Abstract
Both coronal holes and active regions are source regions of the solar wind. The distribution of these coronal structures across both space and time is well known, but it is unclear how much each source contributes to the solar wind. In this study we use photospheric magnetic field maps observed over the past four solar cycles to estimate what fraction of magnetic open solar flux is rooted in active regions, a proxy for the fraction of all solar wind originating in active regions. We find that the fractional contribution of active regions to the solar wind varies between 30% to 80% at any one time during solar maximum and is negligible at solar minimum, showing a strong correlation with sunspot number. While active regions are typically confined to latitudes ±30∘ in the corona, the solar wind they produce can reach latitudes up to ±60∘. Their fractional contribution to the solar wind also correlates with coronal mass ejection rate, and is highly variable, changing by ±20% on monthly timescales within individual solar maxima. We speculate that these variations could be driven by coronal mass ejections causing reconfigurations of the coronal magnetic field on sub-monthly timescales.
Highlights
The solar wind is a flow of hot tenuous plasma, driven by the large pressure difference between the Sun’s corona and the interplanetary medium
This agrees with observations of active region lifetimes, and we have manually checked some of the multiple-rotation trails in Figure 3 to verify that they match with active regions observable in extreme ultra-violet (EUV) images over multiple rotations
The latitude distributions agree with previous observations of active regions, notably the butterfly diagram that shows a reduction in active region latitudes as a solar cycle progresses (Carrington, 1858; Maunder, 1922)
Summary
The solar wind is a flow of hot tenuous plasma, driven by the large pressure difference between the Sun’s corona and the interplanetary medium. As magnetic flux emerges through the photosphere it starts out closed, but as the field strength increases the closed loops can reconnect with adjacent open field lines (van Driel-Gesztelyi et al, 2014; Ma et al, 2014; Kong et al, 2018), redistributing regions of existing open flux (Sheeley, Wang, and Harvey, 1989; Baker, van Driel-Gesztelyi, and Attrill, 2007) and in the process opening up previously closed flux (Wang and Sheeley, 2003a; Attrill et al, 2006) This allows plasma originating in active regions to flow out from the corona and form part of the solar wind
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