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| Titel |
Anisotropy of the He+, C+, N+, O+, and Ne+ Pickup Ion Velocity Distribution Function |
| VerfasserIn |
Christian Drews, Lars Berger, Andreas Taut, Robert F. Wimmer-Schweingruber |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250122864
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| Publikation (Nr.) |
 EGU/EGU2016-1999.pdf |
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| Zusammenfassung |
| Interstellar and inner-source PickUp Ions (PUIs) are produced by the ionization of neutral
atoms that originate either outside or inside the heliosphere. Just after the ionization, the
singly charged ions are picked up by the magnetized solar wind plasma and develop strong
anisotropic toroidal features in their Velocity Distribution Functions (VDF). As the plasma
parcel moves outward with the solar wind, the pickup-ion VDF gets more and more affected
by resonant wave-particle interactions, changing heliospheric conditions, and plasma drifts,
which lead to a gradual isotropization of the pickup ion VDF. Past investigations of the
pickup ion torus distribution were limited to He+ pickup ions at 1 Astronomical Unit
(AU).
The aim of this study is to quantify the state of anisotropy of the He+, C+, N+, O+, and
Ne+ pickup ion VDF at 1 AU. Changes between the state of anisotropy between PUIs of
different mass-per charges can be used to estimate the significance of resonant wave particle
interactions for the isotropization of their VDF, and to investigate the numerous
simplifications that are generally made for the description of the phase space transport of
PUIs.
Pulse height analysis data by the PLAsma and SupraThermal Ion Composition instrument
(PLASTIC) on board the Solar Terrestrial RElations Observatory Ahead (STEREO A) is used
to obtain velocity spectra of He+, C+, N+, O+, and Ne+ relative to the solar wind, f(wsw).
The wsw-spectra are sorted by two different configurations of the local magnetic field - one in
which the torus distribution lies within the instrument’s aperture, φ⊥, and one in which the
torus distribution lies exclusively outside the instrument’s field of view, φ∥. The ratio of the
PUI spectra between φ⊥ and φ∥ is used to determine the degree of anisotropy of the PUI
VDF.
The data shows that the formation of a torus distribution at 1 AU is significantly more
prominent for O+ (and N+) than for He+ (and Ne+). This cannot be explained by resonant
wave-particle interactions as the sole mechanism for the isotropization of the PUI
VDF. The anisotropy of the O+ VDF compared to He+ is highly fluctuating but
consistently higher over an observation period of 6 years and therefore unlikely to be
related to either specific heliospheric conditions or solar activity variations. To
our surprise, we also found a clear signature of a C+ torus distribution at 1 AU
very similar to the one of He+, although as an inner-source PUI, C+ should have a
considerably different spectral and spatial injection pattern than interstellar PUIs. |
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