Response of the expanding/contracting polar cap to weak and strong solar wind driving: Implications for substorm onset

Abstract

We quantify the amount of open magnetic flux in the magnetosphere from observations of the auroral polar cap on a near‐continuous basis for a period of 18 days, 20 August to 6 September 2005. This interval encompasses periods of weak, moderate, and strong solar wind driving, including two geomagnetic storms. We identify 49 substorms during the interval and determine the response of the polar cap to growth and expansion phases of the substorms. We find that the frequency of substorms and the flux closed by substorms both increase during enhanced solar wind driving, each approximately as the square root of the dayside reconnection rate. In addition, the average size of the polar cap increases during intervals when there is strong driving and especially when the SYM‐H index indicates that the ring current is enhanced. We suggest that this occurs for two reasons: because there is a delay between substorm onset and the closure of open magnetic flux in the magnetotail (while closed flux is pinched off), during which dayside reconnection can lead to further growth in the size of the polar cap, and also because the magnetotail is more stable to reconnection when the ring current is enhanced.