Fast single-dish scans of the Sun using ALMA

TL;DR

This paper presents the development and testing of high-speed scanning software for ALMA antennas to observe the Sun, enabling detailed, high-temporal-resolution solar brightness maps and variability analysis.

Contribution

It introduces a novel control and data acquisition system for ALMA that allows complex, high-rate solar scans with detailed characterization of noise and signal fluctuations.

Findings

Achieved Nyquist sampling of the Sun at 230 GHz in 60 seconds

Mapped brightness temperature variations up to 1000 K

Detected time-series variability of about 0.5% RMS over minutes

Abstract

We have implemented control and data-taking software that makes it possible to scan the beams of individual ALMA antennas to perform quite complex patterns while recording the signals at high rates. We conducted test observations of the Sun in September and December, 2014. The data returned have excellent quality; in particular they allow us to characterize the noise and signal fluctuations present in this kind of observation. The fast-scan experiments included both Lissajous patterns covering rectangular areas, and double-circle patterns of the whole disk of the Sun and smaller repeated maps of specific disk-shaped targets. With the latter we find that we can achieve roughly Nyquist sampling of the Band~6 (230~GHz) beam in 60~s over a region 300$"$ in diameter. These maps show a peak-to-peak brightness-temperature range of up to 1000~K, while the time-series variability at any given point appears to be of order 0.5 percent RMS over times of a few minutes. We thus expect to be able to separate the noise contributions due to transparency fluctuations from variations in the Sun itself. Such timeseries have many advantages, in spite of the non-interferometric observations. In particular such data should make it possible to observe microflares in active regions and nanoflares in any part of the solar disk and low corona.