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Astron. Astrophys. 335, 303-308 (1998)

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4. Conclusions

We have presented a model (based on a generalization of McKenzie et al. 1997) which describes the acceleration of [FORMULA] particles and the minor species in high-speed component of the solar wind. The main assumption is that all the heating needed to accelerate the minor species corresponds to the temperature profiles per ion mass equal to those of the protons augmented by an extra factor of K. In order to obtain results consistent with observations of helium and minor species in high speed solar wind (differential speed [FORMULA] and high temperatures roughly [FORMULA] mass) K must exceed one and perhaps be as large as 2.0. That is, the minor species must be heated more efficiently than the protons. This could be associated with a form of cyclotron resonance with a wave power spectrum which decreases with increasing frequency. As a consequence, the (perpendicular) temperatures of ions in the inner corona (r) reach 30-50 MK for [FORMULA] particles and 120-200 MK for [FORMULA]. Furthermore, if K the minor species accelerate faster than the protons and for the preferred value K may exceed the proton speed by up to 280 km/s at r. The relative flow speed reaches the Alfven speed only at large distance (r for K). As a consequence of the specific assumptions of the model, there is no build-up of minor species density in the corona.

Comparing these results with the measurements of UVCS at SOHO (Cranmer et al. 1997, Kohl et al. 1997) we find that, despite some common features (high heavy ion temperatures, flow speeds of [FORMULA] km/s reached already at [FORMULA]2 [FORMULA]), there are important differences in detail. The UVCS/SOHO data imply the ratio between [FORMULA] and the proton temperatures to be much higher than our upper limit, although the maximum [FORMULA] temperature is close to the value used in the model. The shape of the temperature profiles inferred from the observations are quite different from our model. In particular, the rise of temperatures starts apparently not at 1 [FORMULA] but only at [FORMULA] [FORMULA] and the peak is not reached within the observation limits (up to [FORMULA] [FORMULA]). On the other hand, the preferential heating of the transverse ion motion is confirmed by the analysis of the data (Kohl et al. 1997). It is possible that the ion-cyclotron heating becomes more effective after some minimum distance from the coronal base. It would be worthwhile to explore this possibility in future work so as to obtain a better agreement with the UVCS observations very close to the Sun.

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© European Southern Observatory (ESO) 1998

Online publication: June 12, 1998