Since the discovery of hot plasma upflows from the edges of solar active regions (ARs) by Hinode/XRT (Sakao et al., 2008) and EIS (Harra et al. 2008) it has been suspected that these upflows are in fact outflows, i.e. sources of the slow solar wind (SW). In an article to be published in Solar Physics (van Driel-Gesztelyi et al., 2012) we make an attempt to step beyond anecdotic evidence and show that this is indeed the case - at least for some of the flows.
We analyse a complex of two active regions (AR1 and AR2) flanked by two coronal holes (CH1 and CH2) of opposite magnetic polarity using observations from Hinode/EIS and XRT, SOHO/MDI and EIT, as well as STEREO/EUVI taken in January 2008. We employ three levels of magnetic topology modelling from local to global and find that although some of the upflows observed with EIS remain confined along closed loops, there are indeed plasma upflows originating in AR2 bordering CH2, which have the means to get out of the Sun. Magnetic field extrapolations (LFFF) show that AR plasma outflows observed with EIS are co-spatial with quasi-separatrix layer locations (Figure 1A), confirming the results by Baker et al. (2009). Some of the flow-active QSLs in AR2 include the separatrix of a null point (Figure 1B). Global potential field source-surface modeling indicates that the spine field line and field lines in the vicinity of the null point extend up to the source surface, i.e. `open' towards the heliosphere (Figure 1C,D). At the null point, which is 120 Mm high in the corona, `interchange' reconnection takes place between closed high-density magnetic loops of the AR and open evacuated field lines of the neighboring coronal hole in agreement with Del Zanna et al. (2011). The resulting pressure gradient drives the flows: upflows along the reconnected `open' field lines and downflows along the newly created closed loops, as proposed by Baker et al. (2009) and Bradshaw et al. (2011). Along the open field lines coronal plasma from the AR can gain access to the SW, presumably creating a stream of slow SW.
To ascertain that this is indeed the case, we then follow the flows into the interplanetary space using in situ measurements by ACE (Figure 2A,B), and indeed find the slow wind streams corresponding to AR1 and AR2 flanked by fast-wind streams originating in CH1 and CH2. Increased ion ratios indicate that the slow SW stream originated at lower height from AR2 than from AR1, which is consistent with the difference in source-surface PFSS magnetic topology between the two ARs: as AR1 is fully envelopped by a streamer, the slow SW stream should originate from larger loops and the tip of the streamer. On the other hand, part of AR2 is not fully covered by the streamer, the null-point above it is being out in the open field domain of CH2. Magnetic reconnection takes place low in the corona, and the frozen-in ion ratios indicate hotter plasma than for the AR1-related slow wind. We also find a peak in proton temperature and speed in the AR2-related slow wind, which we link to reconnection taking place at the coronal null point. We conclude that properties of these SW streams are consistent with the magnetic topology of their source.
Our results suggest that magnetic interchange reconnection is possible not only at the source surface as proposed by Wang et al. (2007), but well below the source surface along null-points in pseudo-streamer configurations, which can be jointly present with the streamer. In such cases, AR plasma can gain access to open field lines and be released into the solar wind, i.e. outflows from ARs become sources of the slow solar wind. Therefore the answer to the fundamental question: "What are the sources of the slow solar wind?" can only be answered by the analysis of the magnetic topology.
Figure 1 (A) Hinode/EIS observations of AR2 bordering CH2 on 10 Jan. 2008. Local LFFF modelling confirms the results by Baker et al. (2009) that AR upflows originate from quasi-separatrix layers (QSLs, dark red traces). (B) Larger-scale potential extrapolations of the AR1-AR2 complex reveal a null-point at ~120 Mm above AR2, and field lines from the vicinity of QSLs found in the local model run along the spine field line reaching the top of the 600 Mm high computational box - but are they open towards the IP field? This is examined in (C) and (D): PFSS topological model of the Sun, which shows separatrix surfaces (colored semi-transparent surfaces) separating closed and open field line domains, high-altitude null-points (red dots) and shows that AR2 is not fully covered by a streamer (yellow semi-transparent surface), but its pseudo-streamer-type separatrix dome (light green) is partially out in the open-field region (C/b). The spine field line from the null point above AR2 reaches the source surface, i.e. it is open towards the heliosphere. Therefore, some of the QSLs found in the local model (A/c) are in fact separatrices and flows emanating from them can reach the solar wind. The view of the model shown in (D) shows that AR1 is fully covered by the streamer.
Figure 2 (A) Hinode/XRT image of the Sun showing the AR-CH complex, which is the source of the solar wind shown in (B) based on ACE in situ measurements. The slow wind stream following the Heliospheric Plasma Sheet (HPS) before the arrival of the fast wind stream from CH2 has higher ion ratios and a velocity & Temperature peak, which we link to AR2 and reconnection at the null-point above it. The flows really reached the SW! The slow wind streams related to AR1 (indicated by a blue ellipse) and AR2 (red) show different ion ratios, which we link to the large-scale magnetic topology.
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The research leading to these results has received funding from European Commission's Seventh Framework Programme under grant agreement No. 284461 (eHEROES project).