An auroral substorm commences when a discrete auroral arc brightens and subsequently expands poleward and azimuthally. The arc that brightens is usually the most equatorward of several quiescent auroral arcs present for a few to tens of minutes before the break-up commences. This arc is referred to as the "pre-existing auroral arc (PAA)" or the "growth-phase arc". Till now, the magnetospheric manifestation of the PAA during the growth phase and at substorm onset are not well understood. This dissertation addresses the magnetospheric location and source of the field-aligned current (FAC) of the PAA by analyzing measurements from magnetic and optical observatories on the ground and spacecraft at low and high altitudes.

In order to understand the magnetospheric location of the PAA relative to the inner edge of the electron plasma sheet (IEEPS), 2 years of THEMIS plasma data are surveyed to characterize the location of IEEPS on the equatorial plane. The IEEPS is found to lie at 7~8 RE in the pre-midnight sector and 6~7 RE in the post-midnight sector at geomagnetically quiet times. Next, from 5 events obtained between 2007 and early 2009 in which FAST crosses the flux tube linking to a PAA imaged by THEMIS all-sky imagers before an auroral substorm, the PAA is found to be located 1~2 degree poleward of the equatorward boundary of the 1 keV trapped electrons in the ionosphere. This latitudinal separation maps to a 2~3 RE radial separation from the IEEPS at the equator by the T96 model, and it places the PAA at 8~10 RE for typical growth phase conditions and further tailward for a very stretched magnetotail. In addition, the PAA is found to be located very close to the boundary between the Region 1 and Region 2 FACs but shifted into the upward FAC region.

The source of the FAC of the PAA is examined from observations in the ionosphere and in the magnetosphere. ~200 PAA events identified from the FAST measurements in 1998 show that an ionospheric flow shear is present in the vicinity of the PAA, where westward flow increases within 0.5 degree equatorward of the PAA in the premidnight sector and eastward flow increases within 0.5 degree poleward of the PAA in the postmidnight sector. Corresponding magnetospheric flow shear is observed by the THEMIS probes in a region estimated to be located near the PAA's source. Two events with THEMIS in-situ and ASI observations show that flow shear is the primary source of the FAC of the PAA during the growth phase and contributes significantly to the FAC system of the onset arc shortly before the auroral breakup.