Field-Line Tracing from Locations of Polar Cap Neutral Density Anomalies to the Magnetosphere

Abstract:

Localized neutral density enhancement in the polar cap above 70^o magnetic latitude have been frequently observed during major geomagnetic storms. It has been suggested that energy input responsible for producing localized neutral density spikes is the dominant energy deposition in the polar cap. To better understand the origin of polar cap neutral density anomalies (PCNDAs) we trace magnetic field lines from the polar cap region at about 400 km to the magnetosphere using the data-based Tsyganenko magnetic field model TS05 [Tsyganenko and Sitnov, 2005] for the periods when CHAMP detected PCNDAs during major magnetic storms with the minimum Dst < -100 nT. The magnetopause boundary is specified according to the three-dimensional asymmetric magnetopause model recently developed by Lin et al. [2010]. The closest distance to the magnetopause along the traced field line path is determined as a function of time. The tracing results indicate that depending on Dst and locations PCNDAs could be connected through magnetic field lines either to the nightside magnetopause or to the magnetotail lobe. For some events field lines originating from a portion of the PCNDA region are found to cross the equatorial plane in the near earth tail region. We discuss the results to help elucidate the coupling between the magnetosphere and the thermosphere and its roles in producing polar cap density anomalies. 

References

Lin, R. L., X. X. Zhang, S. Q. Liu, Y. L. Wang, and J. C. Gong (2010), A three‐dimensional asymmetric magnetopause model, J. Geophys. Res., 115, A04207, doi:10.1029/2009JA014235.

Tsyganenko, N. A., and M. I. Sitnov (2005), Modeling the dynamics of the inner magnetosphere during strong geomagnetic storms, J. Geophys. Res., 110, A03208, doi:10.1029/2004JA010798.