1. ESWW4, Brussels, Novembre 2006 Retrieving the EUV solar spectrum from a selected set of lines for space weather purposes: A review of theories, models and experiments J. Lilensten, T. Dudok de Wit, M. Kretzschmar, P.- O. Amblard, S. Moussaoui, J. Aboudarham and F. Auchère (ias)

2. ESWW4, Brussels, Novembre 2006 Importance of EUV flux for space weather purposes Influences the near space environment (ionosphere / thermosphere) Wavelength

3. ESWW4, Brussels, Novembre 2006 Result (amongst other phenomena) in Dilatation of the thermosphere (density may increase by a factor of 10 at the altitude of the International Space Station) rapid variations, creations of small scale disturbances, winds

4. ESWW4, Brussels, Novembre 2006 Pap and Frölich, 1999; Nesme-Ribes and Thuiller, 2000 Variations are not linear

5. ESWW4, Brussels, Novembre 2006 CELIAS/SEM experiment, Bochsler, 1999 14 July 1998 27 days Their characteristic variation times range from the second to several years

6. ESWW4, Brussels, Novembre 2006 2) Existing models for Aeronomy. Poorly sampled and / or based on too short data sets (2 years with TIMED) Torr and Torr (1985) SC#21REF, f10.7 = 70 F79050, f10.7 = 243 He + et Si 9+ Fe 14+ Si 10+ et He + Mg 10+ Ne 6+ O5+O5+ He O 3+ et Mg 9+ O4+O4+ O ++ N 3+ et Ne 7+ O3+O3+ C ++ Lyman  O5+O5+

7. ESWW4, Brussels, Novembre 2006 SERF / HFG Torr, M.R., and D.J. Torr, J. Geophys. Res. 90, 6675-6678, 1985 EUV / SOLAR 2000 Tobiska, W. K. and F. Eparvier, Sol. Phys., 177, 147-159, 1998 EUVAC / HEUVAC Richards, P. G., J. A. Fennelly, and D.J. Torr, J. Geophys. Res. 99, 8981-8992, 1994 P. G. Richards, T. N. Woods, and W. K. Peterson, Adv. Spa. Res., 37, 315- 322,2006 NRLEUV Lean et al, J. Geophys. Res. 108, 1059, 2003 Warren et al., J. Geophys. Res. 106, 15 745, 2001 Other: Nusinov A. A., models for prediction of EUV and W-ray solar radiation based on 10.7 cm radio emission, Ed. R.F. Donnely, NOAA ERL, 1992

8. ESWW4, Brussels, Novembre 2006 2) Existing models for Aeronomy. Based on indices (Ri, f10.7, Mg …) or proxies (E10.7, X10.7, I u … ) (see poster P2.7, Menvielle) Year

9. ESWW4, Brussels, Novembre 2006 2) Existing models for Aeronomy. The proxies are NOT well correlated to the solar activity See poster P3.14, Dudok de Wit et al. T. Dudok de Witet al., Which solar EUV proxies are best for reconstructing the solar EUV irradiance?, submitted to Adv. Spac. Res., 2006

10. ESWW4, Brussels, Novembre 2006 3) To measure the full spectrum ?. Difficult and expensive Schmidtke et al., Tiger program, Adv. Sp. Res., 10, 1553-1559, 2002

11. ESWW4, Brussels, Novembre 2006 Today : only Solar Extreme Ultraviolet Experiment (SEE) onboard TIMED (NASA) XPS EGS EUV Grating Spectrograph (EGS) 25 à 200 nm 0.4 nm spectral resolution. XUV Photometer System (XPS) 0.1 to 35 nm in 12 spectral bands Woods, et al., Solar EUV Experiment (SEE): mission overview and first results. J. Geophys. Res., 110, 1312- 1336, 2005.

12. ESWW4, Brussels, Novembre 2006 Retrieve the flux variation through their effects -On electron density profiles (Mikhailov and Schlegel, Ann. Geoph., 18, 1164 – 1171, 2000) -On the E region critical frequency (Nusinov, Adv. Space Res., 37, 426- 432, 2006) -On the thermospheric emission (Singh and Tyagi, Adv. Space Res., 30, 255è-2562, 2002) -On wind or temperatures (Zhang et al., Geoph. Res. Lett., 29, 10.029/2001GL013579, 2002) …  Several attempts to retrieve the full spectrum

13. ESWW4, Brussels, Novembre 2006 These method depend all on a given model: - They cannot be general - They cannot be used as operationnal tools - They are absolutely required for tests

14. ESWW4, Brussels, Novembre 2006 Reduce the solar spectrum to a limited (3) set of characteristic spectra: very promissing method through positive source separation SEE (TIMED) : reconstruction < 2% Quiet sun contribution? Active zone contribution? Hot lines contribution?

15. ESWW4, Brussels, Novembre 2006 Physics through Differential Emission Measure: 6 to 10 lines are necessary See poster P6.4, Kretzschmar et al. M. Kretzschmar et al., Retrieving the Whole Solar EUV Flux from 6 Irradiance Line Measurements, Advances in Space Research, 37, 341–346, 2006

16. ESWW4, Brussels, Novembre 2006 Use statistics (Singular Value Decomposition): 6 to 10 lines are necessary. Same conclusion through 2 totally different approaches T. Dudok de Wit, et al., Retrieving the solar EUV spectrum from a reduced set of spectral lines, Annales Geophysicae, 23, 3055–3069, 2005

17. ESWW4, Brussels, Novembre 2006 How to choose the lines to be observed? Dendrogram of 38 spectral lines using an average distance linkage between all lines. Statistical analysis of TIMED/SEE data. Using two years of daily EUV spectra and classification techniques,

18. ESWW4, Brussels, Novembre 2006 This table allows to select the best set of lines to observe in order to reconstruct the total flux with the best accuracy. The relative global error for the best combination of 6 lines yields to 3.7% and less than 0.25% with 10 lines

19. ESWW4, Brussels, Novembre 2006 However, the « best » set may depend on the application. An example : the ionosphere How to choose an observed set of solar lines for aeronomy driven applications, J. Lilensten et al., submitted to Ann. Geoph., 2006

20. ESWW4, Brussels, Novembre 2006 Using a multidimensional scaling technique : H I at 102.572 nm CIII at 97.702 nm OV at 62.973 nm HeI at 58.433 nm FeXV at 28.415 nm HeII at 30.378 nm Allows to retrieve the full solar spectrum with a relative global error of 6.8 % and still fulfill ionospheric physics requirements.

21. ESWW4, Brussels, Novembre 2006 Near future: LYRA, the Solar VUV radiometer on-board PROBA II J.-F. Hochedez et al., Adv. Space Res., 37, Iss 2, 303-312, 2006 http://lyra.oma.be/index.php 1/ Lyman-alpha (115-125 nm) 2/ the 200-220 nm range 3/ Al filter channels (17-70 nm 4/ MgF2 windows (120-220 nm), And EVE on-board SDO http://lasp.colorado.edu/eve/eve_home.html