CDS Users Meeting The Coseners House 21/22 September 2005

CDS Users Meeting 21-22 Sep. 2005 In-flight monitoring and validation of the CDS NIS radiometric calibration Lang et al. Page 1

The in-flight monitoring and validation

of the CDS NIS radiometric calibration

J Lang, DH Brooks, AC Lanzafame, R Martin, CD Pike and WT Thompson

CDS Users Meeting The Coseners House 21/22 September 2005


Status of CDS Radiometric Calibration

• Calibration as presently implemented in the CDS software is as defined by Lang et al (2002) in “The Radiometric Calibration of SOHO”, ISSI Scientific Report SR-002, ed. A Pauluhn, MCE Huber and R von Steiger, p105.

• Radiometric calibration built on:

– laboratory calibration using a secondary standard source

– joint observations with the NASA LASP rocket

– joint observations with the NASA SERTS rocket

– intercalibration observations with SUMER

– papers by Landi et al and Del Zanna et al

• The burn-in corrections for NIS and GIS are also detailed in the ISSI paper.


Present Work

• The present work is to look at the calibration as a function of time by measuring line ratios. Three classes of line ratio are investigated.

– branching ratios (Griffin and McWhirter)

– density and temperature insensitive line ratios (Neupert and Kastner)

– density and/or temperature sensitive line ratios

• Data – use NISAT_S (ID7, variation 2) which gives the full NIS spectra and builds up a 20” by 240” raster using the smallest slit (2” by 240”). This has been run at least once a week on QS and at least once a month on AR and CH. Area of Sun observed checked using EIT Fe XI 195 Å. The corrected and calibrated data were averaged over the 10 mirror positions in the raster. To avoid any possible edge effects the lowest 15 and highest 9 pixels were ignored and the remaining 120 were divided into 4 equal segments along the slit. Only one segment was used in this work.


Monitoring NIS-2 O III 2s22p2 1D – 2s2p3 1P (525.8 Å) and 2s22p2 1S – 2s2p3 1P (597.8 Å)

Branching ratio with well-known A values and good wavelength span.

597.8 Å is blended with 3/2-3/2 member of 3s23p2 2P - 3s3p3 2D multiplet of Ca VIII. Blend corrected using adjacent 3/2-5/2 component at 596.4 Å. The correction is density sensitive. However, average correction is 3 % so even 50 % uncertainty in correction has a very small effect.In graphs, points just have fitting uncertainties, while theory is shown as solid line with dashed line having theory uncertainties, burn-in uncertainties and calibration uncertainties.

Quiet Sun observations are shown. The corresponding results for AR are about 10 % higher which could indicate a blend affecting the 525.8 Å line. The agreement between theory and observations is reasonable. If the blend also affects QS observations, this would account for the ratio being slightly high. The loss of attitude period is easily identified. The ratio is slightly lower after the loss of attitude. However, the agreement between theory and measurement has been maintained for six years.


Monitoring NIS-2 O II 2s22p3 4S – 2s22p23s 4P (539 Å) and 2s22p3 2D – 2s22p23s 2P (616 Å)

The ratio of the intensity of the 3/2-5/2 transition (539.1 Å) or the blend of the 3/2-3/2 and 3/2-1/2 components (539.6 Å) of the 4S - 4P transition to the blend of the 5/2-3/2 and 3/2-3/2 components of the 2D – 2P multiplet at 616.3 Å gives a good wavelength span but is temperature sensitive. O II has maximum population in ionisation balance at 3.2 104 K and is zero by 1.0 104 K and 1.6 105 K.


Monitoring NIS-2 O II 2s22p3 4S – 2s22p23s 4P (539 Å) and 2s22p3 2D – 2s22p23s 2P (616 Å)

The ratios I(539.1 Å)/I(539.6 Å) before and after the loss of spacecraft attitude are constant but different. Comparison with the other two ratios indicates the change can be ascribed to an increase in the intensity of the 539.1 Å line.

The ratio I(539.6 Å)/I(616.3 Å) shows little scatter. The difference between theory and measurements is attributed to the atomic data, most likely the 2-3 effective collision strengths. There is a slight fall in the ratio in the first year of operations. However, the ratio remains the same after the attitude-loss of the spacecraft albeit with a larger scatter. This indicates that the calibration has been maintained over the six years of observations.


Monitoring NIS-2 O IV 2s22p 2P3/2 – 2s2p2 2P3/2 (554.5 Å) and 2s22p 2P1/2 – 2s2p2 2S1/2 (608.4 Å)

or 2s2p2 2D – 2p3 2P (617.0 Å)

I(554.5 Å)/I(608.4 Å) is temperature sensitive:Ratio Te4.7 6.0 104 K (0)5.9 1.6 105 K (Peak IB)6.4 4.0 105 K (0)

Ratio agrees with theory. Loss-of attitude gap is obvious, but the ratio changes although still within the uncertainties. The 608.4 Å 2s22p 2P – 2s2p2 2S line is in the wings of a very strong line and the problem is the burn-in correction compounded by the non-Gaussian shape of the post-recovery lines. The spectra moved on the detector following the attitude-loss and this put the 608.4 Å line where the strong 609.8 Å Mg IX line previously caused detector burn-in. The 608.4 Å line is the most difficult NIS-2 burn-in case.

The I(554.5 Å)/I(617.0 Å) ratio is more temperature dependent than the first ratio. The 617.0 Å feature is a blend of O IV and O II. The O II line is 2s22p3 2D3/2 – 2s22p23s 2P1/2 and was corrected using the intensities of the other members of the multiplet at 616.3 Å. This ratio shows that agreement between predicted and measured ratio is satisfactory and also that there was no change over the the loss-of attitude period. The radiometric calibration at 608.4 Å will stand as the problem is burn-in.


Monitoring NIS-2 Second Order Si XI 2s2p 1P - 2p2 1D (604.1 Å) and 2s2 1S - 2s2p 1P (2*303.4 = 606.8 Å)

In NIS-2 the Si XI 2s2p 1P - 2p2 1D (604.1 Å) line is seen in 1st order and the 2s2 1S - 2s2p 1P resonance line in 2nd order at 2303.4 = 606.8 Å, in the wings of the second order He II 304 Å line.

For this ratio at high electron density (3 1016 cm-3) theory and a theta-pinch measurement agree, in the regime where stepwise excitation of 2p2 1D via 2s2p 1P predominates and gives density sensitivity. The solar ratio is independent of density.

The measured ratio agrees with theory within the uncertainties. Most of the data points lie below the theoretical ratio. The loss-of-attitude made no discernable change to the ratio, i.e. to the relative calibration of NIS2 in first and second order. There is some variation with time for the later data well after the loss-of-attitude. This may be a burn-in effect as the 303.4 Å line moved to where the red wing of the strong second order He II 303.8 Å line was prior to the loss-of-attitude.


Monitoring NIS-2 and NIS-1 Intercalibration Si XI2s2p 3P2 – 2p2 3P1 (371.5 Å) and 2s2 1S – 2s2p 3P1 (580.92 Å)

The 3P – 3P multiplet is observed in active region spectra and the 2-1 371.5 Å line is the only unblended member. The ratio I(371.5 Å)/I(580.9 Å) is insensitive to plasma conditions for active regions. The measured and theory ratios are shown. Lang et al pointed out that there could be difficulties with the atomic model for triplet lines at theta-pinch densities (> 1 1016 cm-3).

The figure shows the measured active region ratios. The measurements lie below the theory, with some agreeing within the uncertainties. There is no trend over the six year period and no apparent change before and after the period of loss-of-attitude of SOHO.

The Si XI 580.92 Å line could be affected by a blend from O II and this is still being investigated.


Monitoring NIS-2 and NIS-1 Intercalibration O III 2s22p2 3P – 2s22p3s 3P (374 Å)

This multiplet is observed as three lines and the ratios formed are shown. The weighted means of the ratios are all higher than theory. The ratios with the 1-2 or the sum of the 1-0 and 2-1 components in the numerator are higher than the ratio between the lines, it could be argued that these lines have unknown blends.


Monitoring NIS-2 and NIS-1 Intercalibration O III 2s22p2 3P – 2s22p3s 3P (374.1 Å, blend of 0-1,2-2,1-1 lines) and 2s22p2 1D2 – 2s2p3 1D2 (599.6 Å)

The ratio I(374.1 Å)/I(599.6) is temperature sensitive and slightly density sensitive, the densities shown in the figure being 1 108 cm-3, 1 109 cm-3 and 1 1011 cm-3. The temperature of maximum ion population in ionisation balance is 1 105 K. Below 3.2 104 K and above 3.2 105 K the populations are zero. Thus a range of values from about 0.014 to 0.3 is expected.

The spread of values is as expected apart from one point. The majority of measured ratios lie within a much narrower range. There is no trend in the ratio over the six year period and no apparent change before and after the period of loss-of-attitude of the spacecraft.


Monitoring NIS-2/NIS-1 Intercalibration Ne V2s22p2 3P – 2s2p3 3P (2-3, 2-2, 572.3 Å) and2s22p2 3P – 2s2p3 3S (2-1, 359.374 Å)

The theory ratio is sensitive to temperature. It is insensitive to density in the range from 2 108 cm-3 to 1 1015 cm-3 and then increases with density. The temperature of maximum ion population in ionisation equilibrium is 3.2 105 K and zero below 1 105 K and above 8 105 K. The experimental point is from Lang (1988) from a theta-pinch measurement at a density of 6.2 1015 cm-3. If uncertainties of around 30 % in the theory are taken into account, then experiment and theory agree within the uncertainties.


Monitoring NIS-2/NIS-1 Intercalibration Ne V2s22p2 3P – 2s2p3 3P (2-3, 2-2, 572.3 Å) and2s22p2 3P – 2s2p3 3S (2-1, 359.374 Å)

From the theory a ratio from about 0.12 to 0.50 is expected. For the period before the loss of attitude of SOHO the ratios are above that expected from theory for the temperature of maximum ion population in ionisation equilibrium and many are above the temperature at which the ion population should be zero. However, there is still agreement within the uncertainties. The results taken before the loss-of-attitude of SOHO indicate the usefulness of this temperature sensitive line ratio for calibration purposes. After the loss-of-attitude the results are more sparse and varying. This is a direct result of the loss of resolution of NIS-1 after the loss-of-attitude. The adjacent Fe XIII lines are difficult to resolve. However, the results indicate that there is no obvious change in the ratio after the loss-of-attitude.


Monitoring NIS-1 Fe XI3s23p4 3P – 3s3p5 3P 2-2 at 352.7 Å and 1-2 at 369.2 Å

These lines form a branching ratio. In a critical evaluation Shirai et al gave A(2-1)/A(1-2) = (1.1 109)/(3.5 108) = 3.14. In papers not included in that work Bhatia and Doschek gave A(2-1)/A(1-2) = (2.529 109)/(8.007 108) = 3.16and Fritzsche et al gaveA(2-1)/A(1-2) = (2.23 109)/(6.98 108) = 3.20Although the ratios are the same there are factors of two differences in the A values.

There is no discernable change in the ratio after the loss-of-attitude or with time over the six years of observation.

The weighted mean of the ratios before and after the loss-of-attitude are 2.23±0.09 and 2.41±0.14. The spread in this branching ratio is greater than the spread for ratios dependent on plasma conditions. This ratio has also been observed by SERTS. Again, there is a similar scatter in the results.

SERTS-89 SERTS-91 SERTS-93AR 3.28±0.58 1.53±0.43 2.90±0.72QS 2.46±0.78 3.48±0.87Off limb 4.00±1.88


Monitoring NIS-2 Fe XII3s23p3 4S – 3s3p4 4P, 3/2-1/2 at 346.9 Å and 3/2-5/2 at 364.5 Å

The intensity ratio I(346.9 Å)/I(364.5 Å) is shown for quiet Sun observations.

The ratio is insensitive to electron density and temperature for the quiet Sun and active regions.

The pre-loss data are lower and less variable than the later data.

In active region spectra the Si XI 2s2p 3P1 – 2p2 3P1 line will blend with the 364.5 Å line. Young et al estimate a 7% contribution and Landi et al <5%. Del Zanna et al say the line is blended in active regions but not in the quiet Sun. The blend was not observed in the quiet Sun spectra of Brooks et al.

The lines are weak and hard to fit, especially for the post-loss data. SERTS data are also variable. Many of the post-loss results agree with theory and it is difficult to conclude that a calibration change is needed post-loss.

SERTS results SERTS-89 SERTS-91 SERTS-93AR 0.27±0.05 0.56±0.11 0.43±0.07QS 0.53±0.11 0.35±0.06Off-limb 0.55±0.11


Monitoring NIS-2 Mg VIII2s22p 2PJ – 2s2p2 2PJ (311.8 Å, 313.7 Å, 315.0 Å and 317.0 Å)

Ratios involving the lines of the multiplet are shown. For pre-loss spectra, Lanzafame et al found that the 315 Å line was blended and the fitted pre-loss data here have been reduced by a factor 1.5. Brooks and Warren found a smaller factor.

I(313.7 Å)/I(317.0 Å) branching ratios agrees with theory and observations before and after loss-of-attitude agree.

I(315.0 Å)/I(311.8 Å) branching ratio, data are sparse and the 311.8 Å line is difficult to fit post-loss.

For the density sensitive line ratio I(315.0 Å)/I(317.0 Å), the results are lower than theory but there is no change with time, although the scatter is greater post-loss.

For I(311.8 Å )/I(317.0 Å ) the post-loss 311.8 Å line is difficult to fit. Active region results show that this ratio differs before and after the loss-of-attitude and the ratio is outside the uncertainties post-loss. The conclusions are that the pre-loss sensitivity at 315 Å should be increased and that the post-loss sensitivity at 311.8 Å should also be increased.


Monitoring NIS-2 Mg VIII2s22p 2P1/2 – 2s2p2 2P1/2 (313.7 Å) and 2s22p 2P3/2 – 2s2p2 2S1/2 (338.98 Å)

This line ratio is independent of temperature and density in active region spectra. Active region line ratio measurements are shown. The pre-loss data yield a steady ratio while the post loss data show larger variation and are higher with most points agree with theory within the uncertainties.

It is difficult to draw conclusions. The 338.98 Å line is difficult to fit, particularly post-loss. Ascribing the change to the 313.7 Å line is inconsistent with the ratios just discussed. If the change is in the 338.89 Å line, it has to reduce the intensity which rules out blending. It could well be that the post-recovery responsivity needs to be changed at 338.98 Å.


Monitoring NIS-2 Fe XVI2p63s 2S – 2p63p 2P (1/2-3/2 at 335.4 Å and 1/2 -1/2 at 360.7 Å)

The active region ratio I(335.4 Å)/(360.7 Å) is shown. The theory ratio is insensitive to electron temperature and density for active region conditions. The 335.4 Å line is blended with Mg VIII and Fe XII. The corrections for each average at 2 % and thus the uncertainty in the corrected ratio is insensitive to the uncertainty in the correction.

The agreement with theory is within the uncertainties. There is a 17 % increase in the weighted means of the ratios taken before and after the the loss-of-attitude period. However, this cannot be attributed directly to changes in responsivity. As noted by Lang et al. the wide-slit burn-in is not well known for these lines and could well account for the differences in the ratios before and after the the loss-of-attitude.

SERTS Results:

SERTS-89 2.2±0.5SERTS-91 1.9±0.3SERTS-93 2.0±0.3


Discussion and conclusions

The responsivities of NIS-2 and NIS-1 are shown along with the line pairs used in the present work.

For NIS-2, the O II, O III and O IV lines give good wavelength coverage. For data taken before the loss-of-attitude, all ratios agree with theory within the uncertainties, validating the calibration. For post-loss data, there are small changes in O II and O III ratios while the O IV ratio involving the 617.0 Å line increases slightly with time.

For NIS-2 in second order, the measurement of the Si XI 303.4 Å line could be affected by burn-in post-recovery. However, data taken before the loss-of-attitude validates the calibration.

For NIS-2/NIS-1 inter-comparison, line ratios for Si XI, O III and Ne V validate the calibration. The ratios also indicate that the loss-of-attitude did not cause any change in the inter-calibration



For NIS-1, the results from Fe XI and Fe XII are found to be variable as in other measurements. Both could be affected by unknown blends. The Mg VIII ratios reveal some problems. The calibration post-loss at 338.98 Å could well be in error. The calibration pre-loss at 315 Å is also causing problems and needs revising. The same is true at 311.8 Å. Difficulties with the Fe XVI lines at 335.4 Å and 360.7 Å are most probably a result of incomplete burn-in correction.

.



The line ratios measured in the present work indicate that overall, apart from three specific wavelengths, the relative radiometric calibration is validated, yielding results which agree with theory within the uncertainties and being maintained over the years of observation. Problems with the burn-in correction account for difficulties at 608.4 Å, 303.4 Å (seen in second order), 335.4 Å and 360.7 Å.

It can be considered that the line ratios used here relate the calibrations of NIS-1, NIS-2 second order and NIS-2 low wavelengths to the long wavelength end of NIS-2

The radiometric inter-calibration of CDS and SUMER was done for lines in the longer wavelength range of NIS-2 for the first 4.6 years of operations and indicate that the long wavelength part of the NIS-2 calibration was maintained. The present results thus extend this comparison to the whole NIS wavelength range.

The line ratios method has been extended successfully to use ratios which depend on the plasma conditions.

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CDS Users Meeting The Coseners House 21/22 September 2005