[Paper Review] Nature of the energy source powering solar coronal loops driven by nanoflares

Chitta, L. P., Peter, H.+1 more|arXiv (Cornell University)|Jun 28, 2018

Solar and Space Plasma Dynamics58 citations

TL;DR

This study identifies chromospheric magnetic reconnection triggered by photospheric flux cancellation as a key energy source powering solar coronal loops in active regions. Using coordinated SDO/AIA and HMI observations, it demonstrates that 10^15 Mx s⁻¹ flux cancellation at mixed-polarity magnetic sites drives localized coronal brightenings in EUV, indicating energy release via reconnection that powers nanoflare-heated loops. The findings suggest this mechanism could also sustain the quiet-Sun corona.

ABSTRACT

Magnetic energy is required to heat the corona, the outer atmosphere of the Sun, to millions of degrees. We study the nature of the magnetic energy source that is probably responsible for the brightening of coronal loops driven by nanoflares in the cores of solar active regions. We consider observations of two active regions (ARs), 11890 and 12234, in which nanoflares have been detected. To this end, we use ultraviolet (UV) and extreme ultraviolet (EUV) images from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) for coronal loop diagnostics. These images are combined with the co-temporal line-of-sight magnetic field maps from the Helioseismic and Magnetic Imager (HMI) onboard SDO to investigate the connection between coronal loops and their magnetic roots in the photosphere. The core of these ARs exhibit loop brightening in multiple EUV channels of AIA, particularly in its 9.4 nm filter. The HMI magnetic field maps reveal the presence of a complex mixed polarity magnetic field distribution at the base of these loops. We detect the cancellation of photospheric magnetic flux at these locations at a rate of about $10^{15}$ Mx s$^{-1}$. The associated compact coronal brightenings directly above the cancelling magnetic features are indicative of plasma heating due to chromospheric magnetic reconnection. We suggest that the complex magnetic topology and the evolution of magnetic field, such as flux cancellation in the photosphere and the resulting chromospheric reconnection, can play an important role in energizing active region coronal loops driven by nanoflares. Our estimate of magnetic energy release during flux cancellation in the quiet Sun suggests that chromospheric reconnection can also power the quiet corona.

Motivation & Objective

• Understand the energy source responsible for heating coronal loops in solar active regions, particularly those driven by nanoflares.
• Investigate the connection between coronal loop brightenings and photospheric magnetic field evolution in active regions.
• Determine whether flux cancellation and chromospheric reconnection at the magnetic base of loops can supply sufficient energy to power coronal heating.
• Assess the viability of this mechanism for powering both active region and quiet-Sun coronae.

Proposed method

• Analyzed co-temporal EUV and UV images from SDO/AIA (12 s cadence) to detect coronal loop brightenings in active regions AR 11890 and AR 12234.
• Used line-of-sight magnetic field maps from SDO/HMI (45 s cadence) to track photospheric magnetic flux evolution beneath bright coronal loops.
• Identified locations of magnetic flux cancellation by measuring the monotonic decrease in photospheric magnetic flux at a rate of ~10^15 Mx s⁻¹.
• Correlated the timing and location of coronal brightenings with photospheric flux cancellation and reconnection signatures in the chromosphere.
• Estimated the rate of magnetic energy release using the formula ẋE = (1/4π) B ẋΦ H, with B ≈ 10–100 G, ẋΦ ≈ 8×10^20 Mx s⁻¹, and H ≈ 500 km.
• Compared the estimated energy release rate (~10^28–10^29 erg s⁻¹) with the observed energy requirement of the quiet-Sun corona (~10^28 erg s⁻¹).

Experimental results

Research questions

• RQ1 What is the nature of the magnetic energy source powering coronal loop brightenings in active region cores?
• RQ2 How is photospheric magnetic flux cancellation related to coronal loop heating and nanoflare activity?
• RQ3 Can chromospheric reconnection triggered by flux cancellation supply sufficient energy to heat the corona?
• RQ4 Is this mechanism viable for powering the quiet-Sun corona as well as active region coronae?

Key findings

• Coronal loop brightenings in AR 11890 and AR 12234, particularly in the AIA 9.4 nm filter (sensitive to ~7 MK plasma), are associated with photospheric flux cancellation at a rate of ~10^15 Mx s⁻¹.
• The HMI magnetic field maps reveal a complex mixed-polarity magnetic environment at the loop footpoints, with clear signs of flux cancellation and reconnection.
• Compact coronal brightenings directly above the cancelling magnetic features indicate plasma heating via chromospheric magnetic reconnection.
• The rate of magnetic energy release through flux cancellation is estimated at ~10^28–10^29 erg s⁻¹, which matches the energy requirement for sustaining the quiet-Sun corona.
• The observed energy release rate is sufficient to power the quiet-Sun corona, suggesting that chromospheric reconnection during flux cancellation is a viable heating mechanism.
• The findings support the hypothesis that nanoflare-like heating in active regions is driven by reconnection at the chromospheric level due to photospheric flux cancellation, not just coronal braiding or wave dissipation.