A new formulation for the ionospheric cross polar cap potential including saturation effects

Abstract

Abstract. It is known that the ionospheric cross polar cap potential (CPCP) saturates when the interplanetary magnetic field (IMF) B z becomes very large. Few studies have offered physical explanations as to why the polar cap potential saturates. We present 13 events in which the reconnection electric field (REF) goes above 12 mV/m at some time. When these events are examined as typically done in previous studies, all of them show some signs of saturation (i.e., over-prediction of the CPCP based on a linear relationship between the IMF and the CPCP). We show that by taking into account the size of the magnetosphere and the fact that the post-shock magnetic field strength is strongly dependent upon the solar wind Mach number, we can better specify the ionospheric CPCP. The CPCP ( ) can be expressed as =(10 −4 v 2 +11.7B(1−e −M a /3 ) sin 3 (θ/2))r ms /9 (where v is the solar wind velocity, B is the combined Y and Z components of the interplanetary magnetic field, M a is the solar wind Mach number, θ =acos(B z /B), and r ms is the standoff distance to the magnetopause, assuming pressure-balance between the solar wind and the magnetosphere). This is a simple modification of the original Boyle et al. (1997) formulation.

The response of the magnetosphere-ionosphere system to a sudden dynamic pressure enhancement under southward IMF conditions

“…The transient increase in the ionospheric cross polar cap potential during southward IMF is not accounted for in simple empirical relationships between the solar wind/IMF and the ionospheric potential (e.g., Boyle et al, 1997). Further, it has been noted by some that the potential should decrease if the pressure is increased, due to the decrease in the length of the reconnection line (e.g., Ridley, 2005). These simulation results show that under transient conditions, the potential may increase significantly.…”

Polar cap potential saturation, dayside reconnection, and changes to the magnetosphere

“…Pudovkin et al [1985] considers changes in the length of the flow stagnation line with changes in the solar wind; Ridley [2005] considers changes in the magnetosheath electric field with changes in the solar wind. The present findings contradict this explanation in two manners.…”

“…[82] In the X-line-length model [Raeder and Lu, 2005;Ridley, 2005] [see also Pudovkin et al, 1985] changes in solar wind parameters result in changes in the size of the magnetosphere, which changes the length of the dayside reconnection line, which can reduce the total amount of reconnection between the solar wind and the magnetosphere to cause polar cap saturation. The present findings contradict this explanation in two manners.…”

“…[83] In the magnetosheath-flow model [Pudovkin et al, 1985;Ridley, 2005] variations in the solar wind parameters produce variations in the properties of the magnetosheath flow that change the total reconnection rate between the solar wind and the magnetosphere, which can lead to polar cap saturation. Pudovkin et al [1985] considers changes in the length of the flow stagnation line with changes in the solar wind; Ridley [2005] considers changes in the magnetosheath electric field with changes in the solar wind.…”

Numerical considerations in simulating the global magnetosphere

“…While there is an empirical understanding of the expected value of the actual CPCP as a function of interplanetary magnetic field (e.g., Rich and Hairston, 1994;Weimer, 1996;Ruohoniemi and Greenwald, 1996;Boyle et al, 1997;Siscoe et al, 2002;Ridley, 2005), there is not a very good understanding of what ideal MHD (with the allowance for numerical resistivity in the reconnection sites) should give. Some codes tend to give a very large CPCP (e.g., Fedder et al, 1998), while other codes give a value that is smaller than expected (e.g., Palmroth et al, 2005).…”