Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007a%26a...476.1341t&link_type=abstract
Astronomy and Astrophysics, Volume 476, Issue 3, December IV 2007, pp.1341-1346
Astronomy and Astrophysics
Astrophysics
4
Sun: Corona, Sun: Uv Radiation, Sun: Solar Wind
Scientific paper
The purpose of the paper is to measure the degree of temperature anisotropy of the oxygen ions in the outer corona. The ratio of the Doppler dimmed O VI 1037-1032 line intensity as a function of the velocity of the fast solar wind, computed for typical values of coronal density, is consistent with the observed ratio, only when a significant temperature anisotropy is established in polar coronal holes. The oxygen ion velocity distribution is constrained to be bi-Maxwellian from 2R_&sun; to 3.7R_&sun;, where the lowest degree of anisotropy compatible with the observational data increases up to ~7 at 2.9R_&sun;, proving that the oxygen ions are accelerated across the magnetic field, in accordance with a preferential energy deposition perpendicular to the field lines, consistent with the process of ion-cyclotron dissipation of Alfvén waves. The most plausible evolution of the velocity distribution of the O+5 ions departs from the bi-Maxwellian configuration at 2R_&sun;, according to an anisotropy ratio that reaches its maximum value T_⊥/T_∥~14 at 2.9R_&sun;, and further out approaches isotropy, at 3.7R_&sun;. In response to the acceleration across the field, energy redistribution along the magnetic field lines accelerates the oxygen component of the solar wind to velocities of 760 km s-1 at 5R_&sun;. The variation of the anisotropy ratio with the heliocentric distance might be satisfactorily explained by theoretical models of the fast solar wind heating based on the oxygen cyclotron instability or the fast shock mechanism. The observations of the extended corona analyzed in this paper are performed with the Ultraviolet Coronagraph Spectrometer on board the Solar Heliospheric Observatory, during the solar minimum activity period 1996-1997.
Antonucci Ester
Dodero Maria Adele
Telloni Daniele
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