Astron. Astrophys. 363, L21-L24 (2000)

4. Conclusions

We have shown that He⁺ pick-up ion spectra observed at solar distances of the order of 1 AU or smaller clearly reflect the fact that these pick-up ions are injected into phase-space with non-negligible Keplerian velocities. This phenomenon becomes manifest by the fact that, when judged in the solar wind rest frame, the peaks of spectral densities are shifted to values larger (upwind hemisphere) or smaller (downwind hemisphere) than 1 if the spectra are studied as functions of velocities normalized by the solar wind velocity . Hereby the hemispherical behaviour is clearly explained since in the upwind hemisphere He⁺ ions are injected into the solar hemisphere of the velocity space leading to higher initial relative velocities, whereas on the downwind hemisphere injection occurs into the antisolar hemisphere leading to smaller initial relative velocities with respect to the solar wind frame. In the upwind hemisphere the spectral peak in addition shifts to higher velocities the closer to the sun the spectrum is taken, reflecting clearly the increase of the Keplerian injection velocities with decreasing distances from the Sun.

Furthermore one may recognize that the slope of the high velocity wings of the spectra are less steep in the case of low velocity solar winds. This in our calculations clearly arises because the diffusive acceleration process (Fermi-2) operates more efficiently (longer particle exposure periods) in the low velocity wind. There may, however, even be an additional reason why this phenomenon comes up connected with the fact that the turbulence levels in the low velocity wind and the degree of turbulence isotropy are larger in low velocity solar winds making diffusive acceleration even more effective (see Chalov & Fahr 1998, 1999). However, to support this hypothesis more concrete evidence from observations is needed.

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000
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