Solar flare observations with the Radio Neutrino Observatory Greenland (RNO-G)

TL;DR

The RNO-G observatory detects solar flares with high temporal resolution, using solar signals for calibration, and demonstrates its capability to observe impulsive solar radio emissions with potential applications in neutrino astronomy.

Contribution

This work shows RNO-G's ability to detect and analyze solar flares, using them for calibration, and highlights its dual role in neutrino detection and solar radio observations.

Findings

Detected solar flare signals coinciding with Callisto network and SWAVES satellite data.

Achieved sub-degree accuracy in pointing calibration using solar flare signals.

Demonstrated RNO-G's capability for high-resolution solar radio observations.

Abstract

The Radio Neutrino Observatory - Greenland (RNO-G) seeks discovery of ultra-high energy neutrinos from the cosmos through their interactions in ice. The science program extends beyond particle astrophysics to include radioglaciology and, as we show herein, solar observations, as well. Currently seven of 35 planned radio-receiver stations (24 antennas/station) are operational. These stations are sensitive to impulsive radio signals with frequencies between 80 and 700 MHz and feature a neutrino trigger threshold for recording data close to the thermal floor. RNO-G can also trigger on elevated signals from the Sun, resulting in nanosecond resolution time-domain flare data; such temporal resolution is significantly shorter than from most dedicated solar observatories. In addition to possible RNO-G solar flare polarization measurements, the Sun also represents an extremely useful above-surface calibration source. Using RNO-G data recorded during the summers of 2022 and 2023, we find signal excesses during solar flares reported by the solar-observing Callisto network and also in coincidence with $\sim$2/3 of the brightest excesses recorded by the SWAVES satellite. These observed flares are characterized by significant time-domain impulsivity. Using the known position of the Sun, the flare sample is used to calibrate the RNO-G absolute pointing on the radio signal arrival direction to sub-degree resolution. We thus establish the Sun as a regularly observed astronomical calibration source to provide the accurate absolute pointing required for neutrino astronomy.