Hinode/EIS Nugget – Response of the corona to the emergence of "serpentine" magnetic field

by Louise Harra, MSSL


New flux emergence into a pre-existing active region clearly can have dramatic consequences, often producing flaring with hours of the flux appearing. The high resolution of the SOT data also shows the complexity of the flux as it emerges with the complex and small-scale positive/ negative (white/black) features appearing. These small-scale structures are thought to be indicative of a serpentine structure emerging into the atmosphere (e.g. Cheung et al., 2008). There have been evidence of small-scale brightenings corresponding to these complex structures in the chromosphere. In this nugget we look at the response of the emerging flux in the active region observed in 1-2 December 2006. This active region was studied by Magara (2008), and shows a beautiful example of flux emergence (see Figure 1 and movie 1: eis_sot.avi).

Figure 1: The left hand side show the SOT Fe I Stokes-V data showing the evolution of the emergence of flux into the lagging polarity of a preexisting active region. The right hand side shows the TRACE 171 data at the same times with the SIT contours overlaid.  »  Click figure to see full-size image.

We found little evidence of small scale structures corresponding to complexity of the serpentine field, suggesting that by the time the magnetic field reaches the corona it has become a large-scale structure (see for example Strous and Zwaan, 1999). However the larger scale response to the flows in the corona are quite dramatic. Movie 2 shows the TRACE 171 fluctuations with time. As the flux emerges reconnection takes place on one side of the emerging flux, and eventually large scale loops form overlying the whole region. If we look at the Doppler flows in EIS, we can see a gradual enhancement of the outflows at the edges of active regions. These outflows are a potential source of the slow solar wind, and their source is currently under debate with waves, compression, large scale reconnection and small angle reconnection all being possibilities. As the flux emerges a strong enhancement in the Doppler flows are seen to the left-hand side of the emergence (see Figure 2). This is the region where the new positive polarity emerges into preexisting positive polarity. Whatever the process is appears to create stronger outflows, which should help us understand whether this can form part of the solar wind. We are currently comparing the data with simulations and hope to understand the process in more detail soon — watch this space!!

Figure 2: This is the evolution of the Doppler velocity of the Fe XII ion during the flux emergence. The velocities range between ± 30 km/s. Red shows downflow and blue shows upflow.  »  Click figure to see full-size image.

The first movie (eit_sot.avi) shows the EIS Fe XII intensity data overlaid on the SOT magnetic field data. This give an idea how the intensity of the corona changes as the flux emerges into the lagging polarity of the active region.

Figure 3: Screenshot of the movie "eit_sot.avi"  »  Click figure to download the movie.

The second movie shows the TRACE data as the flux emergence proceeds (trace_int.mov). The region to the right of the new flux emergence shows evidence of reconnection occurring with many new small loops being created. The region to the left of the flux emergence doesn't show such clear reconnection as it is dominantly the same polarity interacting.

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Movie 2   » Click here to download the original movie (in MOV format, 11MB)

This nugget is the summary of a paper just published in Solar Physics 2010, 263, 105-119 by Harra, Magara, Hara, Tsuneta, Okamoto and Wallace.

The link:  http://springerlink.com/content/l401392724076773/?p=416fb0a919834bc68c1c2685fc438328&pi=7

Contact the author: Louise Harra

Last Revised: 1-July-2010

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