Widespread nanoflare variability detected with Hinode/XRT in a solar active region

TL;DR

This study provides evidence that nanoflares are widespread in solar active regions, contributing to coronal heating, with observed intensity fluctuations indicating frequent small energy releases consistent with nanoflare activity.

Contribution

The paper demonstrates that nanoflares are common in active regions and introduces a method to detect their signatures through intensity fluctuation asymmetries in X-ray observations.

Findings

Intensity fluctuations show asymmetry with more negative fluctuations.

Monte Carlo simulations indicate part of the asymmetry is due to photon statistics.

Exponential intensity decay suggests cooling plasma from nanoflares.

Abstract

It is generally agreed that small impulsive energy bursts called nanoflares are responsible for at least some of the Sun's hot corona, but whether they are the explanation for most of the multi-million degree plasma has been a matter of ongoing debate. We here present evidence that nanoflares are widespread in an active region observed by the X-Ray Telescope on-board the Hinode mission. The distributions of intensity fluctuations have small but important asymmetries, whether taken from individual pixels, multi-pixel subregions, or the entire active region. Negative fluctuations (corresponding to reduced intensity) are greater in number but weaker in amplitude, so that the median fluctuation is negative compared to a mean of zero. Using MonteCarlo simulations, we show that only part of this asymmetry can be explained by Poisson photon statistics. The remainder is explainable with a tendency for exponentially decreasing intensity, such as would be expected from a cooling plasma produced from a nanoflare. We suggest that nanoflares are a universal heating process within active regions.