The solar community has been trying to understand the mechanism responsible for coronal heating for several decades now. In the past decade, a number of studies have shown that the active regions and the coronal loops are heated impulsively. However, such a consensus is yet to be reached for the quiet sun. Past searches for impulsive events in the EUV and X-ray are yet to provide conclusive evidence in favour of Parker’s nanoflares, hypothesised to explain coronal heating. Here we aim to search for evidence for impulsive radio emissions from the quiet sun using radio observations.  They are an excellent probe for detecting small reconnection events, which by their very nature generate energy impulsively. This work also allows us to check if the detected impulsive events satisfy the key necessary conditions for them to be important from a coronal heating perspective.

We use data from a very quiet day, with no reported X-ray and radio flare, and only one active region present on the visible solar disc. Seventy minutes of Murchison Widefield Array (MWA) data were imaged using the Automated Imaging Pipeline for Compact Arrays for Radio Sun (AIRCARS; Mondal et al. 2019). Images were made in four spectral bands with a time resolution of 0.5 s and frequency resolution of 160 kHz, leading to more than 33,000 images. We tiled the Sun using point-spread-function (psf) sized regions. The regions close to the active region, or those with emission showing a high correlation with that from the active region were not used for the analysis. The flux density time series at each such location was extracted and suitably normalised. The normalised flux density histogram for all the remaining regions is shown in left panel of Figure 1. The red lines are powerlaw fits to the tails of the histograms. Such powerlaw tails are not expected from thermal distributions and are often seen in nonthermal distributions. The powerlaw index of the red line satisfies the Hudson criterion (Hudson 1991). It is well known that the observed powerlaw index has a nonlinear relationship with the powerlaw index of the actual energy deposition events. Simulations (Bingert & Peter 2013) suggest that the powerlaw index of distribution of energy deposition events is likely to be steeper than that for the radiated power.

Figure 1: Left and middle panels: Distributions of the normalised flux density and the temporal widths of the “events” respectively. Right panel: The fraction of observation duration for which “events” were detected in a particular region.

Henceforth, we refer to the points which lie on the powerlaw tail of the histogram as “events”. We have detected events with flux density ~1 mSFU, about two orders of magnitude fainter than the weakest reported in past works. We have also studied the temporal width distribution of these events and their occupancy for a given region. These distributions are shown in middle and right panels of Figure 1, respectively. It is evident that these events are impulsive in nature and are present through out the quiet sun. There is no evidence of clustering in the regions where these events are detected. On an average these events are present for about ~5% of the total observation distribution. The results are similar at all other frequencies analysed. There is significant uncertainty in quantitatively estimating the energy deposited in the corona during a reconnection process from that radiated in the radio band.  However, in the spirit of doing the best possible, we estimate the energy associated with the detected events, and find that it is sufficient to maintain the corona at MK temperatures.

Interestingly, we notice striking similarities between the statistical properties of these impulsive events and the magnetic switchbacks recently characterised using data from the Parker Solar Probe (e.g. Dudok de Wit et al. 2020). Since, both of these are hypothesised to originate due to magnetic reconnections, it is possible that these two are related. It will require efforts both on the observational and numerical fronts to investigate this hypothesis.

Based on a recent paper by Surajit Mondal, Divya Oberoi, and Atul Mohan, “First Radio Evidence for Impulsive Heating Contribution to the Quiet Solar Corona“, 2020 ApJL 895, L39 DOI: 10.3847/2041-8213/ab8817

References:

Bingert, S., & Peter, H. 2013, A&A, 550, A30

Dudok de Wit, T., Krasnoselskikh, V. V., Bale, S. D., et al. 2020, ApJS, 246, 39

Hudson, H. S. 1991, SoPh, 133, 357

Mondal S., Mohan A., Oberoi, D., Morgan J., et al., 2019, ApJ, 875, 97

Mondal S., Oberoi D., Mohan A., 2020, ApJ, 895L, 39