An assessment of the NRLMSISE-00 density thermosphere description in presence of space weather events C. Lathuillère and M. Menvielle The data and the method Statistical analysis for year larger events of 2004 Conclusion

STAR/CHAMP densities – May 3 rd, 2003 Densities ( g.cm -3 ) The data to the sun daytime and night time orbit sections are considered separately … between -50° and 50° in latitude, with a 1° sampling rate with respect to the latitude STAR/CHAMP densities – May 3 rd, 2003 STAR atmosphere density along CHAMP trajectory at about 400 km altitude Densities ( g.cm -3 ) ~5:30 pm LT at equator ~5:30 am LT at equator Inclination:87°

TOTAL MASS DENSITY at the satellite altitude ( g/cm 3 ) 10 LT 22 LT The method: Running SVD analysis over 15 consecutive orbits (about 1 day) The projection on the first component accounts for large scale variations: spatial variations are captured by the first principal component, and time variations are captured by the associated projection coefficient: C1. Residuals account for smaller scales, as tides and gravity waves

Comparison with NRLMSISE-00 model NRLMSIS: with MgII proxy and ap CHAMP data Normalized coefficient: C1 CHAMP / C1 NRLMSIS_quiet C1 NRLMSIS/ C1 NRLMSIS_quiet NRLMSIS_quiet: with MgII proxy and Ap=4 MgII proxy: The composite MgII index (Viereck et al, 2004) is used as a proxy for solar EUV instead of F10.7

NRLMSIS_quiet is used as a reference, that accounts for LT, seasonal variations, solar activity…

- Statistical analysis: binning of normalized C1 coefficients as a function of ap - Analysis of the height larger events

Day timeNight time x : CHAMP data x : NRLMSIS model correlation coefficient between CHAMP data and ap ap index Density perturbation Binning is done using the value of ap from the previous 3 hour interval

x Day time + Night time

Linear fit y =1.9 x – 0.9 x Day time + Night time

x Day time + Night time Day and Night time with ap <100

x Day time + Night time Day and Night with ap <100 Quadratic fit y =1.92 x 2 – 3 x

Density perturbation: CHAMP data and NRLMSIS model Day of 2004 ap magnetic index Day time Night time - the relative density increase is greatly underestimated by the model - the model seems to correctly represent the shape of the perturbation 3 days

The perturbations appear later in the data than in the model and this timing discrepancy is slightly larger during night ( hours) than during day time. Density perturbation: CHAMP data and NRLMSIS model normalized to the amplitude of the data perturbation Normalization factor mean value = days

Conclusion NRLMSISE-00 correctly estimates the main features of the thermosphere density response to geomagnetic activity: - the morphology of UT variations - the larger relative increase during night than during day time But it underestimates : the amplitude of the density response (by about a factor 2) and its phase lag (up to 4.5 hours) Altitudes about 400km. Latitudes between 50S and 50N