# Observing the Sun with the Atacama Large Millimeter-submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping

## TL;DR

This paper details the development and calibration of a fast-scanning single-dish mapping technique for the ALMA solar observations, enabling accurate temperature measurements of the Sun's atmosphere.

## Contribution

It introduces a novel fast-scanning method for ALMA solar observations and provides a detailed calibration procedure for absolute temperature measurements.

## Key findings

- Derived quiet-Sun temperature values at disk center: 7300 K at 3 mm and 5900 K at 1.3 mm.
- Single-dish images reveal temperature variations of about 2000 K across the solar disk.
- Calibration uncertainties are around 100 K, with potential larger systematic errors.

## Abstract

The Atacama Large Millimeter-submillimeter Array (ALMA) radio telescope has commenced science observations of the Sun starting in late 2016. Since the Sun is much larger than the field of view of individual ALMA dishes, the ALMA interferometer is unable to measure the background level of solar emission when observing the solar disk. The absolute temperature scale is a critical measurement for much of ALMA solar science, including the understanding of energy transfer through the solar atmosphere, the properties of prominences, and the study of shock heating in the chromosphere. In order to provide an absolute temperature scale, ALMA solar observing will take advantage of the remarkable fast-scanning capabilities of the ALMA 12m dishes to make single-dish maps of the full Sun. This article reports on the results of an extensive commissioning effort to optimize the mapping procedure, and it describes the nature of the resulting data. Amplitude calibration is discussed in detail: a path that utilizes the two loads in the ALMA calibration system as well as sky measurements is described and applied to commissioning data. Inspection of a large number of single-dish datasets shows significant variation in the resulting temperatures, and based on the temperature distributions we derive quiet-Sun values at disk center of 7300 K at lambda=3 mm and 5900 K at lambda=1.3 mm. These values have statistical uncertainties of order 100 K, but systematic uncertainties in the temperature scale that may be significantly larger. Example images are presented from two periods with very different levels of solar activity. At a resolution of order 25 arcsec, the 1.3 mm wavelength images show temperatures on the disk that vary over about a 2000 K range.