Long-term variation of the polar wind velocity and its implication for the ion acceleration process: Akebono/suprathermal ion mass spectrometer observations

Details Long-term variation of the polar wind velocity and its implication for the ion acceleration process: Akebono/suprathermal ion mass spectrometer observations Long-term variation of the polar wind velocity and its implication for the ion acceleration process: Akebono/suprathermal ion mass spectrometer observations

Sep 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004jgra..10909305a&link_type=abstract

Journal of Geophysical Research, Volume 109, Issue A9, CiteID A09305

Physics

11

Ionosphere: Ionosphere/Magnetosphere Interactions (2736), Ionosphere: Particle Acceleration, Ionosphere: Polar Cap Ionosphere, Magnetospheric Physics: Polar Cap Phenomena, Magnetospheric Physics: Magnetosphere/Ionosphere Interactions

Scientific paper

We present the solar activity dependence and seasonal variation of H+ and O+ polar wind velocity profiles observed by the suprathermal ion mass spectrometer (SMS) on Akebono. These observations spanned a solar cycle and covered a wide range of altitudes and invariant latitudes (ILAT) in the polar ionosphere and a variety of geomagnetic activity conditions from 1500 km to 8500 km altitude and from the poleward edge of the ionospheric trough (~60° ILAT) to the polar cap (>85° ILAT). At low (high) altitudes below (above) 4000 km, the increase of the averaged H+ and O+ ion velocities with altitude was larger (smaller) at solar minimum than at solar maximum. For example, the averaged H+ velocity on the dayside at 4000 km altitude was approximately 8 km s-1 at low solar activity but ~5 km s-1 at high activity. This suggests that the averaged polar wind velocity correlates differently with solar activity and the dominant acceleration process may be different at low and high altitudes, respectively. For both H+ and O+ the observed ion velocity at high altitude was largest in the summer under essentially all magnetic and solar activity conditions. The O+ velocity at high altitude (>4000 km) was significant and largest in the summer at solar maximum, when the solar energy input into the polar cap was largest; theoretically, the velocity of O+ ions in the polar wind is expected to be negligible below 10,000 km. We consider geophysical processes that may contribute to the observed velocities and their solar activity and seasonal dependences, including the possible contributions of photoelectrons and elevated electron temperatures to the ambipolar electric field that drives the polar wind.

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