Active Region Irradiance During Quiescent Periods: New Insights from Sun-as-a-star Spectra

TL;DR

This study estimates the radiative energy losses of solar active regions during quiescent periods using Sun-as-a-star spectra from the EVE instrument, revealing their significance relative to flare energies and magnetic energy input.

Contribution

It introduces a novel method to analyze Sun-as-a-star spectra for active region energy estimates, overcoming spatial resolution limitations.

Findings

Active regions radiate significant energy during quiescence, comparable to large flare energies.

AR radiative losses are about 100 times smaller than magnetic energy injection rates.

This is the first detailed thermal analysis of ARs using Sun-as-a-star spectra.

Abstract

How much energy do solar active regions (ARs) typically radiate during quiescent periods? This is a fundamental question for storage and release models of flares and ARs, yet it is presently poorly answered by observations. Here we use the "Sun-as-a-point-source" spectra from the EUV Variability Experiment (EVE) on the Solar Dynamics Observatory (SDO) to provide a novel estimate of radiative energy losses of an evolving active region. Although EVE provides excellent spectral (5-105nm) and temperature (2-25MK) coverage for AR analysis, to our knowledge, these data have not been used for this purpose due to the lack of spatial resolution and the likelihood of source confusion. Here we present a way around this problem. We analyze EVE data time series, when only one large AR 11520 was present on the disk. By subtracting the quiet Sun background, we estimate the radiative contribution in EUV from the AR alone. We estimate the mean AR irradiance and cumulative AR radiative energy losses in the 1-300A and astronomical standard ROSAT-PSPC, 3-124A, passbands and compare these to the magnetic energy injection rate through the photosphere, and to variations of the solar cycle luminosity. We find that while AR radiative energy losses are ~100 times smaller than typical magnetic energy injection rates at the photosphere, they are an order of magnitude larger or similar to the bolometric radiated energies associated with large flares. This study is the first detailed analysis of AR thermal properties using EVE Sun-as-a-star observations, opening doors to AR studies on other stars.