Probing the Magnetic Structure of a Pair of Transpolar Arcs With a Solar Wind Pressure Step

Abstract

We present observations of the Northern Hemisphere auroras taken with the far ultraviolet cameras onboard the Imager for Magnetopause‐to‐Aurora Global Exploration spacecraft during a compression of the magnetosphere by a solar wind pressure step on 30 December 2001. The compression occurs during a period of northward interplanetary magnetic field which has given rise to the presence of a pair of transpolar arcs (TPAs) near the dawnside oval. The compression causes a brightening of the oval, from dayside to nightside over the course of 10 min, followed by a brightening of the midnight sector oval and TPAs from nightside to dayside, again over 10 min. We suggest that the brightening is caused by pitch angle scattering of particles trapped on closed magnetic field lines and that the sequence of the brightening tracks the solar wind pressure step as it progresses along the length of the magnetotail. Traveling at 600 km s , the step reaches up to 90 down‐tail over the period of brightening, suggesting that the magnetic field lines which map to the TPAs are closed and stretch almost this length down‐tail. The auroras usually take the form of ovals surrounding the geomagnetic poles, but occasionally an auroral feature bisects the dim region within the ovals: a transpolar arc. Although the geomagnetic conditions that give rise to transpolar arcs (TPAs) are well‐understood, there is continued controversy regarding how TPAs are formed and the structure of the magnetosphere during their presence: Are the magnetic field lines associated with the TPA connected into the interplanetary medium outside the magnetosphere (are open), or do they link from one hemisphere to the other (are closed). In this study we use observations of the brightening of the auroral oval and a pair of TPAs in response to a sharp increase in the pressure of the solar wind. The oval first brightens from the dayside to the nightside, and then the TPAs brighten from nightside to dayside, allowing us to track the progression of the solar wind step along the length of the magnetotail. This confirms that the TPA field lines are closed and stretch for up to 90 Earth radii down‐tail. This allows for the first time the magnetic structure of a TPA to be deduced, probing a region of the distant magnetotail that is rarely accessed by spacecraft. A solar wind pressure step causes a brightening of the auroral oval and a pair of preexisting transpolar arcs (TPAs) The oval first brightens from dayside to nightside, and then the TPAs brighten from nightside to dayside The observations suggest that the TPAs comprise closed field lines which stretch up to 90 down‐tail