Rudi Komm
National Solar Observatory, Tucson, AZ 85719, USA

Some sunspots are deceptively simple like the one in active region NOAA 11092, observed on August 2010, which was nearly round and not particularly large. However, its magnetic field was twisted in such a way as to resemble a pinwheel that is rotating counter-clockwise (see Figure 1). High in the solar atmosphere, sunspot magnetic fields can be seen as a mixture of structures that are usually twisted in all directions. A twist in the same direction happens only on rare occasions as in the case of AR11092. The pattern persists throughout the disk passage of the active region. While the orientation of the twist is clearly visible in the chromosphere and the corona, what is the orientation of the twist at photospheric heights and below the solar surface?

Figure 1 | Active region NOAA 11092 is a medium region in the northern hemisphere with a counter-clockwise whirl as shown in chromospheric and transition region structures (left) and coronal loops (right) observed on August 2, 2010 by the SDO/AIA instrument in He II 304 Å and Fe IX 171 Å. The curvature of the whirl is defined by following the magnetic field lines; the sunspot has negative polarity. (Adapted from Ref. [3])

At the photospheric level, vector magnetograms from SDO/HMI have been used to determine a simple and robust measure of magnetic twist, the spatially averaged signed shear angle (SASSA)^[1]. This parameter produces negative values for AR11092 consistent with the counter-clockwise orientation of the whirl seen in the higher solar atmosphere. Below the solar surface, the twist of the magnetic field cannot be directly measured but might be estimated by using that of subsurface flows as a substitute. The subsurface flows associated with this active region have been measured using ring-diagram analysis applied to SDO/HMI Dopplergrams. The kinetic helicity density has been derived as a measure of the twist in these flows^[2].

Figure 2 | Active region NOAA 11092 with a counter-clockwise whirl is associated with subsurface locations of predominantly positive kinetic helicity density, as shown in the slice in latitude and depth at 75 degree longitude (top: magnetic activity; middle: kinetic helicity density; bottom: signal-to-error ratio). The subsurface flows have been derived from SDO/HMI Dopplergrams.

The twist in the lower atmosphere, as determined by SASSA, is in the same direction for NOAA 11092 as seen higher up in the solar atmosphere.  However, below the surface the twist is in the opposite direction, as indicated by the positive kinetic helicity density (Figure 2).  Another sunspot with a clockwise whirl (AR11084) shows the same result of having different signs of helicity below and above the solar surface. In a control experiment of six sunspots without persistent whirl pattern, the orientation of the twist of the magnetic field in the solar atmosphere turned out to be the same as that of the flows below the solar surface for four of the six regions. The regions without whirls follow overall the same hemispheric rule in their kinetic helicity as in their current helicity with positive values in the southern and negative values in the northern hemisphere. This suggests that opposite directions of twist above and below the solar surface is indeed a characteristic of active regions with whirls.  But, could this be a coincidence due to the small sample size?  We plan to analyze many more active regions and find out.

More details of this work can be found in Ref [3].

References

[1] Tiwari, S.K., Venkatakrishnan, P., & Sankarasubramanian, K. 2009, ApJ, 702, L133.
[2] Komm, R.W., Corbard, T., Durney, B.R., González Hernández, I., Hill, F., Howe, R., & Toner, C. 2004, ApJ, 605, 554.
[3] Komm, R., Gosain, S., & Pevtsov, A. 2014, Solar Phys., 289, 475