Emily Mason1, Kara Kniezewski2, David Fritz3
1. Predictive Science Inc.
2. Air Force Institute of Technology
3. Charles E. Smith Jewish Day School
The National Oceanic and Atmospheric Administration (NOAA) identifies and tracks active regions (ARs) due to their role in producing geoeffective space weather, numbering every AR that appears or rotates into view as the Sun rotates. NOAA does not, however, track which ARs persist for multiple solar rotations. This becomes a challenge for researchers looking to study the life cycle of ARs, as older ARs are given new numbers that don’t relate to their previous designations. We have constructed a database of linked NOAA AR numbers for the years 2011 – 2019 to lower the barrier for long-term AR study.
We developed a Python code that searched for ARs first identified on-disk and then used SunPy’s[1] differential rotation tool to determine the time the AR would re-emerge on the east limb. The code then listed any ARs that reappeared within 2 days of that time and in that approximate location (a range of +/- 20 arcsec deviation from the expected y coordinate was allowed for drift as the AR decayed). This was applied to every NOAA AR from 2011 through 2019.
From this list of candidates, a list of 599 unique pairs were identified. The full set of pairs were assessed two ways: master classifications as described below, and volunteer classifications conducted via a citizen science project on Zooniverse (Solar Active Region Spotter). The volunteer classifications are still under review, and are not included in this data release.
For the master classifications, we manually confirmed each pair through inspection of SDO HMI[2] data; an example of a positive match is shown in the figure. The final confirmed list of 119 multi-rotation ARs (248 NOAA AR designations, 119 unique ARs) was uploaded as a CSV to Zenodo: https://doi.org/10.5281/zenodo.8309944. This includes ARs that received NOAA designations on 2 to 4 Carrington rotations; none were identified that lasted more than 4 rotations.
Figure 1| An example of a positive match between an active region candidate pair (NOAA AR 11416 and 11428).
There are several aspects of the list which should be kept in mind. First and foremost, this is not an exhaustive list of multi-rotation ARs. If an AR initially received a NOAA designation near the western limb, it was discounted from the list due to the line-of-sight effects precluding positive identification on subsequent rotations. Additional AR pairs were also added to the final list when it was apparent from the visual inspection that neighboring ARs were also matches across rotations, showing that the Python script did not identify every possible candidate pair. We maintained a high index of suspicion necessary to confirm a pair; some candidates were likely to be matches given the timing and location, but there was not sufficient nearby context to be certain, and so they were not included in the list of matches. Finally, some ARs were still apparent in the HMI data but had decayed so much that they did not receive NOAA designations, so these were not included in the final list. We caution against drawing statistical conclusions on the frequency or overall lifespan of ARs based solely on this list.
This dataset will be part of an upcoming paper studying heating patterns across several sets of the multi-rotation ARs identified in this project. However, since the main goal was to create a list that can be used by the community to easily identify long-lived ARs, we are publicizing the database now. It will be updated periodically throughout solar cycle 25, as more multi-rotation ARs appear.
References
[1]The SunPy Community, Barnes, W. T., Bobra, M. G., et al. 2020, ApJ, 890, 68
[2] Scherrer, P. H., Schou, J., Bush, R. I., et al. 2012, Solar Phys., 275, 207