The solar wind as seen by SOHO/SWAN since 1996: comparison with SOHO/LASCO C2 coronal densities

摘要

We update the SOHO/SWAN H Lyman-alpha brightness analysis to cover the1996-2008 time interval. A forward model applied to the intensity maps providesthe latitude and time dependence of the interstellar Hydrogen ionisation rateover more than a full solar cycle. The hydrogen ionisation, being almostentirely due to charge-exchange with solar wind ions, reflects closely thesolar wind flux. Our results show that the solar wind latitudinal structureduring the present solar minimum is strikingly different from the previousminimum, with a much wider slow solar wind equatorial belt which persists untilat least the end of 2008. We compute absolute values of the in-ecliptic Hionisation rates using OMNI solar wind data and use them to calibrate ourionisation rates at all heliographic latitudes. We then compare the resultingfluxes with the synoptic LASCO/C2 electron densities at 6 solar radii. The twotime-latitude patterns are strikingly similar over all the cycle. Thiscomparison shows that densities at 6 solar radii can be used to infer the solarwind type close to its source, with high (resp. low) densities tracing the slow(resp. fast) solar wind, simply because the density reflects at which altitudeoccurs the acceleration. The comparison between the two minima suggests thatthe fast polar wind acceleration occurs at larger distance during the currentminimum compared to the previous one. This difference, potentially linked tothe magnetic field decrease or(and) the coronal temperature decrease should bereproduced by solar wind expansion models.