The Solar Wind as Seen by SOHO/SWAN Since 1996: Comparison with SOHO/LASCO C2 Coronal Densities
Résumé
We update the SOHO/SWAN H Lyman-α brightness analysis to cover the 1996-2008 time interval. A forward model applied to the intensity maps provides the latitude and time dependence of the interstellar hydrogen ionization rate over more than a full solar cycle. The hydrogen ionization, being almost entirely due to charge-exchange with solar wind ions, reflects closely the solar wind flux. Our results show that the solar wind latitudinal structure during the present solar minimum is strikingly different from the previous minimum, with a much wider slow solar wind equatorial belt which persists until at least the end of 2008. We compute absolute values of the in-ecliptic H ionization rates using OMNI solar wind data and use them to calibrate our ionization rates at all heliographic latitudes. We then compare the resulting fluxes with the synoptic LASCO/C2 electron densities at 6 solar radii. The two time-latitude patterns are strikingly similar over all the cycle. This comparison shows that 6 Rs densities can be used to infer the solar wind type close to its source, with high (low) densities tracing the slow (fast) solar wind, simply because the density reflects the altitude at which the acceleration occurs. The comparison between the two minima suggests that the fast polar wind acceleration occurs at a larger distance during the current minimum compared to the previous one. This difference, potentially linked to the magnetic field decrease and/or the coronal temperature decrease should be reproduced by solar wind expansion models.
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