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| Titel |
Finite-wavelength Alfvén waves and proton-alpha particle differential speed in the solar wind |
| VerfasserIn |
B. Li, X. Li |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250024286
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| Zusammenfassung |
| The complex solar wind plasma comprises the electrons, protons, and a non-negligible flux of
alpha particles. The differential speed between the protons and alpha particles is prominent in
the fast solar wind, and may also be so in the slow one. Alfvén waves, which abound in the in
situ solar wind measurements, are widely accepted as being responsible for accelerating the
solar wind. From the theoretical perspective, usually the short-wavelength WKB
approximation is adopted to describe the interaction between these waves and ion flows.
However, the WKB approximation may not hold in the inner corona in view of the large
Alfvén speeds there. In this presentation we examine the interaction of the finite-wavelength
(non-WKB) Alfvén waves with ion fluids, paying special attention to the wave effect on the
proton-alpha differential speed. We show that with decreasing frequency the waves
demonstrate a transition from a genuinely wave-like to a quasi-static behavior,
with the critical frequency fc being 0.5 ~ 1 Ã 10-5Â Hz. Whichever category the
waves belong to, they tend to reduce the proton-alpha speed difference. We also
demonstrate that in the super-Alfvénic regions of the solar wind, in the presence of
Alfvénic fluctuations, an apparent break should exist around fc in the velocity
fluctuation spectrum for alpha particles, provided that the spectrum for protons is
smooth. Such a break is entirely a linear property, and has nothing to do with the
non-linearities that may also shape the fluctuation spectra. Introducing realistic ion thermal
anisotropies will not make the apparent break disappear. Direct samples from the Solar
Orbiter and Solar Probe will make the observational test of this prediction possible. |
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