Dayside open field line region boundary at high altitudes

The Polar satellite with its high apogee and high latitude orbit offers a unique opportunity to chart the dayside open/closed field line boundary (OCB). The data from energetic particle and plasma measurements are examined to obtain the position of the OCB for a range of IMF and magnetic disturbance conditions. The Polar observations were taken within two hours of local noon in the spring and fall 1996 and spring 1997. These data were examined for evidence of IMF and solar wind control of the OCB. Some evidence of control was observed in the Polar data. However, the OCB position was found to depend more strongly on the magnetic activity levels than on IMF B(Z) and solar wind pressure, for example. Examination of the invariant (Λ) and magnetic (λ) latitude of OCB for B(Z) < 1 gave better results than using the whole data set. For B(Z) < 1 the OCB Λ ~ 78.9 + 0.38 B(Z) and examination of the VB(Z) dependence gave Λ ~ 78.9 + 1.023 VB(Z). The comparison with K(P) gave Λ ~ 80.2 - 0.88 K(P). These dependencies are similar to those previously observed for the auroral zone boundaries.