , 

Ca X 557.77\AA , Fe XIII 348.18\AA , He I 584.33\AA , 

O III 599.59, Mg IX 368.07\AA , Mg X 624.85, O V 629.73\AA 

Pointing: Pre-selected disk filaments

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