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| Titel |
Solar wind contribution to the average population of energetic He+ and He++ ions in the Earth's magnetosphere |
| VerfasserIn |
G. Kremser, R. Rasinkangas, P. Tanskanen, B. Wilken, G. Gloeckler |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 12, no. 2/3 ; Nr. 12, no. 2/3, S.152-168 |
| Datensatznummer |
250010317
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| Publikation (Nr.) |
 copernicus.org/angeo-12-152-1994.pdf |
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| Zusammenfassung |
| Measurements with the ion charge-energy-mass
spectrometer CHEM on the AMPTE/CCE spacecraft were used to investigate the
origin of energetic He+ and He++ ions observed in the
equatorial plane at 3 ≤ L ≤ 9. Special emphasis was laid on the
dependence of long-term average distributions on magnetic local time (MLT)
and the geomagnetic activity index Kp. The observations are
described in terms of the phase space densities f1 (for He+)
and f2 (for He++). They confirm preliminary results
from a previous study: f1 is independent of MLT,
whereas f2 is much larger on the nightside than on the
dayside. They show, furthermore, that f1 increases slightly
with Kp on intermediate drift shells, but decreases on high
drift shells (L ≥ 7). f2 increases with Kp
on all drift shells outside the premidnight sector. Within this sector a
decrease is observed on high drift shells. A simple ion tracing code was
developed to determine how and from where the ions move into the region of
observations. It provides ion trajectories as a function of the ion charge, the
magnetic moment and Kp. The ion tracing enables a distinction
between regions of closed drift orbits (ring current) and open convection
trajectories (plasma sheet). It also indicates how the outer part of the
observation region is connected to different parts of the more distant plasma
sheet. Observations and tracing show that He++ ions are effectively
transported from the plasma sheet on convection trajectories. Their distribution
in the observation region corresponds to the distribution of solar wind ions in
the plasma sheet. Thus, energetic He++ ions most likely originate in
the solar wind. On the other hand, the plasma sheet is not an important source
of energetic He+ ions. Convection trajectories more likely constitute
a sink for He+ ions, which may diffuse onto them from closed drift
orbits and then get lost through the magnetopause. An ionospheric origin of
energetic He+ ions is unlikely as well, since the source mechanism
should be almost independent of Kp. There is considerable
doubt, however, that a plausible mechanism also exists during quiet periods that
can accelerate ions to ring current energies, while extracting them from the
ionosphere. It is concluded, therefore, that energetic He+ ions are
mainly produced by charge exchange processes from He++ ions. This
means that most of the energetic He+ ions constituting the average
distributions also very likely originate in the solar wind. Additional
ionospheric contributions are possible during disturbed periods. |
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