What drives impulsive coronal heating?

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Nugget
Number: 395
1st Author: Pradeep CHITTA
2nd Author: et al.
Published: 30 November 2020
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Introduction

Active regions host concentrations of strong magnetic fields including sunspots at the solar surface. The plasma trapped by these magnetic fields, in the cores of the active regions, is often impulsively heated to over 5 MK. These hot plasma loops appear bright in coronal extreme ultraviolet (EUV) and X-ray emission. Observational signatures of impulsive heating include rapid variability of "moss" emission in the i UV and EUV at the footpoints of hot loops on timescales below 60 s, along with near-simultaneous moss brightening at both footpoints [1, 2].

How are these impulsive heating events driven? One of the well-studied models to explain such impulsive coronal heating to high temperatures is based on nanoflares, as hypothesized by Parker. In these models, magnetic fields that thread coronal loops are continually braided by random motions of their footpoints due to turbulent convection at the solar surface. These braided magnetic fields then reconnect and relax and, in the process, nanoflares are impulsively triggered in the corona. In this traditional view, footpoints of hot loops are thought to be rooted in patches of unipolar magnetic fields at the solar surface. Consequentlny, impulsive heating is presumed to result from the coronal reconnection of braided magnetic fields. A closer examination of recently reported nanoflares, however, has revealed that footpoints of those nanoflaring loops end not just in unipolar magnetic field patches, but in regions of mixed-polarity regions at the solar surface [2]. The UV bursts, which are thought to be the signatures of magnetic reconnection in the lower atmosphere (i.e. chromosphere and transition region), are observed to directly overlie those mixed-polarity regions at the footpoints of hot loops. Furthermore, emission from the coronal part of the loop and the rapid moss variability at one of its footpoints are observed to be correlated with intensity variations of UV bursts at the conjugate footpoint [2]. These observations suggest that impulsive coronal heating may well be closely related to chromospheric magnetic reconnection. However, this earlier study was limited to only two cases.

What drives the impulsive coronal heating?

We have now conducted a statistical analysis of the magnetic setting at the footpoints of hot coronal structures. If the magnetic field patches at their footpoints are unipolar, then nanoflares are likely to be produced by the traditional view, namely, reconnection at coronal heights. If the footpoints are instead rooted in mixed-polarity regions, then interactions of opposite-polarity magnetic patches might lead to reconnection in the lower atmosphere that could play a key role in the impulsive heating. We analyzed magnetic footpoints of a large number of heating events (137 in total) from the cores of seven active regions, in an automated way. We used data from the (HMI) on board SDO magnetic field maps of the active regions, and AIA for UV (1600 Å and 1700 Å filters) and EUV (171 Å, 193 Å, and 94 Å filters) diagnostic images. We combined these data with the co-temporal solar-disk integrated X-ray flux recorded by the 1-8 Å soft X-ray GOES data. When available, we also analyzed UV imaging (1400 Å and 2796 Å slit-jaw images) and spectroscopic (Si IV, Mg II k and Mg II triplet lines) observations obtained with the IRIS to investigate signatures of chromospheric magnetic reconnection. The main results of our new work [3] are as follows.

Simple but complex

We found that even those hot loops that exhibit seemingly simpler morphological features in the corona end in complex magnetic footpoints that are often not unipolar. We briefly discuss this apparent complexity with an example (Figure 1). A hot loop system recorded by AIA, with footpoints E, W1, W2, and R, is observed in active region AR 12692 on 2017 December 23 at 10:52 UT (panel a). Magnetic field lines traced from a linear force free field extrapolation reveal the connectivity between different footpoints (i.e. E-R, E-W1, and E-W2). At footpoint-E, the dominant magnetic polarity is positive, while at footpoint-R, it is negative. Nevertheless, both footpoints have adjacent, minor respectively opposite-polarity magnetic patches, clearly discernible in the HMI magnetic field map (panel b). The UV spectroscopic observations of the lower atmosphere recorded by the IRIS at footpoints E and R show broad or enhanced line profiles (panels c-e). In particular, at footpoint-E, a double peaked Si IV line profile is observed; panel c. this indicates a bi-directional jet in the transition region. At footpoint-R, the Mg II profiles (magenta; panels d-e) are significantly enhanced compared to the quiet-Sun profiles (black; panels d-e), suggesting chromospheric heating. The interaction of mixed-polarity magnetic fields at footpoints-E and R could be responsible for the magnetic reconnection signatures in the lower atmosphere. This process could then inject energy into the loop to impulsively heat the corona to over 5 MK, as suggested by the flux-cancellation model [4].

Figure 1: Complex loop system in AR 12692. Maps of AIA 94 Å Fe XVIII (panel a), and with a grey-scale an HMI line-of-sight magnetic field map (white: positive; dark: negative; saturated at 300 G; panel b) are displayed. Various footpoints (namely, E, W1, W2, and R) of the loop system are marked. Footpoint-R is detected by our automated method [3]. The yellow colored curves represent magnetic field lines traced from a linear force-free field extrapolation. Panels (c), (d), and (e) show profiles of the Si IV, Mg II k and Mg II triplet lines recorded by the IRIS at the footpoints of the hot loop system. Both footpoints E and R that overlie mixed-polarity magnetic patches at the solar surface (panel b) exhibit broad or enhanced line profiles consistent with lower atmospheric reconnection and heating. In panel (c) the dotted vertical line marks the Ni II absorption line superimposed on the Si IV line profile. This figure is adapted from Fig. 4 (Ref. [3]), and reproduced with permission © ESO.

Conclusions

Based on a statistical analysis, we found that a majority of hot coronal structures (> 73%) have at least one of their footpoints rooted in mixed-polarity magnetized patches at the solar surface. The detected opposite-polarity patch is within 3 Mm of the loop footpoint in most cases. Moreover, magnetic flux at the footpoints changes at a rate on the order of 1015 Mx s-1 (Figure 2). Our statistical analysis points to a key role for footpoint reconnection driven by interacting mixed magnetic polarities at the solar surface in impulsive coronal heating. The main future observational challenge is then to investigate whether it is also the dominant process. For more detail, please see the full paper (Ref. [3)].

Figure 2: Statistical overview of the magnetic properties of the footpoints of hot coronal loops. Panel (a) is a histogram of the heating events, quantifying the type of magnetic structures detected at the coronal base (total 137 cases). Mixed cases (101/137) are those in which the impulsively heated coronal structures are rooted in mixed-polarity magnetized regions at the solar surface. In the unipolar cases (34/137), the detected footpoints of the hot loops are rooted in unipolar regions in the photosphere. The remaining unclear cases (2/137) are loops with footpoints that could not be determined by our method. Panel (b) shows the distribution of the minimum distance of the minor opposite-polarity field from the detected footpoints of loops in mixed cases. Panel (c) shows the distribution of the rate of change of magnetic flux, for dominant (black) and minor (blue) magnetic polarities, at footpoint regions of mixed events.

Acknowledgements

The co-authors of this Nugget, and of Ref. [3], are Hardi PETER, Eric PRIEST, and Sami SOLANKI

References

[1] "Evidence of nonthermal particles in coronal loops heated impulsively by nanoflares"

[2] "Nature of the energy source powering solar coronal loops driven by nanoflares"

[3] "Impulsive coronal heating during the interaction of surface magnetic fields in the lower solar atmosphere"

[4] "A Cancellation Nanoflare Model for Solar Chromospheric and Coronal Heating"

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