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 ∼10 V, reaching ∼90% (small field-aligned current (FAC) case) and ∼83% (all data). A typical magnitude of the potential drop above ∼3800 km altitude is ∼20 V. This value is significantly larger than the potential drop below ∼3800 km altitude (probably ∼1-3 V), although the typical potential drop is smaller by a factor of ∼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.
ASJC Scopus subject areas
- Space and Planetary Science