Comparison of Solar EUV Irradiance Measurements from

CDS and TIMED/EGS

W. T. ThompsonL3 Communications EER, NASA GSFC

P. BrekkeESA Space Science Department

T. N. Woods, F. EparvierUniv. of Colorado, Lab. for Atmospheric and Space Physics

The CDS Normal Incidence Spectrometer (NIS) measures the solar EUV irradiance by rastering over the entire Sun. The whole process takes ~13 hours and is done roughly monthly since March 1997.

The TIMED satellite was launched on Dec. 7, 2001. One of the instruments in the Solar EUV Experiment (SEE) package is the EUV Grating Spectrometer (EGS), which measures the solar spectrum between 25 and 200 nm.

TIMED/SEE/EGS

The original CDS calibration was based on two sounding rocket flights in 1997, one of which included an earlier version of EGS. (The other was SERTS.) The current measurements allow us to explore the CDS calibration after SOHO’s accident in 1998, and to compare it to measurements made before the accident.

Instrumental comparisonThe two instruments have very different wavelength resolutions, and different analysis techniques. The CDS data depend on individual line widths to separate out emission lines from the instrumental background. The TIMED/EGS data are lower resolution, with more blending of lines, so the area under the curve is used for these data.

Monitoring of CDS intensities from synoptic measurements show an apparent drop in the quiet-Sun intensity of the Mg IX line at 36.8 nm, suggesting a drop in the short wavelength bandpass sensitivity of about a factor of 2.

No such drop was seen in the long wavelength bandpass, except possibly a small one in second-order He II at 30.4 nm.

A sounding rocket underflight by SERTS in 2000 also suggested a drop in the CDS sensitivity. However, that comparison has not yet been adopted because of concerns over the 30.4 nm comparison.

Do the TIMED/EGS measurements show the same intensity drop?

The comparison between the simultaneous CDS and TIMED/EGS measurements supports these inferences. The total integrated intensity over the CDS long wavelength bandpass (NIS-2) is approximately the same for both CDS and TIMED/EGS. However, in the short wavelength band (NIS-1) the CDS values are about a factor of 2 lower than those of TIMED/EGS.

Detailed analysis of these data will allow us to correct the CDS NIS-1 post-recovery calibration.

We will also be able to more accurately explore any changes in NIS-2.

Second-order 30.4However, the comparison at the second-order wavelength of 30.4 nm is not quite as expected. The TIMED/EGS instrument is showing lower emission at this wavelength than predicted by CDS. It should be noted that the results are sensitive to the wavelength range used to analyze the EGS data.

Individual linesThe comparison of individual lines shows possible variation as a function of wavelength in NIS-1. In the long wavelength channel NIS-2, all the lines agree well, except He I 58.4 nm. The reason is not yet clear.

Future workWe have only just received the TIMED/EGS data to compare with the CDS data. More work will be needed to precisely quantify the implications for the CDS calibration. The remaining slides outline some work which has been previously done with the CDS irradiance data, which can be improved with the comparison to TIMED/EGS.

This older analysis included correction factors of 1.447 for NIS-1, and 1.167 for second-order 304Å, such as in this comparison to EIT and SEM measurements. (The bold curves represent non-standard SEM analysis.)

We can reanalyze these and the following data to take the new TIMED/EGS comparison into account.

Comparison with EUV modelsWe compared our results against four standard models for the EUV irradiance:

•Hinteregger (Hinteregger et al., 1981)

•EUVAC (Richards et al., 1994)

•EUV97 (Tobiska and Eparvier, 1998)

•SOLAR2000/v1.20 (Tobiska et al., 2000)

These models use proxy indices, such as the solar radio flux at 10.7 MHz, to predict the EUV irradiance. This is important, because the EUV flux is an important driver for ionization in the upper atmosphere.(A newer version of the SOLAR2000 model is now available, but does not significantly change the results)

Black: CDS

Red: Hinteregger

Green: EUVAC

Blue: EUV97

Orange: Solar2000

This comparison over the two full CDS bandpasses helps distinguish the various models. Solar2000 came closest in absolute values, while EUVAC came closest to the shape of solar cycle variation.

Black: CDS

Red: Hinteregger

Green: EUVAC

Blue: EUV97

Orange: Solar2000

Correlations

Average behavior of each model after recovery is an extrapolation of pre-accident behavior. Solar2000 has significant solar minimum flux not seen in the other models, or in the CDS data. All the models have correlations of 0.85-0.89. If the correction factor of 1.447 had not been applied, the correlations would be slightly smaller (except Solar2000).

Black: CDS

Red: Hinteregger

Green: EUVAC

Blue: EUV97

Orange: Solar2000

Variation of individual lines

•Mg IX 368Å like full NIS-1

•Considerable variation from line to line within NIS-2, but mostly flat

•Mg X 610Å blend acts like NIS-1 rather than other NIS-2 lines.

Black: CDS

Red: Hinteregger

Green: EUVAC

Blue: EUV97

Orange: Solar2000

He II, Si XI 304Å

We can also compare second-order 304 to the models. SOLAR2000 stands out as significantly different. All the other models would be consistent with CDS if we did not apply the 1.167 post-recovery correction factor. This may also explain the step seen in the CDS to SEM comparison.

Conclusions• We have successfully compared CDS irradiance values to

simultaneous measurements by the SEE/EGS instrument aboard TIMED.

• This comparison confirms that the CDS post-recovery calibration in the NIS-1 channel requires adjustment.

• Such an adjustment is consistent with comparisons with other solar EUV measurements from EIT, SEM, and SERTS.

• It is also consistent with internal CDS monitoring of quiet-Sun intensities in Mg IX at 36.8 nm.

• We have also demonstrated how these data can be used to differentiate between solar EUV irradiance models.