Photoelectron flows in the polar wind during geomagnetically quiet periods

Characteristics of photoelectron flows and presence of a field-aligned potential drop on the open magnetic field lines in the polar cap are systematically investigated using the data obtained by the FAST satellite during geomagnetically quiet periods in July 2002. We found high occurrence frequencies of the potential drop larger than similar to 10 V, reaching similar to 90% (small field-aligned current (FAC) case) and similar to 83% (all data). A typical magnitude of the potential drop above similar to 3800 km altitude is similar to 20 V. This value is significantly larger than the potential drop below similar to 3800 km altitude (probably similar to 1-3 V), although the typical potential drop is smaller by a factor of similar to 2-3 in comparison to the modeling results that suggested presence of a field-aligned potential jump at several earth radii. The net escaping electron number flux negatively correlates with the upward electron number flux and with the magnitude of the potential drop. This relation is contrary to expectation from photoelectron-driven polar wind models that an increase in the photoelectrons drives the larger polar wind flux, since the net escaping electron number flux balances the flux of polar wind ions under zero net FAC conditions. An increase in the upward backscatter of reflected electrons with an increasing potential drop may explain the negative correlations. A potential drop at high altitudes would provide a polar wind system regulated by a negative feedback, and the most appropriate balance for polar wind ions would be achieved near the median of the reflection potential.