Cross-phase-derived plasma mass density trends during the Hallowe'en 2003 geomagnetic storms are presented for 38° magnetic latitude 63° (1.61 ≤ L ≤ 5.10), using data from the SAMNET (Subauroral Magnetometer Network), BGS (British Geological Survey), and SEGMA (South European Geomagnetic Array), ground-based magnetometer arrays in Europe. At all latitudes monitored, a rapid increase of total mass density is observed immediately following the initial storm sudden commencement at 0611 UT on 29 October, believed to be due to rapid ionospheric O+ outflow. Plasmaspheric density depletion to at least 50° magnetic latitude (L ∼ 2.4) took place over the next 3 days. Poststorm refilling began on 2 November. Following the sudden commencement of another storm on 4 November, a density enhancement was monitored at 2.79 ≤ L ≤ 3.84, with subsequent plasmaspheric depletion occurring by 6 November. Plasma mass density values are compared to empirical plasmapause location model predictions, with reasonable agreement for most days, but density depletion and refilling were monitored 1 day after they are predicted. During poststorm plasmaspheric refilling, some extremely low early morning resonance frequencies are monitored and appear be due to quarter mode standing waves. This study also highlights that care must be taken in the choice of assumed geomagnetic field geometry when deriving plasma mass densities from observed field line resonances during severe geomagnetic storms.