Abstract

The accurate determination of the open/closed field line boundary in the ionosphere is important for quantifying the open magnetic flux content of the magnetosphere, a diagnostic of magnetospheric dynamics. We present a statistical comparison of the latitude of the open/closed field line boundary (OCB) as estimated by the three different wavelength FUV cameras onboard the IMAGE spacecraft and from particle precipitation boundaries from the DMSP satellite. The latitude differences between the three FUV OCB proxies is less than a degree except in the predawn sector where the electron aurora is found up to two degrees equatorward of the proton aurora and vice versa in the evening sector. Systematic differences are found between DMSP and FUV OCB proxies, which may be used to correct FUV OCB proxies to give a more accurate estimation of the OCB .

Using auroral images from the IMAGE Wideband Imaging Camera, we are able to make an accurate global estimation of the latitude of the OCB and hence quantify the amount of open magnetic flux in the magnetosphere. Making use of the Frey substorm database we compare a parent distribution of the average open magnetic flux content of the magnetosphere, calculated from over 22000 auroral images, with the open magnetic flux at the time of substorm onset, showing that substorms occur in the upper quartile of the parent distribution and supporting the view that the magnetosphere must be preloaded with open magnetic flux as a necessary condition for substorm onset.