Modeling of ion composition in the topside ionosphere with emphasis on extremely low solar activity

Knowledge of an accurate ion composition in the outer ionosphere and plasmasphere is very important for fully understanding the propagation of radio waves with frequencies near the lower hybrid resonance (LHR) in this region and for many applications that use ray-tracing through this region. The IRI model describes ion composition in the topside ionosphere and it has been employed in many such studies. However, using new C/NOFS CINDI data it was found that IRI predicted values of ion composition did not fully reflect the extremely low solar activity conditions during the years 2008 and 2009 (Heelis et al., 2009; Klenzing et al, 2011). Particularly, large differences were found between the observed and predicted upper transition heights. C/NOFS measurement showed transition heights as low as 700km during daytime and about 500km during nighttime at the magnetic equator, while the IRI model predicted daytime transition height of about 900-1000km. In IRI-2012, there are two options for the ion composition in the topside ionosphere: a) TTS-2003 model (Triskova et al., 2003) and b) DY-1985 model. First we compare both models with the CINDI data to evaluate how well these models describe solar activity variation of the ion composition with special emphasis on the upper transition height. Next we focus on possible revision of the TTS-2003 model (Triskova et al., 2003). This model is based on ion mass spectrometer data from AE-C and AE-E for low solar activity and on Bennett ion mass spectrometer data from the Intercosmos-24 satellite for high solar activity. We study the consistency of ion composition data from AE satellites and from C/NOFS and discuss possible improvements of the ion composition model with C/NOFS data. In addition we will also report on model improvements using a different solar proxy (currently it is daily F10.7), a better interpolation scheme in altitude and a better suited latitudinal coordinate. Our main goal is to propose an improved ion composition model for IRI that better matches the measured ion composition during the lowest levels of solar activity. Heelis, R.A., Coley, W.R., Burrell, A.G., Hairston, M.R., Earle, G.D., Perdue, M.D., Power, R.A., Harmon, L.L., Holt, B.J., Lippincott, C.R., 2009. Behavior of the O+-H+ transition height during the extreme solar minimum of 2008. Geophys. Res. Lett. 36, doi:10.1029/2009GL038652. Klenzing, J., F. Simões, S. Ivanov, R. A. Heelis, D. Bilitza, R. Pfaff, and D. Rowland (2011), Topside equatorial ionospheric density and composition during and after extreme solar minimum, J. Geophys. Res., 116, A12330, doi:10.1029/2011JA017213. Triskova, L., V. Truhlik. and J. Smilauer (2003), An empirical model of ion composition in the outer ionosphere, Adv. Space Res., 31, No. 3, pp. 653-663, doi:1O.1016/S0273-1177(03)00040-1