Long Term Variation of Driven and Unloading Effects on the Polar Cap Index

Physics – Plasma Physics

Scientific paper

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[7899] Space Plasma Physics / General Or Miscellaneous

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 virtually identical 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. Thus the temporal variation of the response bears little resemblance to that of the input. The polar cap index, which has been used as a monitor of the ionospheric activity, can be decomposed into a driven component and an unloading component. A simple decomposition is achieved through a linear regression with the polar cap index as response and EK-R, the solar wind electric field modified as suggested by Kivelson and Ridley (2008), and AL as predictor. Such an analysis has been systematically applied day-by-day to most of the days in solar cycle 23. The driven/unloading ratio 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 for the responses of the drive component to the unloading component is smaller at solar maximum than at solar minimum. There is also a strong seasonal variation with stronger driven/unloading ratio in summer than in winter. The seasonal variation is as high as ± 30%. Finally, the driven/unloading ratio is not sensitive to the magnitude of the solar wind electric field, unless a threshold value close to 12 mV/m has been reached.

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