Quiet time observations of the open-closed boundary prior to the CIR-induced storm of 9 August 2008

The open-closed magnetic field line boundary (OCB) is an important indicator of magnetospheric dynamics and can be used to identify locations of particle precipitation at the edge of the magnetosphere. The OCB can fluctuate during geomagnetic events, and the extent of this variability is a vital component of space weather research and modeling. There was a unique opportunity to identify and study the synoptic variability of the OCB during the extended 2007-2009 solar quiet period through use of the Polar Experiment Network for Geophysical Upper-atmosphere Investigations-Automatic Geophysical Observatory (PENGUIn-AGO) network of ground-based fluxgate magnetometers on the Antarctic continent. The fluxgates, which measured the occurrence of standing Pc5 modes on closed field magnetic field lines, allowed for identification of the OCB structure and study of the synoptic behavior of the OCB before and during a corotating interaction region (CIR)-driven magnetic storm. Observations were compared with results from the BATSRUS space weather model and show 83% agreement for over ∼2 days before the CIR event. It is shown that such synoptic magnetometer data sets of the OCB during these storms allow for a careful test of current space weather models. The current study investigates the pre-storm time period, while a future paper will address the storm time evolution of the OCB.