Ground magnetometer observation of a cross-phase reversal at a steep plasmapause

The cross‐phase technique employs ground‐based magnetometer data in order to determine the resonance frequency of a geomagnetic field line. Typically, a positive cross‐phase maximum identifies the field line resonance frequency, but occasionally, a negative cross‐phase maximum is observed and is believed to be a feature of the steep density gradient at the plasmapause. For a few hours during the local morning of 14 May 2001 the cross‐phase maximum, observed using two pairs of ground‐based magnetometer stations from the European sector, with midpoints at L = 3.16 and L = 3.34, reversed polarity from positive to negative. All other British Geological Survey, Sub‐Auroral Magnetometer Network, and International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer array station pairs examined between L = 2.39 and L = 6.54 showed a positive cross‐phase maximum throughout the day. The Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) satellite made an excellent close magnetic conjunction with these ground‐based magnetometer arrays on this day, and data from the IMAGE Radio Plasma Imager instrument show a very steep plasmapause in this region during this UT interval. IMAGE Extreme Ultraviolet Imager global plasmasphere images show that the plasmapause moved outward through the day, passing through the region of the observed negative cross‐phase maxima. This rare observation of a negative cross‐phase maximum occurs at the location of a plasmapause with a gradient steeper than r−8 and thus is in agreement with theory. The two cross‐phase peak polarity reversals are explained by the evolution of the local density profile.