Long-term variation of driven and unloading effects on polar cap dynamics

Details Long-term variation of driven and unloading effects on polar cap dynamics Long-term variation of driven and unloading effects on polar cap dynamics

Feb 2012

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012jgra..11702203g&link_type=abstract

Journal of Geophysical Research, Volume 117, Issue A2, CiteID A02203

Physics

Magnetospheric Physics: Magnetosphere/Ionosphere Interactions (2431), Magnetospheric Physics: Polar Cap Phenomena, Magnetospheric Physics: Solar Wind/Magnetosphere Interactions

Scientific paper

The influence of the solar wind on the magnetosphere-ionosphere system can be described in terms of driven and unloading processes. In a driven process, the response is proportional to the input with at most a small time delay. In an unloading process, the input energy accumulates without a corresponding output until, after a significant time delay, the energy is unloaded. The polar cap index, which has been used as a monitor of the polar cap activity, can be decomposed into a driven component and an unloading component through a linear regression with the polar cap index as response and EK-R, an electric field proposed by Kivelson and Ridley (2008) as representative of the electric field imposed on the ionosphere by magnetopause reconnection when saturation is taken into account, and AL index as predictors. Such an analysis has been systematically applied day-by-day to most of the days in solar cycle 23. The driven-to-unloading ratio (DUR) is characterized in terms of the ratio of the regression coefficients for an appropriately normalized representation of the predictors. We have found that the ratio of the responses of the driven component to the unloading component is smaller near solar maximum than near solar minimum with variation of around ±5%. There is also a strong annual variation of as much as ±15% with stronger driven-to-unloading ratio in summer than in winter. The variation of driven-to-unloading ratio in solar cycle 23 is explained in terms of enhancement of ionospheric conductance by electron precipitation during strong solar wind high-speed streams and the annual variation can be understood as seasonal variation of conductance caused by solar illumination.

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