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Ann. Geophys., 23, 3533-3547, 2005
www.ann-geophys.net/23/3533/2005/
© European Geosciences Union 2005
A new formulation for the ionospheric cross polar cap potential including saturation effects
A. J. Ridley
University of Michigan, Ann Arbor, Michigan, USA
Abstract. It is known that the ionospheric cross polar cap potential (CPCP)
saturates when the interplanetary magnetic field (IMF) Bz 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 12mV/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 (Φ) can be expressed as Φ=(10-4v2+11.7B(1-e-Ma/3)sin3(θ/2)) {rms/9 (where
v is the solar wind velocity, B is the combined Y and Z
components of the interplanetary magnetic field, Ma is the solar
wind Mach number, θ=acos(Bz/B), and rms is the stand-off
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.
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