The Sun’s corona, its outermost layer, is millions of degrees hotter than its surface—a puzzle that has intrigued scientists for decades. Various mechanisms have been proposed to explain this extreme heating, with wave dissipation and magnetic reconnection being the most prominent contenders. While both are theoretically capable of supplying the necessary energy, neither has been rigorously tested in models directly tied to observations with measured inputs—until now.
Coronal loops show the measurement of the non-ideal velocity. The letters p and n stand for the conjugate footpoints of a loop and the subscripts 0 and 1 refer to time labels for the start and end time, t0 and t1, for one measurement, respectively. The yellow loop represents the initial loop at t0 and the transparent yellow loop is the hypothetical version at time t1 under the assumption of ideal MHD. The footpoints are advected to new places by the photopsheric plasma flow, as indicated by the red and blue arrows in positive and negative polarities, respectively. The orange loop is the actual coronal loop at time t1. The distance denoted by the two-headed arrow is the non-ideal distance δn due to magnetic diffusion (reconnection).
In our recent work, we demonstrate that impulsive magnetic reconnection can quantitatively and qualitatively reproduce the observed characteristics of an active region corona. By calculating the heating power based on the “non-ideal velocity,” the velocity difference between magnetic footpoints and the photospheric plasma, we identify a critical scale: flux elements reconnect in the corona at a length scale of approximately 160 km.
DEM distribution from the model and the discrepancy χ2 between the model and observations. a, The image at EUV 171 Å observed by AIA. The red box indicates the area where the DEM is calculated and shown in b. b, The DEM distribution calculated from the model by fixing R = 0.3 but varying the scale length of the cross section of the loop, L (see the color bar). The solid black curve is for the DEM inferred from the AIA observations at six wavelengths averaged over 12 minutes. The gray areas with different transparencies represent the results with 1σ, 2σ and 3σ from the Monte-Carlo test. The dash-dotted line is the curve from the model with L = 160 km. c, The discrepancy χ2 between the modelled DEM and the observed one plotted as a color scale over parameter space.
Intensity histogram from the model and the discrepancy χ2 between the model and observations.
a, The image at EUV 171 Å observed by AIA. The blue box refers to the area over which the discrepancy is
computed between the intensity histograms from the model and observations as shown in b–g. b–g, The intensity
histogram calculated from the model by fixing R = 0.3 but varying the scale length of the cross section of the
loop, L (see the color bar). The solid black curve stands for the intensity histogram from the AIA observations
at six wavelengths averaged over 12 minutes. h, The sum of the Pearson's χ2 of the intensity histogram between model and observations over all of the six wavelengths, χ2 = ∑i χ2i, where i is the index of the six AIA channels. It is plotted as a color scale over parameter space.
Our model’s predictions align remarkably well with observations. The differential emission measure, derived from multi-wavelength extreme-ultraviolet (EUV) images, matches the emission characteristics of the model corona. Furthermore, the synthesized EUV images not only mirror the loop-dominated structures seen in real observations but also replicate their intensity histograms.
Observations and synthesized EUV images showing the loop structures. The left column (a, b, and c) shows the AIA observations at three channels. The right column (d, e, and f) shows the synthetic images at the same channels generated with L = 160 km and R = 0.3. All images use linear color scales with units of DN s−1, and corresponding pairs use the same color scale. The numbers indicate the corresponding structures, including brightening loops (1), moss structures (2 and 3), and large loops (4), respectively.
This work provides support that impulsive reconnection events, rapid, localized energy releases triggered by magnetic field realignments, are a viable mechanism for heating the Sun’s corona. It offers a compelling step forward in solving one of solar physics’ most enduring mysteries.
