The Subterahertz Solar Cycle: Polar and Equatorial Radii Derived from SST and ALMA

Abstract

At subterahertz frequencies—i.e., millimeter and submillimeter wavelengths—there is a gap in measurements of the solar radius, as well as other parameters of the solar atmosphere. As the observational wavelength changes, the radius varies because the altitude of the dominant electromagnetic radiation is produced at different heights in the solar atmosphere. Moreover, radius variations throughout long time series are indicative of changes in the solar atmosphere that may be related to the solar cycle. Therefore, the solar radius is an important parameter for the calibration of solar atmospheric models enabling a better understanding of the atmospheric structure. In this work, we use data from the Solar Submillimeter-wave Telescope (SST) and the Atacama Large Millimeter/submillimeter Array (ALMA) at frequencies of 100, 212, 230, and 405 GHz to measure the equatorial and polar radii of the Sun. The radii measured with extensive data from the SST agree with the radius-versus-frequency trend present in the literature. The radii derived from ALMA maps at 230 GHz also agree with the radius-versus-frequency trend, whereas the 100 GHz radii are slightly above the values reported by other authors. In addition, we analyze the equatorial and polar radius behavior over the years by determining the correlation coefficient between solar activity and subterahertz radius time series at 212 and 405 GHz (SST). The variations of the SST-derived radii over 13 yr are correlated to the solar activity when considering equatorial regions of the solar atmosphere and anticorrelated when considering polar regions. The ALMA-derived radius time series for 100 and 230 GHz show very similar behaviors with those of SST.

Subterahertz Radius and Limb Brightening of the Sun Derived from SST and ALMA

“…SST solar maps are obtained from parallel or radial raster scans of the Sun covering an area of about a 1°× 1°in the sky with a tracking speed in the range of 0.1°and 0.2°per second. A few solar maps are reconstructed from radial and RA-dec 1 raster scans, whereas the vast majority of the solar maps are reconstructed from azimuth-elevation scans with a 2 separation between scans obtained (Giménez de Castro et al 2020;Valle Silva et al 2021;Menezes et al 2021). For a typical integration time of 0.04s, the rectangular pixels are 0.48 × 2 in size.…”

“…As described by Menezes et al (2021), some criteria are adopted to avoid extracting limb points associated with active regions and instrumental or random observational errors introduced by high Earth's atmospheric attenuation. For the limb point extraction, only those with a center-to-limb distance between 815 (0.85 𝑅 𝜃 ) and 1100 (1.15 𝑅 𝜃 ) are considered.…”

“…Since then, measurements at radio frequencies began to take place (Coates 1958). By observing the Sun at different radio wavelengths, the radius measurements present an exponential trend, as showed by Menezes et al (2021), decreasing with frequency. Therefore at a specific wavelength, the difference of ★ E-mail: menezes.astroph@gmail.com the measured radius from the optical radius can be understood as the height above the photosphere where most of the emission is being created in the limb regions.…”

“…At radio wavelengths, the radius can be measured by the inflectionpoint method (Alissandrakis et al 2017;Menezes et al 2021), and by the half-power method (Costa et al 1999;Selhorst et al 2011;Menezes & Valio 2017). The measurement of the solar radius by any of theses two methods is considerably affected by the presence of limb brightening in the solar disk.…”

“…This work is a follow-up of Menezes et al (2021), in which we measured the average equatorial and polar radius at 212 and 405 GHz (using all receivers of the Submillimeter Solar Telescope) from 2007 to 2019, and at 100 and 230 GHz from 2015 to 2018. Also it was focused on the relation between solar radius time series and solar activity.…”