Insights into chromospheric large-scale flows using Nobeyama 17 GHz radio observations: I. The differential rotation profile

Abstract

Context. Although the differential rotation rate on the solar surface has long been studied using optical and extreme ultraviolet (EUV) observations, associating these measurements with specific atmospheric heights remains challenging due to the temperature dependent emission of tracers observed in EUV wavelengths. Radio observations, being primarily influenced by coherent plasma processes and/or thermal bremsstrahlung, offer a more height-stable diagnostic and thus provide an independent means to test and validate rotational trends observed at other EUV wavelengths. Aims. We aim to characterise the differential rotation profile of the upper chromosphere using cleaned solar full-disc 17 GHz radio imaging from the Nobeyama Radioheliograph spanning a little over two solar cycles (1992–2020). Methods. A tracer-independent method based on automated image correlation was employed on daily full-disc 17 GHz radio maps. This method determines the angular velocities in 16 latitudinal bins of 15◦ each by maximising the 2D cross-correlation of overlapping image segments. Results. The best-fit parameters for the differential rotation profile are A = 14.520 ± 0.006◦ /day, B= –1.443 ± 0.099◦ /day, and C = –0.433 ± 0.267◦ /day. These results suggest that the upper chromosphere rotates significantly faster than the photosphere at all latitudes, with a relatively flatter latitudinal profile. We also observed a very weak anti-correlation, ρs = −0.383 (94.73%), between the equatorial rotation rate and solar activity. Conclusions. Our findings reaffirm the potential of radio observations to probe the dynamics of the solar chromosphere with reduced height ambiguity. The overlap of the equatorial rotation rate (A) found in this study with that for 304 Å in the EUV regime lends additional support to the view that the equatorial rotation rates increase with height above the photosphere. Future coordinated studies at wavelengths with better-constrained height formation will be crucial for further understanding the complex dynamics of the solar atmosphere.