Observation of Chromospheric Sunspot at Millimeter Range with the Nobeyama 45 m Telescope

TL;DR

This study presents the first high-resolution single-dish millimeter observations of a sunspot, revealing brightness temperature characteristics that challenge existing atmospheric models and correlating well with UV emissions.

Contribution

It provides new millimeter-range observational data of a sunspot using the Nobeyama 45 m telescope, highlighting discrepancies with current models.

Findings

Sunspot umbra brightness temperature is lower than quiet regions.

Brightness temperature correlates strongly with UV 1700 Å emission.

Results challenge existing atmospheric models.

Abstract

The brightness temperature of the radio free-free emission at millimeter range is an effective tool for characterizing the vertical structure of the solar chromosphere. In this paper, we report on the first single-dish observation of a sunspot at 85 and 115 GHz with sufficient spatial resolution for resolving the sunspot umbra using the Nobeyama 45 m telescope. We used radio attenuation material, i.e. a solar filter, to prevent the saturation of the receivers. Considering the contamination from the plage by the side-lobes, we found that the brightness temperature of the umbra should be lower than that of the quiet region. This result is inconsistent with the preexisting atmospheric models. We also found that the brightness temperature distribution at millimeter range strongly corresponds to the ultraviolet (UV) continuum emission at 1700 {\AA}, especially at the quiet region.