Active Region Modulation of Coronal Hole Solar Wind
Understanding the mechanisms which produce the solar wind; opening the coronal magnetic field, accelerating the plasma, and imbuing it with a range of compositional and dynamical properties, is a major challenge in solar and heliospheric physics. Coronal holes (CHs) are established as the source of the fast solar wind (Cranmer, 2009). The origins of the slow solar wind, however, are still unclear, particularly as its properties imply origins in closed magnetic field regions (see review by Kepko et al. 2016). We investigate one candidate slow wind source: active regions (ARs).
Evidence has been found that interchange reconnection (reconnection between an open field line and a closed loop, which releases plasma confined on the loop) takes place between closed AR loops and open CH field lines (Baker et al. 2007). This process is a strong candidate for ARs contributing plasma to or otherwise modulating the solar wind, as explored by e.g., Del Zanna et al. (2011). Other processes, such as the over-expansion of AR loops into the heliosphere (Uchida et al. 1992; Morgan et al. 2013), and complex chains of reconnection eventually leading plasma onto open field lines (Culhane et al. 2014; Mandrini et al. 2014) have also been argued to produce AR solar wind. We present a case study on the origins of solar wind associated with an active region. The study connects detailed remote sensing observations from Hinode-EIS and SDO-AIA with in situ solar wind data from the ACE spacecraft at the L1 point.
Figure 1: AIA-193 Å images of the Sun during the first (a; R1) and second (b; R2) rotation. Mapped solar wind source points are shown as green crosses. The white and blue boxes show the fields of view of relevant EIS observations.
In April 2016, an AR emerged at the eastern boundary of an equatorial CH. The CH had produced Earth-directed solar wind one solar rotation prior when it was bordered by quiet Sun (QS). The configuration of the CH and AR for the two rotations is shown in AIA-193 Å images in Figure 1. Despite the emergence of the AR, both the general structure of the CH, and the solar wind source point locations linked to the ACE spacecraft within it (as determined by combing ballistic solar wind propagation with potential field source surface coronal modelling) are very similar between the two rotations. This configuration allows us to contrast the solar wind streams associated with each rotation in order to isolate the effects of the AR on solar wind properties.
Figure 2: Doppler velocity and non-thermal velocity maps derived from EIS Fe XII observations of the CH and its surroundings during the first (top) and second (bottom) rotations. Pixels of low Fe XII intensity are masked. A solid line indicates the locations of mapped solar wind source points for each image. Numbered boxes indicate regions where FIP bias is measured in upflowing pixels only..
EIS observed the regions of interest during both rotations, producing rastered spectral images with fields of view indicated by the boxes in Figure 1. Figure 2 shows line of sight doppler velocity and non-thermal velocity derived from fits to the Fe XII line for the CH images for each rotation. (Equivalent maps are produced for the AR itself and can be found in the full paper.) Clear upflows are present in the CH (as expected) and also near the CH-QS (rotation 1) and CH-AR (rotation 2) boundaries. A subset of the upflows are located near the predicted ACE solar wind source locations. Enhancements in non-thermal velocity are also observed in several of the upflow locations.
Composition measurements provide a powerful tool to link in situ solar wind data to remote source region observations. EIS in particular allows direct comparison of these regimes through measurement of the first ionisation potential (FIP) bias in relative abundance of minor ions in the corona. Plasma from CHs features ‘photospheric’ composition (FIP bias of ~1) while plasma in closed loops has ‘coronal’ composition (FIP bias >2). We estimate the FIP bias in the pixels which contain upflowing plasma in the small boxed areas of Figure 2 by combining lines including the high FIP S X 264.22 Å line, and the low FIP Si X 258.38 Å line, in a procedure as employed by e.g., Brooks et al. 2011. The chosen upflow regions, in both the CH-QS and CH-AR boundaries, exhibit mild to strongly enhanced FIP bias values of 1.6—3.4 relative to the CH FIP bias of 1. The upflows at the CH-AR boundary do not feature stronger FIP bias than those at the CH-QS boundary. Enhancements in FIP bias relative to CH values are thus anticipated in the corresponding in situ data for both periods.
Figure 3: In situ observations from the ACE spacecraft of the solar wind associated with the CH and its boundary region during the first (left panel) and second (right panel) rotations. The parameters from top to bottom are: bulk speed, vsw; carbon charge state ratio, C6+/C5+; and the iron to oxygen abundance ratio, Fe/O. FIP bias calculated from Fe/O is shown on the opposite axis. For the first rotation, streams associated with the CH, CH boundary (CHB) and QS are labelled, and coloured by their mean C6+/C5+ value. For the second rotation, the stream associated with the CH is labelled, along with smaller sub-streams (identified based on composition measurements) which are associated with the CH-AR boundary. These are coloured using the same mean C6+/C5+ scheme.
The relevant in situ L1 solar wind data from ACE for the two rotations are shown in Figure 3. The first rotation exhibits solar wind typical of a CH which borders QS; an initial fast stream with relatively low C6+/C5+ (coronal temperature proxy) and Fe/O (FIP bias indicator) values transitions gradually through decreasing speed into a characteristic slow wind stream in which a current sheet (not shown) is embedded. There are three distinct regions which we can identify based on the compositional parameters.
The second rotation, associated with the CH and CH-AR boundary, contrasts strongly with the first. An initial, significantly shorter, fast stream is followed by highly variable and structured solar wind which fluctuates between different characteristic compositional properties on timescales of around 8—12 hours. These structures include unique compositional signatures (combinations of C6+/C5+ and Fe/O values) which are not present in the first rotation. The highly variable composition is also accompanied by numerous strong deflections in the interplanetary magnetic field (shown in the full paper).
Enhanced FIP bias relative to the observed CH levels is found in both the CH-QS and CH-AR periods, in agreement with the EIS FIP bias observations. The maximum value of in situ FIP bias is also consistent between the two rotations, which again supports that EIS is identifying the correct source regions for these periods.
Through combining in situ solar wind and remote sensing coronal observations, we have found that the primary differences between solar wind originating from this CH-QS and CH-AR boundary are in its structure, variability, and composition. The more rapidly changing compositional structures in the CH-AR case are consistent with an increased importance of interchange reconnection for this period. This process can sporadically open closed loops which contain plasma of distinct composition to the heliosphere, leading to the variable properties observed. Transfer of open flux away from the CH through interchange reconnection can also explain the reduced duration of the CH stream during the second rotation. Interchange reconnection may also be related to the deflection in the in situ magnetic field, and the enhancements in non-thermal velocity observed at the CH boundaries with EIS.
This case study demonstrates the effectiveness of observations which connect the Sun and solar wind in addressing questions of solar wind origin. Future connectivity studies with Solar Orbiter and Parker Solar Probe stand to build upon this substantially, through improved instrumentation, and observations of the Sun from unique vantage points, and of the solar wind in yet-unexplored regions of the heliosphere. Observatories located along the Sun-Earth line will play an important role in these observations, particularly in conjunction or quadrature configurations. In such cases, the compositional information which can be derived from EIS will be especially valuable for linking the solar wind and coronal observations.
The link to the paper is here:
Active Region Modulation of Coronal Hole Solar Wind
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