H. Schunker1, D. C. Braun2, A. C. Birch1, R. B. Burston1, & L. Gizon1,3
1. Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
2. NorthWest Research Associates, 3380 Mitchell Ln, Boulder, CO 80301, USA
3. Georg-August-Universität Göttingen, Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen
We have generated a dataset of emerging active regions (EARs) observed by SDO/HMI that is specifically suitable for helioseismic analysis. Surveys are useful for studying magnetic flux emergence and evolution as they allow statistical studies and the possibility of revealing subtle signals which are otherwise overwhelmed by noise1. A previous survey dedicated to this goal2,3 used GONG Dopplergrams and MDI line-of-sight magnetic fields to study EARs from about one day prior to emergence, and measured wave travel-times for waves with lower turning points up to 20 Mm below the surface. With the advent of SDO/HMI it is now possible to observe closer to the limb and with full temporal coverage and an excellent duty-cycle: we can track active regions further back in time and cover larger areas and therefore probe deeper with helioseismology.
The SDO Helioseismic Emerging Active Regions (SDO/HEAR) survey consists of 105 emerging active regions observed from 2010 to 2012. We restricted the sample to active regions identified in National Oceanic and Atmospheric Administration (NOAA) solar region reports that emerged into quiet Sun largely avoiding pre-existing magnetic regions. We mapped the data to Postel’s projection and tracked the regions at the Carrington rotation rate up to seven days pre- and post-emergence. The emergence time was defined as in Ref. 2 and 3. The dataset consists of 45 second cadence datacubes of line-of-sight magnetic field, Doppler velocity and intensity continuum with a resolution 1.39 Mm per pixel. The datacubes are at intervals of about 6 hours. As a reference data set we paired a control region (CR), with the same latitude and distance from central meridian, with each emerging active region (EAR). The control regions do not have any strong emerging flux within 10° of the center of the map. Figure 1 shows the average magnetic field during emergence over all active regions for a selection of times. The 15 terabyte dataset was computed by, and is stored at, the German Data Centre for SDO (GDC-SDO) .
Figure 1 | Average line-of-sight magnetic field maps for all EARs at selected time intervals. The southern hemisphere active regions have had their polarity reversed and have been flipped in the y direction to account for Hale’s law (the +y direction is always poleward). The +x direction runs longitudinally. The origin of the axes is at the emergence location.
This dataset enabled the rise speed of active regions approaching the surface from below to be constrained in Ref. 4. We used this dataset to measure the relative east-west velocity of the leading and trailing polarities from the line-of-sight magnetogram maps during the first day after emergence. At each time (six hour cadence) we averaged the line-of-sight magnetic field in the north-south direction over 15 Mm centered on the emergence location. From this, we fit a Gaussian as a function of longitude to each polarity at each time interval. We used the location of the peak of the Gaussian as an estimate of the east-west location. The motion of the peak gives us the east-west velocity of the polarities relative to the Carrington rotation rate (the rate at which the data cubes were tracked).
Figure 2 | Left: mean east-west velocity relative to the Carrington rotation rate of the leading (red crosses) and trailing (black crosses) polarities over the first day after emergence for each active region. The size of the symbols represents the size of the active region (AR 11158 is the largest). The scatter is large; this emphasizes the uniqueness of each active region emergence. The mean velocity of the leading polarity in the first day after emergence is 121±22 m s-1 and the trailing polarity is -70±13 m s-1. Right: the average velocities in two bins of latitude divided by the median latitude (dashed vertical lines) of the EARs. The black curve shows the differential velocity of the surface plasma relative to the Carrington rotation rate. The uncertainties are given by the rms of the velocities in each bin, divided by the square root of the number of EARs in the bin.
Figure 2 shows the leading and trailing velocities in the first day after emergence. On average, the leading (trailing) polarity moves in a prograde (retrograde) direction with a speed of 121±22 m s-1 (-70±13 m s-1) relative to the Carrington rotation rate in the first day. However, relative to the differential rotation of the surface plasma, the east-west velocity is symmetric, with a mean of 95±13 m s-1 (Fig. 2). This work and figures was originally published in Ref. 5.
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 Schunker, H., Braun, D. C., Birch, A. C., Burston, R. B., & Gizon, L. 2016, A&A, in press