Impact of high-speed streams and co-rotating interaction regions on the thermosphere-ionosphere

Friday, November 2, 2012

Changes in the thermosphere-ionosphere system caused by high-speed streams in the solar wind, and the co-rotating interaction regions they engender, were studied using a combination of CMIT and TIEGCM simulations and data analysis. 

A comparison of thermospheric neutral density image
A comparison of thermospheric neutral density at 400 km measured by the STAR accelerometer on the CHAMP satellite with TIE-GCM simulations for the Whole Heliosphere Interval (WHI) during March–April 2008. (a) CHAMP observations from the ascending node, near 21–18 local solar time as indicated by the white dashed lines. (b) Simulations at the local time and latitude of the satellite using the TIE-GCM, with high-latitude forcing derived from the Weimer 2005 model using all measured solar wind and IMF parameters. (c) Simulations with the solar wind speed set to 400 km s-1 and density set to 4 cm-3. (d) Simulations with the IMF y and z components smoothed using a running 72-hour centered mean. (e) Simulations with the IMF y and z components set to zero.

It was found that the interplanetary magnetic field (IMF) is more important than solar wind speed and density per se in controlling magnetosphere-ionosphere coupling. Numerical experiments were conducted to elucidate the mechanisms of solar wind and IMF forcing.

The magnetospheric responses to these structures and consequent ionospheric drivers are simulated using the numerical Coupled Magnetosphere-Ionosphere-Thermosphere model and the empirical Weimer 2005 model, finding that the interplanetary magnetic field (IMF) is more important than solar wind speed and density per se in controlling magnetosphere-ionosphere coupling. The NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model is then employed to calculate neutral density, nitric oxide cooling, and electron density, for comparison to space-based measurements from the STAR instrument on the CHAMP satellite, the SABER instrument on the TIMED satellite, and GPS occultations from the COSMIC mission, respectively. The recurrent, periodic changes observed under solar minimum conditions during 2008, and particularly during the Whole Heliospheric Interval (March–April of 2008), are simulated by the model and compared to these measurements. Numerical experiments were conducted to elucidate the mechanisms of solar wind and IMF forcing, setting the solar wind speed and density to nominal values, smoothing the IMF, and also setting it to zero. The results confirm the importance of IMF variations, particularly its north-south component (Bz), but also show that when the average Bz values are negative (southward), the interaction with increased solar wind speed amplifies the magnetosphere-ionosphere-thermosphere response. Conversely, during events when Bz is on average positive (northward), even large increases in solar wind speed have small effects on the system. 

Reference: Solomon, S. C., A. G. Burns, B. A. Emery, M. G. Mlynczak, L. Qian, W. Wang, D. R. Weimer, and M. Wiltberger (2012), Modeling studies of the impact of high-speed streams and co-rotating interaction regions on the thermosphere-ionosphere, J. Geophys. Res., 117, A00L11, doi:10.1029/2011JA017417.