Tracking Solar Active Region Outflow Plasma from its Source to the near-Earth Environment

Len Culhane (UCL-MSSL)


Following the discovery of persistent hot plasma upflows from the edges of solar active regions (ARs) by Hinode/XRT (Sakao et al., 2007) and EIS (Harra et al. 2008), much effort has been devoted to their study given the possibility that they may in fact be outflows and therefore contributors to the slow solar wind (SSW). Since the original observations, it has been shown that the plasma has temperature (Te) of 1 MK ≤ Te ≤ 2.5 MK, coronal AR composition, morphology differing from fan-loops where plasma is downflowing with Te ~ 0.6 MK and origin mainly at sites of Quasi-Separatrix Layers (QSLs) where magnetic reconnection between loops of high and low plasma density can drive the plasma upwards.

More recently, van Driel-Gesztelyi et al., (2012) reported on observing the upflows from a pair of ARs. While one of these was fully enclosed by the streamer belt, the other was partially uncovered and lay below a magnetic null-point where closed magnetic field from the AR could reconnect with open field from a nearby Coronal Hole (CH). This geometry provided an open field pathway to the Heliosphere and a stream of slow wind plasma with AR composition was indeed detected by the ACE spacecraft at the appropriate time later. This work emphasised the importance of the global magnetic field configuration in identifying contributions to the SSW.

Culhane et al., 2014 have just completed a study of the upflows associated with a single AR that passed disc centre between 10th and 14th December, 2007. Region AR 10978 and its associated East and West upflows are shown in Figure 1. The apparent upflow velocity reversal from West to East occurs because the upflows are significantly inclined to the line-of-sight – East: - 570 , West: +100. A linear force free field (LFFF) magnetic topology analysis confirms the upflow inclinations and also shows that the flows originate from quasi-separatrix layers (QSLs). EIS spectroscopic measurements of the plasma temperature and composition for both upflows shows that they have typical AR values with, in particular a high concentration for elements of low first ionisation potential (FIP) indicating that the material has spent some time in closed magnetic field configurations.

Figure 1:XRT images of the AR 10978 disc transit (top). AR (blue box) is located between two CHs and is shown in inverse contrast. Velocity maps (bottom) obtained from EIS Fe XII/195.12 Ā raster maps where blue indicates upflow velocity. EIS raster is 460" (x) by 384" (y).

On the issue of whether or not the upflowing plasma can leave the AR and reach the heliosphere and earth, a global Potential Field Source Surface (PFSS) magnetic topology analysis (Figure 2) is at first sight not very promising. The closed coronal streamer, bordered by two CHs East and West of the AR, fully encloses AR 10978. The AR-associated field lines (blue in b) do not enter the open field domain but remain closed below the streamer. Thus the structures carrying upflowing plasma are fully contained below the streamer. Hence it is not obvious how plasma with AR composition could gain access to the Heliosphere. In-situ solar wind observations from the ACE spacecraft located at the L1 point (Figure 3), show fast solar wind characteristics associated with wind streams from the two adjacent CHs on opposite sides of the Heliospheric Current Sheet (HCS). The flow characteristics change to those of the slow wind with reduced proton velocity and increases in Np, O+7/O+6, C+6/C+5 and Fe/O – or FIP-bias, from West of the HCS crossing. This material has very similar properties to those of the upflowing plasma as originally measured at AR 10978. While the reduction of He/p is characteristic of a streamer tip contribution to the overall flow, the slow solar wind plasma sampled by ACE before the HCS crossing appears to have a significant contribution from active region plasma. However it is apparent from the global PFSS magnetic field model shown in Figure 2 that there is no obvious open field pathway from the AR 10978 neighbourhood to the L1 point near earth.

Figure 2: PFSS model showing large-scale topological magnetic structures for 12-DEC-2007 surrounding AR 10978. Semi-transparent yellow separatrix surface a) shows AR fully covered by helmet streamers. Grey open field areas E and W of the AR are the CHs. b) with yellow surface removed for clarity shows +ve (white) and –ve (black) AR polarities. The helmet streamer covering allows no topological link between plasma upflows and open magnetic field.

Mandrini et al., 2014 analysed the global magnetic topology for this solar rotation and located four high altitude magnetic null points within a Ī 200 longitude range around AR 10978. Only one of these, labelled N1 in Figure 4, had associated open field lines originating from the northern CH.

Figure 3: STEREO-B EUVI 195 A synoptic map showing AR 10978 (a). ACE data back-mapped to 2.5 RS include (b) proton speed and density, (c) He/p, (d) O+7/O+6 and C+6/C+5 ratios, (e) Fe/O and FIP-bias, (f) Bradial, (g) Babsolute. The HCS crossing is shown by the vertical red line.

In a first step (Figure 4a), AR expansion drives reconnection at QSLs between the AR closed loops and the large-scale network fields East of the AR. In a second step (Figure 4b), additional diffusion of the photospheric AR field forces interchange reconnection at null N1 of the large-scale field lines anchored to the North-West of the AR with the open ones in the neighborhood of the northern negative CH. The reconnected field lines bend towards the ecliptic (Figure 4c). This is shown more clearly from the perspective of Figure 4d. The ACE detection of the plasma began on 17 December or five days after the AR reached Sun centre. This interval should be around three days for a typical slow-solar wind speed but the longer elapsed time can be explained by the addition of plasma travel time from AR 10978 to Null 1 (Figure 4 a,b), thus further supporting the suggestion of Mandrini et al. (2014).

Figure 4: a) AR loops (blue) reconnect with large network loops (red) East of the AR. Reconnection, driven by AR expansion puts upflow plasma into the large low density network loops. b) Large loops reconnect with the open field lines (pink) at N1 that are associated with the northern CH. c) Open field lines bend towards the ecliptic plane and can carry the original AR upflowing plasma to the Sun – ACE line. d) View from three days earlier with AR 10978 at 450 East shows downward curvature more clearly.

Culhane, J.L., Brooks, D.H., van Driel-Gesztelyi, L., Démoulin, P. Baker, D., DeRosa, M.L., et al.: 2014, Solar Phys., 289, 3799.
Harra, L.K., Sakao, T., Mandrini, C.H., Hara, H., Imada, S., Young, P.R., et al.: 2008, Astrophys. J. Lett. 676, 147.
Mandrini, C.H., Nuevo, F.A., Vasquez, A.M., Démoulin, P., van Driel-Gesztelyi, L., Baker, D., et al.: 2014, Solar Phys., 289, 4151.
Sakao, T., Kano, R., Narukage, N., Kotoku, J., Bando, T., DeLuca, E.E., et al.: 2007, Science 318, 1585.
Harra, L.K., Sakao, T., Mandrini, C.H., Hara, H., Imada, S., Young, P.R., et al.: 2008, Astrophys. J. Lett. 676, 147.

For more details, please contact: Dr. Deborah Baker.

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Last Revised: 01-Oct-2014

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