The Solar Radius at 37 GHz through Cycles 22 to 24

TL;DR

This study analyzes the variation of the solar radius at 37 GHz over solar Cycles 22 to 24, revealing correlations with solar activity and comparing results across different wavelengths.

Contribution

It provides a long-term analysis of the solar radius at 37 GHz, normalizes data across instrumental changes, and compares findings with other wavelength observations.

Findings

Positive correlation between solar radius and solar flux at 37 GHz.

Correlation increased when considering data without instrumental changes.

Results align with previous mm/cm wavelength studies but differ from submillimetre findings.

Abstract

To better understand the influence of the activity cycle on the solar atmosphere, we report the time variation of the radius observed at 37 GHz ($\lambda$=8.1 mm) obtained by the Mets\"ahovi Radio Observatory (MRO) through Solar Cycles 22 to 24 (1989-2015). Almost 5800 maps were analyzed, however, due to instrumental setups changes the data set showed four distinct behaviors, which requested a normalisation process to allow the whole interval analysis. When the whole period was considered, the results showed a positive correlation index of 0.17 between the monthly means of the solar radius at 37 GHz and solar flux obtained at 10.7 cm (F10.7). This correlation index increased to 0.44, when only the data obtained during the last period without instrumental changes were considered (1999-2015). The solar radius correlation with the solar cycle agrees with the previous results obtained at mm/cm wavelengths (17 and 48 GHz), nevertheless, this result is the opposite of that reported at submillimetre wavelengths (212 and 405 GHz).