Picoflares in the Quiet Solar Corona: Solar Orbiter Observations Halfway to the Sun

Abstract

X-ray observations of the Sun led Eugene Parker to propose nanoflares as the basic energy-release units that heat the solar corona. Decades later, Solar Orbiter's Extreme Ultraviolet Imager (HRIEUV), operating halfway between Earth and the Sun, revealed thousands of even smaller brightenings in the quiet corona - tiny "campfires" that are smaller and far more frequent than the fundamental nanoflares observed from 1 AU. We analyze over 12,000 of these events, deriving their thermal energies using multiple geometric models to account for volume uncertainties. Although absolute values vary, all models yield consistent power-law energy distributions and ranges, confirming their flare-like behavior. These picoflares, spanning $10^{20}$--$10^{24}$ erg, were detected by the Solar Orbiter EUI Imager while the spacecraft was at 0.56 AU from the Sun. They occur up to sixty times more often than nanoflares seen from Earth orbit and supply about 1% of the quiet-Sun coronal heating power. This previously unseen energy source may be a missing component in the solar energy balance. Their discovery extends the flare energy spectrum to smaller scales, and future Solar Orbiter observations at 0.28 AU may reveal the most fundamental flare events that sustain the million-degree solar corona.