Two New Methods for Counting and Tracking the Evolution of Polar Faculae
- Creators
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Hovis-Afflerbach, B.
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Pesnell, W. Dean
Abstract
Polar faculae are the footpoints of magnetic-field lines near the Sun's poles that are seen as bright regions along the edges of granules. The time variation in the number of polar faculae has been shown to correlate with the strength of the polar magnetic field and to be a predictor of the subsequent solar cycle. Due to the small size and transient nature of these features, combined with different techniques and observational factors, previous counts of polar faculae differ in magnitude. Further, there were no scalable techniques to measure the statistical properties of the faculae, such as the variation of the facular lifetime with time or solar activity. Using data from the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO), we present two new methods for tracking faculae and measuring their properties. In the first, we calculate the pixel-by-pixel standard deviation of the HMI continuum intensity images over one day, visualizing the faculae as streaks. The lifetime of the facula is found by dividing the angular length of the streaks by the latitude-dependent rotation rate. We apply this method to the more visible pole each day for a week every six months, from September 2010 to March 2021. Combining all of the measured facular lifetimes provides a statistical distribution with a mean of 6.0 hours, a FWHM of 5.4 hours, and a skew towards longer lifetimes, with some faculae lasting up to 1 day. In the second method, we overlay images of the progressive standard deviation with the HMI magnetogram to show the close relationship between the facular candidates and the magnetic field. The results of this method allow us to distinguish between motion due to the Sun's rotation and "proper motion" due to faculae moving across the Sun's surface, confirming that faculae participate in convective motions at the poles. Counts of polar faculae using both methods agree with previous counts in their variation with the solar cycle and the polar magnetic field. These methods can be extended to automate the identification and measurement of other properties of polar faculae, which would allow for daily measurements of all faculae since SDO began operation in 2010.
Additional Information
© The Author(s) 2022. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Received: 17 October 2021; Accepted: 14 March 2022; Published 20 April 2022. This article belongs to the Topical Collection: Celebrating a Solar Cycle of Discovery with SDO, Guest Editors: Dean Pesnell, Ryan Milligan and Shin Toriumi. This research was supported, in part, by the SDO Project. The HMI data is courtesy of the SDO HMI Science Investigation Team. Some of the computing was performed at the JSOC, which is located at Stanford University and operated by the HMI Science Investigation Team. This research made use of version 4.2 of Astropy (http://www.astropy.org), a community-developed core Python package for Astronomy (Astropy Collaboration et al., 2013, 2018). This research used version 2.1.3 of the SunPy open source software package (Mumford et al., 2021; The SunPy Community et al., 2020). This work was performed while B. H.-A. was a research support assistant at the Catholic University of America. The authors declare that they have no conflicts of interest. Data Availability: The data will be made available on reasonable request.Attached Files
Published - Hovis-Afflerbach-Pesnell2022_Article_TwoNewMethodsForCountingAndTra.pdf
Accepted Version - 2204.10863.pdf
Supplemental Material - 11207_2022_1977_MOESM1_ESM.mp4
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Additional details
- Eprint ID
- 115111
- Resolver ID
- CaltechAUTHORS:20220609-226935000
- NASA
- Created
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2022-06-10Created from EPrint's datestamp field
- Updated
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2022-06-13Created from EPrint's last_modified field