Impact of nanoflare heating in the lower solar atmosphere

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Nugget
Number: 458
1st Author: Helle BAKKE
2nd Author:
Published: October 30, 2023
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Contents

Introduction

The solar corona has been continuously studied through observations and numerical models due its unexpectedly high temperatures. Understanding the mechanisms responsible for heating the solar atmosphere from a few thousand Kelvin in the photosphere to millions of degrees in the corona has proved to be a difficult task. It is generally accepted that the corona is heated by processes related to the solar magnetic field, and one of the leading candidates to explain coronal heating is called the "nanoflare" theory (Ref. [1], the original Parker conjecture). Nanoflares are small-scale heating events associated with magnetic reconnection, and this heating mechanism is based on the idea that nanoflares occur frequently in the corona and heat it consistently.

Unfortunately, this theory has been difficult to prove because the energy released from nanoflares could well lie are below the detection threshold of current instrumentation and nanoflares have not been observed directly yet. Furthermore, in large numbers the individual events may be so abundant that their effects merge together into a featureless continuum. We may search more fruitfully via indirect observations.

Flare mechanics

Flares are brightenings associated with magnetic reconnection in the solar atmosphere. As field lines reconnect, magnetic energy is converted into kinetic energy and heat, and the released energy is transported through the atmosphere by either thermal conduction or electrons accelerated to non-thermal energies during the reconnection event. The non-thermal electrons carry energy as they travel along the magnetic field, which they eventually lose through Coulomb collisions with the ambient plasma. Figure 1 shows an example of an idealised flare, where reconnection happens at the top of a coronal loop. Flaring events where larger brightenings are produced usually emit X-ray radiation, however not all reconnection events are energetic enough to be detected through X-ray observations (i.e. with RHESSI or NuSTAR). Accordingly there is only limited knowledge about the presence and properties of nanoflares in the solar atmosphere.

Figure 1: Standard model of a solar flare where reconnection happens at the top of a coronal loop. As the magnetic field lines reconnect, electrons are accelerated to non-thermal energies. They travel along the loop towards the footpoints and impact with the ambient plasma through Coloumb collisions, emitting hard X-rays in the process. Hot plasma fills the flaring loops via ablation (" evaporation"), and soft X-rays are emitted as a result of particles interacting within the high-temperature plasma. This figure is adapted from similar flare cartoons in the literature.

Numerical models

We have explored the role of nanoflares in the solar atmosphere by utilising numerical simulations to investigate the atmospheric response to small-scale heating events. Instead of focusing on the actual nanoflare event, we have explored the impact of non-thermal electrons in the lower atmosphere by probing the signatures in the spectral lines forming at the sites where the electrons deposit their energy. We have studied the effects of non-thermal electrons in synthetic chromospheric and transition region spectra both from 1D RADYN flare simulations and the more realistic 3D MHD Bifrost simulation. Signatures in the synthetic spectra can give an indication of what to look for in observations. The inclusion, for example, of the Ca II H and K lines to the diagnostics gives potential for observing small-scale heating events with ground-based telescopes (such as SST), allowing for higher spatial resolution compared to millimetre and EUV observations. A comparison of an observed small-scale coronal heating event to a 1D RADYN nanoflare simulation is seen in Figure 2. The RADYN flare time is much shorter than the observed event, hence the flare time cannot be directly compared. Instead, we compared the shapes and features of the observed and synthetic lines. Even though RADYN is not able to reproduce the single-peaked Ca II K observed profile, the redshift of the observed lines at locations (a) and (b) is comparable to that of the simulation. Additionally, the synthetic Hα profile is very similar to the observed profile at location (b), both in shape and temporal evolution. The comparison of the observed event to the results of our numerical models generally suggests that there is a presence of non-thermal electrons in the flaring loops.

Figure 2: Spectral evolution of the Ca II K and Hα emission lines from a RADYN nanoflare simulation, and from an SST observation of a small coronal heating event. The top panels show intensity maps of the observed Ca II K line core at the peak of the flare, where the orange crosses represent three different locations at which the spectra are taken. The bottom panels show the spectral evolution of the lines from the simulation and observation, where the overplotted line profiles are indicated at a specific time during the flare (marked along the Y-axis). The time step for the RADYN simulation is chosen at 7 s, while the time step for the observation is chosen at 6.8 min which is the same time as the intensity maps in the top panels are shown. We note that the duration of the simulated flare is much shorter than that of the observed flare, as can be seen from the Y-axis in the bottom panels.


Conclusion

We have analysed signatures arising from non-thermal electrons in a variety of numerical models, and compared the results with an observed small-scale heating event. Since direct signatures of nanoflares are difficult to determine, analysis of synthetic spectra from numerical models gives us the opportunity to provide guidance to future observations. By including lines in the visible to the analysis, we are able to expand the list of possible diagnostics for the presence and properties of nanoflares. The combination of numerical models and observations is beneficial in the continued study of nanoflares; both their impact on the lower atmosphere, and their role in heating the upper atmosphere.

This Nugget has been based on Refs. [2-4], where there is much further information.

References

[1] "Nanoflares and the solar X-ray corona"

[2] "Chromospheric emission from nanoflare heating in RADYN simulations"

[3] "Accelerated particle beams in a 3D simulation of the quiet Sun. Lower atmospheric diagnostics"

[4] "High-resolution Observations of the Low Atmospheric Response to Small Coronal Heating Events in an Active Region Core"

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