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Ann. Geophys., 24, 1099-1112, 2006
www.ann-geophys.net/24/1099/2006/
© European Geosciences Union 2006
Characteristics of high altitude oxygen ion energization and outflow as observed by Cluster: a statistical study
H. Nilsson1, M. Waara1, S. Arvelius1, O. Marghitu2,3, M. Bouhram2,*, Y. Hobara1,**, M. Yamauchi1, R. Lundin1, H. Rème4, J.-A. Sauvaud4, I. Dandouras4, A. Balogh5, L. M. Kistler6, B. Klecker2, C. W. Carlson7, M. B. Bavassano-Cattaneo8, and A. Korth9
1Swedish Institute of Space Physics, Kiruna, Sweden
2Max-Planck-Institut für Extraterrestriche Physik, Garching, Germany
3Institute for Space Sciences, Bucharest, Romania
4Centre d’Etude Spatiale des Rayonnements, Toulouse, France
5Imperial College of Science, Technology and Medicine, London, UK
6University of New Hampshire, Durham, USA
7Space Science Laboratory, University of California, Berkeley, USA
8Istituto di Fisica dello Spazio Interplanetario, Roma, Italy, USA
9Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany
*now at: CETP-CNRS, Saint-Maur, France
**now at: University of Sheffield, Sheffield, UK
Abstract. The results of a statistical study of oxygen ion
outflow using Cluster data obtained at high altitude
above the polar cap is reported. Moment data for both
hydrogen ions (H+) and oxygen ions (O+)
from
3 years (2001-2003) of spring orbits (January to May)
have been used. The altitudes covered were mainly
in the range 5–12 RE geocentric distance.
It was found that O+ is
significantly transversely energized at high
altitudes, indicated both by high perpendicular
temperatures for low magnetic field values as well as
by a tendency towards higher perpendicular than
parallel temperature
distributions for the highest observed temperatures.
The O+ parallel bulk velocity increases with
altitude in particular for the lowest observed
altitude intervals.
O+ parallel bulk velocities in excess of 60 km s-1
were found mainly at higher altitudes
corresponding to magnetic field strengths of less
than 100 nT. For
the highest observed parallel bulk velocities
of O+ the thermal velocity exceeds the bulk
velocity, indicating that the beam-like character of
the distribution is lost.
The parallel bulk velocity of the H+ and O+
was found to typically be close to the same
throughout the observation interval when the H+
bulk velocity was calculated for all pitch-angles. When
the H+ bulk velocity was calculated for upward
moving particles only the H+ parallel bulk
velocity was
typically higher than that of O+. The parallel bulk velocity is close
to the same for a wide range of relative abundance of
the two ion species, including when the O+ ions
dominates.
The thermal velocity of O+ was always well below
that of H+.
Thus perpendicular
energization that is more
effective for O+ takes place, but this is not
enough to explain the close to similar parallel
velocities. Further parallel acceleration must
occur. The results presented constrain the models
of perpendicular heating and parallel acceleration.
In particular centrifugal acceleration of the outflowing
ions, which may provide the same parallel velocity
increase to the two ion species and a
two-stream interaction are discussed in the context
of the measurements.
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