Understanding the Relationship between Solar Coronal Abundances and F10.7 cm Radio Emission

Andy To - UCL/MSSL and Tim Bastian - NRAO


The F10.7 cm radio flux index is one of the most widely used solar indices to characterize solar activity. Daily measurements of the Sun's F10.7 cm flux have been recorded since 1947. Sun-as-a-star coronal plasma composition, derived from full-Sun spectra, and the F10.7 radio flux (2.8 GHz) have been shown to be highly correlated (r = 0.88) during solar cycle 24, suggesting that coronal abundances change with the solar cycle phase (Brooks et al. 2017). This, in turn, suggests that coronal abundances are influenced by magnetic activity and the coronal heating process, with significant implications for solar-like stars as well. However, this correlation becomes nonlinear during increased solar magnetic activity. Sun-as-a-star FIP bias appears to be saturated, while F10.7 cm flux continues to go up with the solar activity. In fact, a saturation of the first ionization potential (FIP) bias is often observed in highly active stars too (Wood & Linsky 2010 and Seli et al. 2022). The reason behind the FIP bias saturation remains poorly understood. Understanding the root cause of the nonlinearity in the F10.7-FIP bias correlation is crucial for gaining insight into solar activity, stellar magnetic fields, and both solar and stellar coronal heating.

In this study, we present co-spatial observations of an active region (AR 12759) using Hinode/EIS, JVLA, and SDO/AIA. We employed two region-defining methods to investigate the relationship between the FIP bias and the F10.7 radio flux: one based on the JVLA F10.7 cm Stokes I and V maps, and the other utilizing the estimated gyroresonance region isolated with the help of both the Stokes I map and AIA DEM. See Figure 1.

Figure 1: Small bipolar active region AR 12759, observed during the JVLA/20A-047 observing campaign on April 3 and 7. Left to right, top to bottom: AIA 193 A, HMI magnetogram, Si X 258.38 A/S X 264.23 A intensity ratio map, F10.7 radio flux map (Stokes V), and F10.7 radio flux map (Stokes I). Four F10.7 cm regions are used in this paper to calculate the FIP bias: (1) a Stokes I region with a brightness temperature >80,000 K subtracted by strong Stokes V emission (black contour), (2) negative Stokes V regions with a brightness temperature <-10,000 K (blue contour), (3) a positive Stokes V region with brightness temperature >10,000 K (orange contour), and (4) the estimated gyroresonance region on April 3 defined using ((Stokes I-modeled free-free)/Stokes I) (red dashed contour; Section 4.1). It can be seen that the negative Stokes V region is associated with the following polarity, whereas the positive Stokes V and gyroresonance regions are associated with the leading polarity. The Si/S intensity ratio maps shown here are for demonstration purposes. In our analysis, we calculated and used the spatially averaged FIP bias. This significantly improves the signal-to-noise ratio.
Using either region-defining method, we consistently found that the following polarity region exhibited a significantly enhanced Si/S ratio compared to the leading region. This suggests that magnetic field strength plays a crucial role in the variation of coronal abundances. In both methods, the leading polarity consistently exhibited a stronger, more concentrated magnetic field. Under this configuration, the strong magnetic flux density inhibits convection, resulting in cooler temperatures, lower ionization rates, and a slightly lower FIP bias. In contrast, in the following spot, the magnetic flux is spread out into a larger area, leading to higher temperatures, higher ionization rates of low-FIP elements, and a higher FIP bias. See Table 1.

Table 1: Calculated FIP Bias Values and Magnetic Flux Density Associated with the Two Region-defining Methods.

This result can be translated into a bigger picture, informing us on the relationship between Sun-as-a-star FIP bias and F10.7 flux, and help interpret the FIP bias saturation observed in both other stars and our Sun (Brooks et al. 2017). As the solar cycle progresses from its minimum, more regions of strong fields and nanoflares contribute to more Alfvén waves being created and reflected along closed magnetic loops (Laming 2015). Consequently, the FIP bias gradually increases with solar activity. Since the F10.7 cm radio flux serves as a proxy for solar activity, we expect a strong correlation between the Sun's FIP bias and F10.7 cm measurements under these conditions. However, during peak solar activity, sunspot areas increase, with spots exhibiting higher field strength and cooler temperatures (Watson et al. 2011; Valio et al. 2020). In such high solar activity conditions, the contribution of gyroresonance emission to the F10.7 cm emission is likely to increase. Hence, we propose that the contrasting behaviors of radio gyroresonance emission and coronal abundances (FIP bias) over strong magnetic field concentrations (sunspots) are the likely cause of the saturation of the Sun's coronal abundances around solar maximum. This result also has implications for stellar coronal composition. Low-activity stars like our Sun have coronae that are dominated by the FIP effect (a more enhanced low-FIP composition). This result highlights the importance of magnetic field strength or magnetic flux density and the F10.7cm emission when linking coronal composition to the different spectral types of stars (Wood & Linsky 2010 and Seli et al. 2022).

For more details, see:
To et al., ApJ, 948:121, 2023:
Understanding the Relationship between Solar Coronal Abundances and F10.7 cm Radio Emission

Brooks, D. H., Baker, D., van Driel-Gesztelyi, L., and Warren, H. P., 'A Solar cycle correlation of coronal element abundances in Sun-as-a-star observations', Nature Communications, vol. 8, 2017. doi:10.1038/s41467-017-00328-7
Wood, B. E. and Linsky, J. L., 'Resolving the Boo Binary with Chandra, and Revealing the Spectral Type Dependence of the Coronal FIP Effect', The Astrophysical Journal, vol. 717, no. 2, pp. 1279-1290, 2010. doi:10.1088/0004-637X/717/2/1279
Seli, B., 'Extending the FIP bias sample to magnetically active stars. Challenging the FIP bias paradigm', Astronomy and Astrophysics, vol. 659, 2022. doi:10.1051/0004-6361/202141493
Laming, J. M., 'The FIP and Inverse FIP Effects in Solar and Stellar Coronae', Living Reviews in Solar Physics, vol. 12, no. 1, 2015. doi:10.1007/lrsp-2015-2
Watson, F. T., Fletcher, L., and Marshall, S., 'Evolution of sunspot properties during solar cycle 23', Astronomy and Astrophysics, vol. 533, 2011. doi:10.1051/0004-6361/201116655
Valio, A., Spagiari, E., Marengoni, M., and Selhorst, C. L., 'Correlations of Sunspot Physical Characteristics during Solar Cycle 23', Solar Physics, vol. 295, no. 9, 2020. doi:10.1007/s11207-020-01691-3

For more details, please contact: Dr. Deb Baker.

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