(a) Plasmapause location given by Moldwin et al. (2002) model for 9–21 December 2003. Horizontal dashed line at L = 2.61 shows location of magnetometer pair midpoint. (b) Mass density at L = 2.61 over 9–21 December 2003. Solid horizontal lines indicate intervals where Kp ≥ 4⁺; dashed horizontal line represents average mass density from the Berube et al. (2005) model. Open circles identify likely quarter mode oscillations and crosses denote VLF-derived electron densities.