High-velocity upflows in solar active regions
Steph Yardley - UCL/MSSL
One of the most important goals of heliophysics and the recently launched Solar Orbiter and Parker Solar Probe missions is to identify the origin of the slow speed solar wind along with the structure and dynamics at its sources. Previous observations from the Hinode satellite have revealed the presence of high-temperature upflows at the peripheries of active regions. These upflows, if they have access to open magnetic fields, could be outflows and a source of the slow solar wind.
In the past, the EUV Imaging spectrometer (EIS) has measured typical bulk flow speeds in these upflows of 10-40 km/s (Brooks & Warren 2011) but there is also a high-speed asymmetric component present in the blue wing of the EIS spectral lines. This high-speed component is often weak with the intensity only amounting to ~10% of the total emission from the upflow regions (Brooks & Warren 2012). In this study, we analyse 12 active regions (three of which are observed later in their evolution) that span a wide range of total unsigned magnetic flux and area (previously analysed for different purporses in Warren et al. 2012). This is in order to determine the following: i) are these high-speed upflows are present in all sizes of active regions and ii) if these upflows are present, is there always a high-speed component present in the blue wing asymmetry in the high-temperature line profiles?
Figure 1: The left panel shows the SDO/AIA 193 Å image of active region (AR) 11150 where the white box corresponds to the EIS field-of-view. The middle panel shows the Fe XII 192.394 Å image along with the maps of the Doppler velocity. The right panel shows the mean spectrum of the pixels in the regions of interest highlighted by the black boxes in the middle panel.
For the analysis of the upflows we use the Fe XII 192.394 Å spectral line to create relative Doppler velocity maps by fitting a Gaussian function to the spectrum across each of the scans. These maps then allow us to easily identify the locations of the upflows and determine whether they exist in every active region. To assess whether a high-speed asymmetric component is present in the line profile we use a double Gaussian function. We select a region of interest in the active region upflows, calculate the mean spectrum and then fit a single Gaussian in order to construct a double Gaussian template. The minor high-speed component is assumed to have 10% of the intensity of the main component and both Gaussians are assumed to have the same width. The centroid of the component is initially placed in the blue wing with a Doppler shift of 125 km/s (based on previous results from Brooks & Warren 2012). This method provides a consistent way of quantifying the area of the minor component compared to the main component across all the different active regions.
Figure 2: The intensity ratio of the asymmetric and main component as a function of the average velocities calculated from the selected regions in the east and west upflows of each active region. Blue (green) circles represent the average velocities associated with the leading (following) polarity of each active region and the numbers correspond to the EIS scans of each active region (given in Table 1 of the paper).
Our results show that:
- High-velocity upflows are present at the east and west boundaries of each active region in our sample (e.g. see the middle panel of Figure 1)
- There is a weak blue wing asymmetry present in all the line profiles constructed from the regions of interest located in the AR upflows
- The average velocities of the AR upflows in the regions of interest are in the range of -5 to 26 km/s (see Figure 2)
- The intensity of the minor high-speed component is relatively small (less than 20% of the main component) however, for eight of the upflows the intensity ratio ranges from 20 to 56%.
Given that the upflows and high-speed components are present in all ARs this suggests that they are likely to be a source of the slow solar wind and that their contribution could be significant.
For more details, see Yardley, Brooks, and Baker, Astronomy & Astrophysics 650:L10, 2021:
Widespread occurrence of high-velocity upflows in solar active regions
Brooks, D. H. & Warren, H. P. 2011, ApJ, 727, L13.
Brooks, D. H. & Warren, H. P. 2012, ApJ, 760, L5.
Warren, H. P., Winebarger, A. R., & Brooks, D. H. 2012, ApJ, 759, 141.
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