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

TL;DR

This study analyzes SOHO/SWAN and LASCO/C2 data from 1996-2008 to compare solar wind structures and densities, revealing differences in solar wind acceleration during solar minima and establishing a link between coronal densities and solar wind types.

Contribution

It provides a novel comparison of solar wind latitudinal structures over two solar minima using combined Lyman-alpha brightness and coronal density data.

Findings

Wider slow solar wind belt during the current minimum.

Densities at 6 solar radii can indicate solar wind type.

Fast polar wind accelerates at larger distances in the current minimum.

Abstract

We update the SOHO/SWAN H Lyman-alpha 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 ionisation rate over more than a full solar cycle. The hydrogen ionisation, 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 ionisation rates using OMNI solar wind data and use them to calibrate our ionisation 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 densities at 6 solar radii can be used to infer the solar wind type close to its source, with high (resp. low) densities tracing the slow (resp. fast) solar wind, simply because the density reflects at which altitude occurs the acceleration. The comparison between the two minima suggests that the fast polar wind acceleration occurs at larger distance during the current minimum compared to the previous one. This difference, potentially linked to the magnetic field decrease or(and) the coronal temperature decrease should be reproduced by solar wind expansion models.