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

Abstract

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. However, this correlation becomes nonlinear during increased solar magnetic activity. Here we use cotemporal, high spatial resolution, multiwavelength images of the Sun to investigate the underlying causes of the nonlinearity between coronal composition (FIP bias) and F10.7 solar index correlation. Using the Karl G. Jansky Very Large Array, Hinode/EIS (EUV Imaging Spectrometer), and the Solar Dynamics Observatory, we observed a small active region, AR 12759, throughout the solar atmosphere from the photosphere to the corona. The results of this study show that the magnetic field strength (flux density) in active regions plays an important role in the variability of coronal abundances, and it is likely the main contributing factor to this nonlinearity during increased solar activity. Coronal abundances above cool sunspots are lower than in dispersed magnetic plage regions. Strong magnetic concentrations are associated with stronger F10.7 cm gyroresonance emission. Considering that as the solar cycle moves from minimum to maximum, the sizes of sunspots and their field strength increase with the gyroresonance component, the distinctly different tendencies of radio emission and coronal abundances in the vicinity of sunspots is the likely cause of saturation of Sun-as-a-star coronal abundances during solar maximum, while the F10.7 index remains well correlated with the sunspot number and other magnetic field proxies.

Identifying Plasma Fractionation Processes in the Chromosphere Using IRIS

“…As the only observable active region, it was chosen as the target for Hinode Observing Plan 390, with observations from both Hinode/EIS and IRIS supporting observations made by the Karl G. Jansky Very Large Array (JVLA; Perley et al 2011). A more detailed discussion of the observing campaign and the relationship between the observations made using Hinode/EIS and JVLA can be found in To et al (2023). As noted by To et al (2023), the JVLA took observations of AR 12759 on 2020 April 3 and 2020 April 7, which were then used to examine the relationship between elemental abundance and F10.7 radio emission.…”

“…A more detailed discussion of the observing campaign and the relationship between the observations made using Hinode/EIS and JVLA can be found in To et al (2023). As noted by To et al (2023), the JVLA took observations of AR 12759 on 2020 April 3 and 2020 April 7, which were then used to examine the relationship between elemental abundance and F10.7 radio emission. However, as the sole active region on the disk, AR 12759 was also the focus of a series of IRIS and EIS rasters during the time period between these two JVLA observations, providing a unique insight into its long-term evolution in both the corona and the chromosphere/transition region.…”

“…The 17 separate Hinode/EIS observations of AR 12759 between 2020 April 2 and 7 described here were taken using the HPW021_VEL_260 × 512v2 study. This study contains the Si X 258.375 Å, S X 264.233 Å, Ca XIV 193.874 Å, and Ar XIV 194.396 Å lines as well as a series of Fe lines, and has previously been used to study active region FIP bias (e.g., Testa et al 2023;To et al 2023). The HPW021_VEL_260 × 512v2 study scans a field of view of 260″ × 512″, with 87 raster positions of 40 s exposure time and uses the 2″ slit width with a raster step of 3″.…”

“…The FIP bias here was estimated using two distinct line ratio diagnostics, namely, the Si X/S X and Ca XIV/Ar XIV ratios. The FIP bias derived from the Si X/ S X diagnostic was calculated using the technique developed by Brooks et al (2015) and subsequently used by Baker et al (2021) and To et al (2021To et al ( , 2023, in which spectral lines from consecutive ionization stages of Fe VIII-Fe XVII were fit with single or multiple Gaussians as appropriate (typically single Gaussians unless the line is blended). These lines have a formation temperature of ∼0.5-5.5 MK, and the diagnostic can be used to estimate the FIP bias assuming a density estimated using the Fe XIII 202.04/203.83 Å line ratio.…”

On Generalized Additive Models for Representation of Solar EUV Irradiance

“…As a result, solar proxies well‐correlated with solar EUV irradiance which can be measured from the ground, such as F10.7 (Tapping, 2013), have seen regular usage due to their operational availability, and are routinely used as inputs for Ionosphere‐Thermosphere models such as NRLMSISE 2.0 (Emmert et al., 2021) and Thermosphere Ionosphere Electrodynamics General Circulation Model (Cai et al., 2022). While these solar proxies have demonstrated applicability in downstream modeling for representing thermospheric and ionospheric climatology, they suffer from some important limitations, including: Each solar index is best described as a proxy for solar processes occurring either in the photosphere, chromosphere, transition region, corona, or a combination of some of these regions, limiting their ability to capture the entire swath of variation throughout the entire EUV range (To et al., 2023). The emissions most strongly correlated with each solar index are absorbed in different regions of the thermosphere and mesosphere, resulting in either increasingly complex parameterization for their ingestion into atmospheric models and non‐trivial impacts on quantification of uncertainty in derived thermospheric temperatures and densities (Thayer et al., 2021).…”