Abstract
Observations from the Solar Dynamics Observatory (SDO) have the potential for allowing the helioseismic study of the formation of hundreds of active regions, which would enable us to perform statistical analyses. Our goal is to collate a uniform data set of emerging active regions observed by the SDO/HMI instrument suitable for helioseismic analysis up to seven days before emergence. We restricted the sample to active regions that were visible in the continuum and emerged into quiet Sun largely avoiding pre-existing magnetic regions. 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). We call this data set, which is currently comprised of 105 emerging active regions observed between May 2010 and November 2012, the SDO Helioseismic Emerging Active Region (SDO/HEAR) survey. To demonstrate the utility of a data set of a large number of emerging active regions, we 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. The latitudinally averaged line-of-sight magnetic field of all the EARs shows that, on average, the leading (trailing) polarity moves in a prograde (retrograde) direction with a speed of 121 +/- 22 m/s (-70 +/- 13 m/s) 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. The SDO/HEAR data set will not only be useful for helioseismic studies, but will also be useful to study other features such as the surface magnetic field evolution of a large sample of EARs.
Highlights
Surface magnetic activity is the most obvious characteristic of the internal solar dynamo
Our goal is to collate a uniform data set of emerging active regions observed by the Solar Dynamics Observatory (SDO)/HMI instrument suitable for helioseismic analysis, where each active region is centred on a 60◦ × 60◦ area and can be observed up to seven days before emergence
We emphasise that studies of subsets and case studies of the SDO/HEAR Survey are important to further characterise the emergence process, for example studying the differences in emergence properties from weak and strong flux active regions
Summary
Surface magnetic activity is the most obvious characteristic of the internal solar dynamo. Helioseismic case studies of individual active region emergences have been carried out for at least two decades Braun 1995; Chang et al 1999; Jensen et al 2001; Hartlep et al 2011) These case studies are useful and some have suggested strong pre-emergence features Birch et al (2010) estimated that by studying approximately 100 emerging active regions, helioseismology should be able to detect the predicted signature (a roughly 100 m s−1 retrograde flow) of a buoyant rising flux tube below the surface before emergence.
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