Asymmetry of the Cusp Position Observed by the Polar Spacecraft

We use 28 months of spacecraft potential measurements, collected with the electric field experiment on Polar, to derive the plasma density distribution in the cusp region. The density lies in the range 1-10 cm(-3) in the center of the cusp (09-14 MLT), and is below 1 cm(-3) dawnward (before 09 MLT) and duskward (after 14 h MLT) of the cusp. It is found that the center of the cusp is displaced by about 0.6 h from local noon toward dawn; this effect is more pronounced in summer than in winter. The summer cusp is found to be wider in both magnetic local time and magnetic latitude than the winter cusp.