Characteristics of Magnetic Sources during Solar Minima
Year: 2026
Type: article
Abstract: A census of magnetograms acquired by the space-based Helioseismic and Magnetic Imager (HMI) during solar activity minimum between 2019 January 1 and 2020 February 29 reveals a population of small, typically ∼1 Mm across, predominantly unipolar magnetic sources, embedded in extensive same-polarity halos. The sources carry ∼10 18 Mx fluxes and have median lifetimes of 400–670 s. Some vary on ∼1 minute timescales, rapidly doubling their magnetic flux densities (MFDs), then returning to the preoutburst level. Emission from a slowly changing mix of network and small-scale sources modulates the total solar irradiance (TSI) at 0.005%–0.01% levels on rotational timescales. We find evidence of a nonlinear TSI–magnetic field relationship at very low activity levels. During the 2019 cycle minimum, as in the prior activity minimum in 2009, the total MFD in ground-based magnetograms measured by the Global Oscillation Network Group (GONG) persistently remained in excess of ∼1.5 G above that of the total flux in the Gaussian core. Using Wilcox Solar Observatory (WSO) synoptic photospheric magnetic field maps, we identify an overall decline of ∼2 G in total MFD from solar minimum in 1976 to the 2009 and 2019 minima, which we attribute to the decline of the Centennial Gleissberg Cycle of 1910 to 2009. We speculate that the MFD during the upcoming solar activity minimum in 2029 may be higher than in 2019 as the next Gleissberg cycle manifests.
Source: The Astrophysical Journal
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
SDG: Climate action
Open Access status: gold
Estimated APC paid: $4,499
OpenAlex ID: W7214972763