Nanoflare distributions over solar cycle 24 based on SDO/AIA differential emission measure observations

¹ Institute of Physics, University of Graz, Universitätsplatz 5, 8010 Graz, Austria
e-mail: stefan.purkhart@uni-graz.at
² Kanzelhöhe Observatory for Solar and Environmental Research, University of Graz, Kanzelhöhe 19, 9521 Treffen, Austria

Received: 1 February 2022
Accepted: 1 March 2022

Abstract

Aims. Nanoflares in quiet-Sun regions during solar cycle 24 are studied with the best available plasma diagnostics to derive their energy distribution and contribution to coronal heating during different levels of solar activity.

Methods. Extreme ultraviolet filters of the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) were used. We analyzed 30 AIA/SDO image series between 2011 and 2018, each covering a 400″ × 400″ quiet-Sun field-of-view of over two hours with a 12-s cadence. Differential emission measure (DEM) analysis was used to derive the emission measure (EM) and temperature evolution for each pixel. We detected nanoflares as EM enhancements using a threshold-based algorithm and derived their thermal energy from the DEM observations.

Results. Nanoflare energy distributions follow power laws that show slight variations in steepness (α = 2.02–2.47), but no correlation to the solar activity level. The combined nanoflare distribution of all data sets covers five orders of magnitude in event energies (10²⁴ − 10²⁹ erg) with a power-law index α = 2.28 ± 0.03. The derived mean energy flux of (3.7 ± 1.6)×10⁴ erg cm⁻² s⁻¹ is one order of magnitude smaller than the coronal heating requirement. We found no correlation between the derived energy flux and solar activity. Analysis of the spatial distribution reveals clusters of high energy flux (up to 3 × 10⁵ erg cm⁻² s⁻¹) surrounded by extended regions with lower activity. Comparisons with magnetograms from the Helioseismic and Magnetic Imager demonstrate that high-activity clusters are preferentially located in the magnetic network and above regions of enhanced magnetic flux density.

Conclusions. The steep power-law slope (α > 2) suggests that the total energy in the flare energy distribution is dominated by the smallest events, that is to say nanoflares. We demonstrate that in the quiet-Sun, the nanoflare distributions and their contribution to coronal heating does not vary over the solar cycle.