Effect of escaping photoelectrons on the polar wind outflows

Friday, 18 December 2015
Poster Hall (Moscone South)
Naritoshi Kitamura1, Kanako Seki2,3, Yukitoshi Nishimura4, James P McFadden5, Takumi Abe1, Manabu Yamada6, Shigeto Watanabe7 and Andrew W Yau8, (1)ISAS Institute of Space and Astronautical Science, JAXA Japan Aerospace Exploration Agency, Kanagawa, Japan, (2)Nagoya University, Nagoya, Japan, (3)The University of Tokyo, Department of Earth and Planetary Science, Graduate School of Science, Tokyo, Japan, (4)University of California Los Angeles, Department of Atmospheric and Ocean Science, Los Angeles, CA, United States, (5)University of California Berkeley, Berkeley, CA, United States, (6)Chiba Institute of Technology, Planetary Exploration Research Center, Narashino, Japan, (7)Hokkaido Information University, Ebetsu, Japan, (8)University of Calgary, Calgary, AB, Canada
A statistical analysis using a long-term (over one solar cycle) photoelectron dataset obtained by the Fast Auroral SnapshoT (FAST) satellite (3000–4200 km altitude) during geomagnetically quiet periods demonstrates that the polar wind ion flux estimated from electron outflows does not change with increasing photoelectron production due to increasing solar activity, while the magnitude of the field-aligned potential drop (~15–25 V), which reflects low-energy photoelectrons back to the ionosphere, increases with increasing solar activity. The magnitude of the field-aligned potential drop is likely developed by photoelectrons themselves so as to equilibrate electron fluxes with the ion fluxes. Another statistical analysis using the thermal ion measurements by the Akebono satellite at solar maximum implies that contribution of O+ ions to the total ion flux in the polar cap is small owing to almost zero upward bulk velocity at least below ~7000 km altitude during geomagnetically quiet periods. Thus, the polar wind ion flux is dominated by H+ ions, and the H+ ion flux has been predicted to be regulated by the production rate in the topside ionosphere. The combination of these results indicates that it is the source region of H+ ions in the topside ionosphere and not the photoelectron flux that control the polar wind outflow.